Historic, archived document Do not assume content reflects current scientific knowledge, policies, or practices. UNITED STATES DEPARTMENT OF AGRICULTURE , BULLETIN No. 1128 Washington, D. C. PROFESSIONAL PAPER February 20, 1923 DECAYS AND DISCOLORATIONS IN AIRPLANE WOODS By J. S. BOYCE, Pathologist Office of Investigations in Forest Pathology Bureau of Plant Industry CONTENTS Page Page Introduction ..... Ey eeretersiictioh a 1 Chemical Discolorations ...... . 23 Genera] Considerations ........ 2 Discolorations Caused by Fungi. ... 24 | Woods Used for Airplane Construction 3 Sap-Stain .....-2-+-+-2-22e- 25 General Defects of Airplane Woods .. 5 Brown-Oak Discolorations .... 29 Color Comparisons .........- 14 | Decay Discolorations ......-- 30 Discolorations Caused by Wounds .. 17 | Decay in Finished Airplanes .... - oe Lightning Wounds ........ 17 | Summary PA REPRER AER et Lg aati Ae 42 Sapsucker Wounds ........ 20 epersarney wes aS Sk ae ale Pitter Mecka|s © .°.). 01.0, 20 Defects of Wood Referred to in This Bulletin, Arranged by Species .... 50 WASHINGTON GOVERNMENT PRINTING OFFICE 1923 u as AES he, ay a res Soe ao i UNITED STATES DEPARTMENT OF AGRICULTURE Washington, D. C. PROFESSIONAL PAPER February 20, 1923 ~ DECAYS AND DISCOLORATIONS IN AIRPLANE WOODS. By J. S. Boycr, Pathologist, Ofice of Investigations in Forest Pathology, Bureau of Plant Industry. CONTENTS. Page. Page. MALPTLOOUCTION — = a ose te eS 1 | Discolorations caused by fungi —----~ 24 General considerations _____-__---_- 2 Sap stares Je Ob i ake ee ees 25 Woods used for airplane construction_ 3 Brown-oak discolorations ___~_~ 29 General defects of airplane woods__-_ 5 Decay. discolorations=——)—— =—-= 30 Color is a natural characteristic of wood while showing a com- While in some the change is negligible. In such eka spruce species as redwood (Sequoia sempervirens (Lam.) caused by an ex- Wndl.), incense cedar, Douglas fir, juniper (Juni- cessive tighten- ? y ing of the ten. perus), white ash, true mahogany, and white ocak sion wires In the there is a decided contrast between the light-colored 3 sapwood and the dark heartwood, while in spruce, fir, western hemlock, and yellow buckeye (Aescutus octandra Marsh) the heartwood more nearly approaches the sapwood in color, and in some cases it is difficult to distinguish between the two. Color is not always uniform in the heartwood. It is necessary to be thoroughly acquainted with woods to be able to recognize normal color variations at a glance. Color should always be observed on a freshly cut surface and the surface (whether radial, tangential, or cross) recorded when making permanent observations. All woods change color on exposure to light and air (54), the most noticeable change occurring in the lighter colored woods, particularly of the conifers. The first change is a yellowing, then a graying, and finally in some conifers a decided browning. These color changes have no weakening effect on the me- chanical properties of wood, since the discolored portion is a very thin surface layer and microorganisms play no role in this change ibe Ye, « 4. DECAYS AND DISCOLORATIONS IN AIRPLANE WOODS. 15 of color. Light is necessary. Ordinarily these color changes are deepened by direct sunlight, which has a greater influence on the color changes than diffused light. Green wood usually differs in color somewhat from air-dry mate- rial of the same species, even on a freshly cut surface. There is a tendency for the more delicate tints to be obscured by drying. A system of color standards is at present sadly needed in describing colors of wood (53). Furthermore, the condition of the wood, that is, Whether green, partly air dry, or fully air dry, invariably should be given consideration. The heartwood of sugar-pine, eastern white-pine (Pinus strobus Linn.), and western white-pine lumber often becomes a pink, light- red, or vinous-red color upon air drying. This color is not confined to the surface layer, but is usually uniform throughout. No reduc- tion in strength results. Wood of this kind is very pleasing to the eye, so it is often desired by pattern makers. This discoloration need not be confused with an incipient decay, since it 1s so uniform throughout. Furthermore, it terminates abruptly in a horizontal direction and does not shade off gradually into the normal light- brown or cream-colored wood. Color is considered an index of strength properties (14, p. 359-360) in certain cases. The French marine department distinguishes two classes of Kuropean oak (Quercus robur L.), inferior wood (bois maigre) and good wood (bois gras). The former, which is straw yellow in color on a fresh cut, is much more subject to atmospheric influences; that is, it shrinks, swells, warps, twists, and splits more readily than the latter, which is pale brown to red brown in color. This is taken into account in specifying in what part of the con- stcuction the two types of wood shall be used. The Danish-Prussian marine specifications distinguish three colors of green oak wood, whitish yellow, brownish yellow, and reddish yellow, all three fre- quently with a tinge of gray. The first color on drying becomes straw-colored or sand gray, the second greenish brown, and the third reddish yellow or a dirty or dusty yellow-brown. It is considered that the unseasoned or fresh wood with any brownish color is de- cidedly poor in quality. The foregoing seems to be somewhat contradictory. In the opin- ion of the writer, trusting to the vagaries of color is an exceedingly uncertain method by which to judge the strength properties of wood within a species or group and has nothing to recommend it as com- pared to the reliable index of the ratio of summer wood to spring wood per annual ring, which is particularly easy to judge in ring- porous woods like oak. There is a widespread opinion in regard to southern bald cypress (Taxodiwm distichum (Linn.) Rich.) that the darker the heartwood the more durable it is, but in reality the color of the heartwood makes no difference. Most woods when dried after a prolonged immersion in water reveal a grayish, lusterless color, much like that caused by steaming (see p. 10). Oak changes to a blue-black or a gray-black color after such treatment. Wood becomes a dirty gray to gray-black color after long exposure to the elements. This is well illustrated by unpainted poles, fence rails, posts, and shingles. The color change is caused by a number 16 BULLETIN 1128, U. S. DEPARTMENT OF AGRICULTURE. of factors (52), but most important is a chemical reaction in which iron plays an important part. Timber is not weakened by this discol- oration, since the action is confined to the surface. In boards cut from red cedar (Juniperus virginiana Linn.) white streaks are frequently found mingling with the normal red heart- wood. Such streaks are the white sapwood, the mingling being due > to the irregular outline of the stem to which the heartwood con- forms or to layers which never change to heartwood. In Sitka spruce the heartwood has a light reddish tinge, slightly distinguishing it from the sapwood. Some trees of Sitka spruce, however, have a pronounced reddish or brownish pink heartwood, which is quite uniform throughout. The color difference is striking in a planed board or timber containing both heartwood of this kind and characteristic white sapwood. The same phenomenon undoubt- edly occurs, in red and white spruce, where it would be even more noticeable, since the heartwood in these species is normally as hight colored as the sapwood. This reddish heartwood is just as strong as wood of the usual color and can be safely utilized. The same condi- tion is reported as being quite common in the Himalayan spruce (Picea morinda Link) in India (16, 29). The brown heartwood of incense cedar (S, p. 22-24) and western red cedar often has a reddish to purplish tinge, varying in intensity even in the same piece, while in other trees it may be completely lack- ing. It is entirely without significance in relation to the strength of wood so affected. In certain softwoods color variations may be connected with changes in the rate of growth. In the heartwood of Douglas fir, which has a distinct reddish or orange-reddish hue, the reddish color may be strongly intensified in long regular bands. A careful exami- nation will show that this color change is confined to a definite group of annual rings, narrower than those on both sides or containing a greater proportion of summer wood. The brown heartwood of the cedars also varies in this way. The so-called “ yellow fir,” from the slowly grown, exceedingly narrow ringed outer layers of the old coast Douglas firs, is another example. The origin of such variations can be readily recognized, since the color is. confined to a definite group of annual rings. | Occasionally an apparent discoloration in heartwood may be due to the failure of the wood to change color uniformly during the transition from sapwood to heartwood. This has been noticed in white ash, Douglas fir, western red cedar, western larch (Larix oc- cidentalis Nutt.), and other woods. The sapwood of white ash is white or straw colored, while the heartwood is grayish brown, some- times with a reddish tinge. Hence, when the condition above men- tioned is found, the grayish brown heartwood will contain sharply delimited straw-yellow areas of various sizes and shapes. The wood is not weakened. How to avoid confusing this condition with the initial stages of white-rot will be considered later. Discoloration may be caused by dirt or dust. Surfaced or sanded white pine or sugar pine is sometimes found covered with tiny little grayish black streaks following the grain of the wood. A close ex- amination will show that this is due to deposition of dust in the numerous resin ducts. This is especially apparent against the almost eee re ee ee ee ee Oe ee ee Niece meer iahge DECAYS AND DISCOLORATIONS IN AIRPLANE WOODS. 17 white wood of these species, whereas in darker woods the streaks would pass unnoticed. Burns or scorches in wood may occur from the use of high-speed saws if the saws are not set properly to provide sufficient clearance. Improperly set planing knives will produce the same effect. Usu- ally such burns, appearing on the face of the piece as dark-brown to blackish blotches, are very superficial and can be planed off. The injurious effect is negligible. Deep burns, extending through a piece one-eighth or even one-fourth of an inch in thickness are rarely encountered and are usually confined to particularly susceptible woods, such as the white pines. These may result when a high-speed sander stops suddenly. The wood is injured and can not be used for highly stressed parts. Burns usually occur in the remanufac- ture of dry lumber and not on green lumber in the mills. DISCOLORATIONS CAUSED BY WOUNDS. The term “wounds” as applied to trees includes not only those scars by which the bark is removed from living trees, exposing the sapwood or heartwood with the death of the cambium over the ex- posed surface, but also those injuries by which the cambium is temporarily damaged but not killed. The cambium, which is very susceptible to injury, is the very narrow layer of delicate growing tissue of a tree situated at the junction of the living bark and sap- wood. When this tissue is injured or killed, a healing or callusing process immediately begins which causes a dip or wave in the grain. Consequently, irregularity of grain in a timber often indicates prox- imity to a wound. : Wounds in living trees result from a variety of causes, among which may be mentioned fire, lightning, insects, birds, and man. All such injuries are usually accompanied by a discoloration of the wood, particularly the sapwood. Such discolorations are most intense in the hardwoods, especially in the sapwood of such species as white ash, hickory, maple, birch, and tulip poplar. When the wood of a living tree is exposed to the air it dries out and changes color. In softwoods the change is to a grayish brown or dead-gray color, while in hardwoods the change ranges from a deep brown to almost black, most noticeable in the sapwood. This color change is an oxidation process. Although the wood is not weakened by this change, wound wood of this type should be avoided, owing to the fact that during its exposure to the air it often becomes infected by wood-destroying fungi and may be weak- ened by incipient decay. LIGHTNING WOUNDS. The general appearance of lightning injury is readily recognized. Spike tops and stag heads, together with the spiral wounds exist- ing for many feet along the trunks of the trees, are unmistakable. Besides such wounds, the cambium is very susceptible to electrical discharge and may be affected for some distance down the tree with- out any outward visible indications. This irritation to the cambium results in the formation of a layer of cells changed in both shape and structure from the normal. Often in the conifers an unusually large number of resin cells or resin ducts are formed within this injured 9997—23——3 18 BULLETIN 1128, U. S. DEPARTMENT OF AGRICULTURE. portion. In ashort time the cell formation returns to normal. Ulti- mately, as the growth of the tree proceeds, these lightning rings, always following one definite annual ring, are deep within the wood, extending completely or partially around the circumference over a varying distance. When the tree is worked up for lumber certain of the boards may have such lightning rings extending completely through, both in width and length. Such a board then consists of two layers of wood held together by a zone of abnormal structure forming a plane of cleavage. Checking often occurs along this line, since the continuity of the medullary rays may be interrupted. Such checks are striking, since they invariably are tangential, following an annual ring on end section or radial face but not visible on the tangential face. This Is not at all an un- common defect in air- plane timber. An ab- normal number of resinducts may be found in the annual ring following many types of mechanical injury, but for prac- tical purposes there is no difference between such so-called trau- matic resin ducts and the abnormal ducts formed as a result of hghtning injury. It isself-evident that wood with these hghtning rings must be used with discre- tion. Eventhough the lightning ring does not check on dry- ing, when a mem- Fig. 4.—Section from a finished interplane strut, showing ber with this defect is a small lightning injury in Sitka spruce. p ut under severe strain and stress a serious check may develop. Of course, every member showing a hghtning ring need not be considered valueless. Such a defect in the stream line of a strut, for example, would be trifling, while a much shorter ring in the butt or inner bay of a wing beam, particu- larly if in the same plane as the bolts, would be serious. The same ring in the tip of such a beam could be overlooked. The detection of lightning rings in rough lumber is exceedingly difficult, unless accompanied by small wounds, which is sometimes the case. Then such wounds must be scrutinized closely for the presence of a lightning ring. Two or more of these wounds, which resemble sapsucker wounds, occurring on the same annual ring and connected by a lightning ring, are sometimes found. Figure 4 shows one of these wounds on an interplane strut, in this case not of | a Te ys oe eee . ‘) “ . ice aaatie eet — — nee Ee ee ee eee eeEeeeeeeEeEeEeEeyeeeeeeeeEeeEeEeEeEeEEeEeEeEeeeee DECAYS AND DISCOLORATIONS IN AIRPLANE WOODS. 19 any importance, since the injury occurred alone with only a very short lightning ring and on the stream lining where high strength is not requisite. Lightning rings are more Teadily detected on a member before it is sanded. In some cases the seriousness of the defect can be determined after planing but before the piece is shaped. This is usually possible when the defect runs entirely through the piece. In white fir the lightning rings are easily detected both on cross section or on the radial face. The normal color of the summer wood is a light brown, while the lightning ring is a pronounced brown or purplish brown, which stands out strongly against the whitish sapwood or heartwood. Abundant resin ducts occur in these rings. Lightning rings in incense cedar are dark brown in color, standing out plainly j in the white sapwood, but are not so apparent, "although still recognizable, against the reddish brown heartwood. Resin ducts do not ac- company lightning rings 1n cedar. Sitka spruce wood is rather susceptible to the effects of elec- tricity. The light- ning rings appear as light to dark brown bnes in the pale pink- ish heartwood or white Fie. 5.—-Cross section from an unfinished elevator beam, p Wegmoe deities are eo criaa ces ecendime Giticels across the section can ne found which appear seen in the summer wood of the fifth annual ring from 2 the bottom. The defect ran the entire length of the to be chiefly composed _ beam. of resin ducts; in fact, when viewed on the end section, it is seen that the resin ducts are so numerous that they almost coalesce. This condition is illustrated in Figure 5. Furthermore, spruce wood is peculiarly susceptible to dis- coloration by hghtning injury. Often in connection with a lightning ring a reddish brown discoloration is found, somewhat tinged with purple. This discoloration rarely extends radially more than 3 or 4 inches from the lightning ring toward the pith, but may extend 2 feet beyond the limits of the ring in a vertical direction. Wood so dis- colored is not weakened. Furthermore, the color is not sufficiently intense to detract from its value for any purpose, particularly since the discoloration when varnished appears merely as a darker tone of the normal heartwood. The lightning rings found in Douglas fir are red-brown in color, darker than the summer wood and consequently are quite apparent in the white sapwood and orange-red or yellowish heartwood. These rings are practically composed ‘of resin ducts. The ducts are smaller than in Sitka spruce. The reader must not get the impression from what has been written that lightning rings are a feature of every piece of wood, but they do occur and must be taken into account. 20 BULLETIN 1128, U. S. DEPARTMENT OF AGRICULTURE. SAPSUCKER WOUNDS. Sapsuckers are a group of woodpeckers which extract the sap from the inner bark and sapwood of living trees and eat the cambium. The final result after the wound has healed or callused over is the so-called bird pecks (15, 35). This injury is often accompanied by extensive staining, particularly in the hardwoods. On the ends of logs or boards the healed wounds appear as stained areas of varying size, each containing a more or less open, short, radial check in con- nection with distorted grain. The general appearance is a T-shaped or triangular mark or check surrounded by a stain varying from brown to almost black. More than one usually occurs in the same annual ring. On the edge-grain or slash-grain faces of sawed lumber these injuries usually appear as small knots or distortions in the erain, surrounded by more or less stain which is usually localized, but the stain may be accompanied by a bleaching which extends for some distance. ‘The stain is always adjacent to the distorted grain, and the more distorted the grain the greater the extent of the stain. | The stain appears to be the most injurious of the two, but in reality the distorted grain is the only cause of weakening in the wood. The strength of the wood is not much affected, so that wood with bird pecks in most cases can be safely utilized: Figure 6 shows a sec- tion from a white-ash longeron with a minor injury of this kind which does not impair the usefulness of the member. Pieces are some- Fic. 6.—Section from a times unsuitable for handles, owing to the ine a “sapekcker’ fury tendency of the grain to roughen up at these or bird’ peck “im white” places when’ planed.-:1i fhe pecksoaregnu. merous 1n one annual ring it is best not to use the piece, for although it has not been determined by comparative tests it is quite probable that such material is reduced in strength. Checks or wind-shakes are very prone to occur along an annual ring containing numerous sapsucker wounds or even at individual in- juries. These often prove to be serious in thin veneer, since pieces of the distorted grain are likely to fall out. Sapsuckers are responsible for much of the curly grain and bird’s-eye found in tulip poplar. Both stain and bird’s-eye in this species are shown in Figure 7. Practically all tree species, both softwoods and hardwoods, are subject to this type of injury, but hard maple, soft maple (Acer sac- charinum Linn.), tulip poplar, and hickory in particular stain badly. The bird pecks are common in white ash, but the accompanying stain is generally closely localized. -PITH-RAY FLECKS. Pith-ray flecks, which are also termed medullary spots and pith flecks, are caused by the larve or grubs of certain insects living in the cambium of living trees during the growing season (10, 17, 20, 21, 44, 67). These insects comprise several species of the genus Agromyza belonging to the order Diptera. On the end section of logs or lumber the flecks appear as small brown crescent or half-moon 7 z DECAYS AND DISCOLORATIONS IN AIRPLANE WOODS. 21 shaped areas, which on the tangential or slash-grain face and the radial or edge-grain face of a board appear as brown streaks, usually running in a vertical direction. (Figs. 8 and 9.) The wood for a little distance around a pith-ray fleck may be darker than normai. This is particularly so in poplars or cottonwoods (Populus spp.). On the whole, the injuries are not at all serious, having no noticeable effect on the strength of the wood unless the flecks are exceedingly Fic. 7.—Slash-grain or tangential surface of a tulip-poplar board, showing stain and bird’s-eye caused by sapsuckers. One-third natural size. (Courtesy of the U. S. Biological Survey.) numerous. Only in the cherries (Prunus spp.) may a weakening be expected, for there the affected wood tissues are broken down, while in the other woods they are but little distorted. Furthermore, the presence of pith-ray flecks is usually hard to detect in the heart- wood of cherries. The color of the heartwood differs but little from the color of the pith-ray flecks. Pith-ray flecks are found in all the common poplars or cotton- woods, birches, maples, cherries, basswood, and many others, but there is considerable variation in their abundance on different closely related species. For example, these flecks are very common in soft maple, while they are rather infrequent in hard maple. In river, gray, and paper birch (Betula negra Linn., B. populifolia Marsh, and B. papyrifera Marsh) they are found in abundance, but are 22 BULLETIN 1128, U. S. DEPARTMENT OF AGRICULTURE. somewhat uncommon in yellow birch, although the writer has found them from time to time in propeller stock of this species. Softwoods Fic. 8.—Transverse section of a branch of river birch, showing pith-ray flecks. Natural size. Fic. 9.—Tangential section of the trunk of a tree of silver maple, showing pith-ray flecks. Natural size. EE EO Eee DECAYS AND DISCOLORATIONS IN AIRPLANE WOODS. 20 are not subject to these pith-ray flecks, but a somewhat similar injury in western hemlock known as black check results from the work ot a different insect (/7). CHEMICAL DISCOLORATIONS. The sapwood of many species of wood is subject to discolorations, varying widely in appearance but fundamentally the same, which are the result of chemical action (3). Sapwood is rich in organic com- pounds and also contains certain soluble ferments which facilitate the oxidation of such compounds. Under favorable temperature con- ditions, for example, when green sapwood is exposed to the oxygen of the air, these ferments, known as oxidizing enzyms, act on the organic compounds in the sapwood. The result of their action, which is an oxidation process, is a discoloration of the sapwood, with the colored substance most noticeable upon microscopic examination in ue cells mainly concerned in the storage and transportation of ood. Hot, humid weather is most favorable for this staining. Cool, dry weather retards it or prevents it entirely. Logs immersed in water are not affected. Light is not necessary for this reaction, as it takes place just as readily in darkness. The stain is confined to the im- mediate surface layer, and the wood is not weakened. The most practical method of prevention, if this is considered necessary, is by dipping the green sap boards into boiling water for a few minutes as they come from the saw. HARDWOODS. Birch, maple, and cherry stain a reddish yellow or rusty color. The wood of alder becomes very intensely red or red-brown on freshly cut surfaces, often within an hour or so after the surface is exposed (40). In the case of red alder (Alnus oregona Nutt.), if the wood dries and remains white, the red color will appear upon the addition of water in the presence of air, provided the temperature is favor- able. A bluish stain often occurs in red gum (Liguidamber styraci- flua Linn.). - The European linden (77lia europaea, Linn.) is subject to a strik- ing discoloration (39), which probably also occurs on basswood in this country. When freshly sawed boards are so closely piled that they dry slowly, a more or less apparent dirty green color ap- pears in from 8 to 10 days. Under very favorable conditions the color is exceedingly bright and intense. The color varies between wide limits, from yellow-green or brown-green through all possible gradations to the purest moss green. Only the outer layers of the wood are colored. Usually the stain extends to a depth of one thirty-second of an inch or rarely to a depth of one-eighth of an inch. The staining, although it is the result of a chemical reaction (an iron- tannin reaction), is not dependent on temperature, since it occurs just as readily in winter asin summer. Too much moisture hinders the reaction, but a certain degree of moisture is essential. If the boards are dried quickly no staining results. SOFTWOODS. - Coniferous woods are not so commonly subject to this type of dis- coloration, but there are a few examples. The ends of incense-cedar 24 BULLETIN 1128, U. S. DEPARTMENT OF AGRICULTURE. logs sometimes have a decided brownish red stain on the sapwood. This is of no importance, because it does not occur on sawed lumber except so faintly as to be almost invisible. A very unsightly discoloration known as brown-stain (43, p. 305-307), which, however, does not weaken the wood, often occurs on sugar pine, but is frequently not noticeable until the lumber has been finished. ‘This appears in the sapwood as a streaky, dirty, light to dark brown or brownish black discoloration, and may be super- ficial or very deep. It is quite striking against the faint yellowish white sapwood in finished lumber. The discoloration cccurs on green sap lumber upon exposure to the air and may appear during air drying or kiln drying. In the last instance it is known as kiln burn, but it does not differ from the brown-stain and is probably sometimes due to defective circulation in the kiln. Brown-stain is particularly bad in lumber cut in early spring. Hot, humid weather and poor circulation of air in the lumber piles favor the staining, while cool, dry weather and proper piling tend to prevent it. This brown stain is an oxidation process similar to the others, but whether it can be prevented by the hot-water treatment is doubtful, since the discoloration often extends deeply into the lumber. The wood of sugar pine in dead trees, standing or down, may be affected by a very brilliant orange stain which occurs in spots or as a solid color, but more often is seen as narrow to broad streaks parallel to the grain of the wood. It is found in both heartwood and sapwood. ‘The exact cause of this discoloration is unknown, but it is probably the result of chemical reaction, since no fungous mycelium has been found associated with it. While the wood is apparently not weakened, the presence of this stain indicates that the lumber came from dead trees, and it should be closely watched for signs of decay and insect borings. DISCOLORATIONS CAUSED BY FUNGI. From an economic standpoint by far the most important discolora- tions in wood are caused by fungi. Fungi are very simple plants which can not live on the simple food elements of the soil and air and build up complex organic matter, as is done by the green plants with which we are familiar, but must have organic matter already prepared in order to sustain life. This they find in the material built up by green plants; hence they may attack living plants, or dead portions of such plants, or any dead vegetable matter. Some live on animal matter, but these do not concern us. The develop- ment of fungi is dependent upon a supply of oxygen, of which there is always sufficient in the air, a certain degree of moisture, a suit- able range of temperature, and the necessary food substances. The maximum and minimum of these requirements vary widely with different fungi. The fungous plant consists of very fine threads (hyphz), which are invisible to the naked eye unless they occur in mass. Individual hyphe require magnification by a compound microscope. Collec-- tively, the hyphe are termed mycelium. The hyphe usually live in the tissues of the substance on which the fungus is growing. The fruiting bodies or sporophores, which vary in size from those so small as to be invisible to the naked eye except in a mass to others quite 2 \ ; — , \ 4 —— =< - J EE DECAYS AND DISCOLORATIONS IN AIRPLANE WOODS. 25 large and conspicuous, appear on the surface after the hyphe have developed vigorously. The fruiting bodies bear the spores, which are microscopically small reproductive bodies of relatively simple structure. The spores, being very light, are borne about by air currents. If they alight in a suitable place under proper conditions, germination takes place and hyphe develop. __ Fungi growing on wood may be roughly divided into two groups, depending on the habit. of growth of hyphe. In the first group are placed thosé fungi whose hyphe live on the substances contained in the various cells of the wood, while to the second group belong those whose hyphe attack the actual wood substance of the cell walls and destroy it. The first group is principally represented by the sap- staining or discoloring fungi, so called because they produce various discolorations which are confined to the sapwood. To the second group belong the wood-destroying fungi. SAP-STAIN. DESCRIPTION. Sap-stain, which has been extensively studied (23, 27, 38, 50, 51), even though it may render wood very unsightly does not reduce its strength for practical purposes. The discoloration is normally limited to the green sapwood, because as a rule there is neither sufficient food material nor moisture in the dry dead heartwood for the development of the fungus. The discoloration is usually most intense in the medullary rays, since in these tissues the bulk of the food material is found. The stain is produced in two ways, either by a reflection of the color of the hyphe through the cell walls of the wood or by an actual color solution excreted by the hyphe, which stains the wood itself. These stains vary in color from blue or blackish to reddish, the former being the most common. Since these fungi do not attack the cell walls in which the strength of the wood reposes, except to a negligible extent, discolored wood is not. appreciably weakened. This has been determined by comparative mechanical tests on stained and unstained wood (4/,; 56, p. 13-14; Pe npe Lh): aah the strength of the wood fibers is not impaired by such stains, the wood is objectionable in places where color is a factor. In a highly varnished interplane strut, for example, a stained streak is unpleasant to the eye. Furthermore, it may lead to a strong prejudice against the airplane having such a member, be- cause while by the uninitiated a dangerous defect not readily ap- parent is passed unnoticed, an unsightly though harmless discolora- tion is considered to indicate a serious weakness. Where the dis- coloration is to be covered up or painted there is no reason to ex- clude it. It must be remembered that the conditions which promote the development of the fungus discoloration are highly favorable to the development of true wood-destroying fungi. These conditions are a comparatively high humidity and warm weather. Sap-stain is at its worst during warm wet weather, when the humidity of the air is relatively high and lumber dries slowly. It is at such periods that the most severe staining may occur if the lumber is not properly handled. The climate of the Pacific Northwest is usually exceed- SB 26 BULLETIN 1128, U. S. DEPARTMENT OF AGRICULTURE. ingly favorable for the development of wood-staining and wood- destroying fungi during the spring and summer months. It is from this region that the three most important airplane woods—Sitka spruce, Douglas fir, and Port Orford cedar—are obtained. Wood containing very severe sap-stain therefore should be care- fully examined for the presence of wood-destroying fungi. If de- cayed, the wood will be brash and may be softer and less tough when the fibers are picked with a knife. If any doubt exists after an inspection, the decision should be based on a microscopical ex- amination or a mechanical test by a qualified expert. The most important of these stains from an economic standpoint is blue-stain, caused by various species of Ceratostomella, which may be found on almost any hardwood or softwood. Softwoods are more commonly affected, and certain species are particularly sus- ceptible. This is due both to the character of the wood and to the climatic conditions of the region where the species occurs. The discoloration may be more or less superficial, occurring as spots or streaks. If the staining is severe, however, the entire sapwood will be affected, so that it can not be surfaced off. The fungi causing these stains are not readily seen, but sometimes if a deeply stained, almost black piece is inspected with a hand magnifying glass, in- numerable bristles with a bulbous base will be observed. ‘These are the fruiting bodies, containing an enormous number of spores, which are exuded and are carried about by air currents. Falling on green sap lumber they sprout, the hyphe develop, and more blue-stain re- sults. Under favorable conditions blue-stain may develop with sur- prising rapidity, appearing on lumber within a day after sawing. Other colors, such as black, brown, gray, red, pink, and violet, are caused by species of Hormodendron, Hormiscium, Graphium, Pen- icilhum, and Fusarium. These discolorations are not nearly so common as blue-stain. Certain other discolorations of sapwood are produced by fungi be- longing to the molds, of which the green mold on fruits or in certain cheeses is an example. Usually such stains: are superficial and readily surface off. They occur on both hardwoods and softwoods. The bluish or blackish stains are difficult to separate by visual inspec- tion from the true blue-stain. CONTROL. Considerable study has been devoted to the development of methods of prevention and control of sap stains caused by fungi (1, 25, 72). Naturally most of this work has been concentrated on blue-stain, and the following paragraphs are most directly applicable to it, but will probably also apply fairly well to the others. Blue-stain may be checked after it has started. but the stain can not be eradicated unless it is so superficial that it can be planed off. Therefore, the keynote of all treatments must be prevention. : | Unfortunately, there is no one principle that can be applied to the prevention of this discoloration. Staining may take place at any time after the trees are felled or, in the case of dead timber, while they are still standing. Hence, in logging operations in regions where blue-stain is of importance, the logs should be removed from the woods as soon as possible after the trees are felled and bucked DECAYS AND DISCOLORATIONS IN AIRPLANE WOODS. 21 (cut up into log lengths). The practice of leaving logs lying in the woods for months can not be too strongly condemned, as this not only causes blue-stain but also promotes the growth of wood-destroying fungi. Furthermore, the inevitable attacks of wood-boring insects assist greatly in the spread of blue-stain and decay. When the trees are bucked the narrow space left by the saw kerf between the logs as they are lying end to end affords an ideal situation for the develop- ment of the blue-stain fungi. Such logs often stain deeply, while those with the ends fully exposed remain ‘entirely free from discolora- tion. As soon as the logs are in the mill pond danger from staining is over for the time being, since the oxygen supply is so reduced that the fungi can not develop. The greatest danger of all is encountered during the process of drying the rough lumber as it comes from the saw. The best method of preventing blue-stain is by kiln drying. If the stock checks easily, so that low ‘temperature and high humidities must be maintained over a considerable period, some of the other staining fungi such as molds, may develop. But these can be checked by raising the tem- perature in the kiln to about 160° F. or shghtly more for an hour by turning live steam into the kiln. When this is done, care must be taken to keep the air saturated while steaming and to reduce the humidity gradually after steaming. When the “stock has once been dried properly the moisture content has been so reduced that there is no more danger from staining, provided it is kept dry. A dispute that arose over the efficiency of a dry kiln was immediately settled by the fact that the blue-stain fungi had resumed vigorous growth the day after the stock was removed from the kiln. This could not have occurred if the lumber had been properly dried. All airplane lumber should be kiln-dried immediately, since this not only prevents blue-stain, but also stops the growth of wood-de- stroying fungi, prevents future checking, and greatly reduces weight without in any way injuring the lumber, provided temperatures that are too high are avoided. In case ‘kiln drying is impossible, treatment with antiseptic solu- tions is of considerable value. As it comes from the saws the ereen lumber is dipped into a hot or cold chemical solution. The solutions most commonly employed are sodium carbonate or sodium bicar- bonate in water. Neither is 100 per cent effective under optimum conditions for staining, but they aid materially in checking discolora- tion. These two chemicals, however, color the tr eated wood a decided yellow or brownish. Sodium fluorid, although it does not stain the lumber and is slightly better for blue-stain, is not so effec- tive against certain molds as the two solutions first mentioned. ‘This chemical is seldom used. It must be remembered that the strength of the solutions must necessarily vary with the conditions. The more favorable the conditions for blue-stain, the stronger the solutions should be. After being dipped in any of these solutions the lumber must be carefully open piled, that is, with spaces between the boards to imsure good ventilation. Narrow cross strips or “stickers” chemically treated should be used, to prevent staining at the points where the boards and cross strips meet. Detailed instructions as to the proper methods of piling lumber may be consulted elsewhere (4, p. 17-21). 28 BULLETIN 1128, U. S. DEPARTMENT OF AGRICULTURE. Salt is of little or no value in preventing blue-stain in comparison with the other chemicals. The application of salt after blue-staining has well started is almost a waste of money. In fact, the application of wet salt or a strong salt solution may prove detrimental in the long run, for if the lumber is dried after such treatment the affinity of the salt for water may cause the moisture content to remain much higher than normal. Mercuric chlorid in a 0.1 per cent solution is exceedingly effective against blue-stain, but on account of its highly poisonous nature and extremely corrosive action when in contact with many metals it is little used. Shipping green stock closely piled in closed box cars during the spring and summer months is almost certain to result in severe stain- ing. Indeed, the writer has seen some stock handled in this way which stained even in winter. On the other hand, any measures taken to prevent staining, such as open piling in gondolas or on flat cars, will almost certainly result in severe checking. Of the two evils, checking is by far the most serious in airplane stock, since checked lumber is greatly reduced in strength, while the stained lumber is only somewhat unsightly. Shipping green lumber in the close hold of a vessel, particularly if tropical seas are to be traversed, is an invitation to swift and sure disaster as far as sap staining is concerned. It is doubtful whether dipping in any chemical solu- tion now used, except possibly mercuric chlorid, would be effective under such severe conditions. But, to repeat, the most effective measure to employ against blue- stain is speed in drying the wood. Get the logs from the woods to the saw with the greatest rapidity and the lumber from the saw di- rectly into the dry kiln. SAP-STAIN ON SOFTWOODS. Certain species are peculiarly susceptible to sap-stain. This is due both to the character of the wood and to the climatic conditions of the region where the species grows. Western white pine, spruce, and southern yellow pine, the last-named wood including longleaf pine (Pinus palustris Mill.), shortleaf pine (P. echinata Mill.), and loblolly pine (P. taeda Linn.), are very subject to sap-stain, especially blue-stain, while true fir and cedar are not so easily affected. Douglas fir occupies an intermediate position. Besides blue-stain, a red stain has been very commonly found on Sitka spruce airplane lumber. It occurred abundantly in the East on stock in cars just arrived from the Pacific coast and also developed on material along the Atlantic coast which had arrived unstained at the port of embarkation but was held over awaiting shipment. The stain appeared as terra-cotta or brick-red spots on the rough lumber, varying from very faint to a pronounced color. In the stock worked up in the factories in this country it was found that the stain was superficial, usually surfacing out during remanufacture; but reports from abroad indicate that the fungus developed very intensively by the time the lumber reached European ports, and the discoloration penetrated deeply into the sapwood. The appearance of the wood is not marred to the same extent that it is by blue- DECAYS AND DISCOLORATIONS IN AIRPLANE WOODS. 29 stain, and as far as is known no reduction in strength results. The fungus causing the discoloration is as yet unknown. Blue-stain is very severe on the white pines and is particularly noticeable because of their white wood. Plate I, left part, shows a section from a sugar-pine rib web in which the sapwood is stained to some extent. The small, darker, bluish black spots are the ends of the medullary rays, in which, as before stated, the fungous myce- lium is most abundant. The longer streaks are the resin ducts. Certain fungi (Penicillium spp.), stain the sapwood of the pines an orange-red to a crimson-red color. Another fungus (/usariwm vosewm Link) is responsible for a pink to lilac color in the same woods. The color is produced by means of a pigment secreted by the hyphe, which actually dyes the wood. A wood-staining fungus (Zythia resinae (Fr.) Karst.) has been reported in Europe (9) as working on finished pine lumber after the wood has been oiled. The discoloration was characterized by violet to dirty red or even dark grayish brown flecks beneath the oiled surface of the wood. The spots were covered with minute pustules varying from violet, orange, and brown to black. These constitute the spore-producing bodies. The discolored areas ex- tend within the wood as streaks closely associated with the medul- lary rays and resin ducts. The report does not state whether the discoloration was confined to sapwood. Apparently the wood was not reduced in strength. As far as is known, this stain has not yet been found in the United States. SAP-STAIN ON HARDWOODS. Hardwoods are not as subject to the stains caused by fungi as are softwoods. In hardwoods, when sap-stain does occur, the discolora- tion is most intense in the medullary rays and large pores or vessels. In a wood such as yellow birch, in which these vessels are not too closely crowded, the stain, if not too severe, appears in longitudinal section aS very narrow bluish black lines or streaks following the erain of the wood. This stain will not necessarily be confined to the surface layers, but may extend entirely through the sapwood. Of all the hardwoods, however, red gum seems to be the most susceptible to stains caused by fungi. BROWN-OAK DISCOLORATIONS. A somewhat different discoloration than those previously de- scribed, in that it is confined to heartwood only, is the “ brown oak” (78, 79) found in Great Britain. This is also known as “red oak” and “ foxiness,” but the name first given is most commonly accepted. Instead of the normal heartwood, certain trees of the common Euro- pean oak have a dull-brown to rusty brown or even rust color in the heartwood. In some cases the color is uniform, while again longi- tudinal streaks of normal-colored heartwood may alternate with those of the brown color. When these brown streaks contain black patches this type of wood is known as “tortoise-shell” oak. This discoloration originates in the heartwood of living trees, the normal heartwood changing first to a faint yellow color, which continues to deepen until the brown stage is attained. The color change is caused by a fungus, but so far as known the infected wood is not weakened. 30 BULLETIN 1128, U. S. DEPARTMENT OF AGRICULTURE. The hyphe attack the cell walls very slightly, presumably living on the tannin, of which oak wood contains a high percentage. The value of the wocd for veneers is very much enhanced. The writer has no record of this discoloration being found on oaks in this country. : DECAY DISCOLORATIONS. The hyphe of wood-destroying fungi ving within the wood feed on the various substances composing the cell walls. They use certain constituents of the cell walls, neglecting others, with the result that these walls are broken down, the wood being thus greatly weakened and more or less destroyed. It is the breaking down of the wood and the change in its physical and chemical qualities that is termed decay. The degree of decay is determined by the energy of growth of the fungus, “the length of time it has been at work, and the type of wood it attacks. Some fungi attack many different kinds of wood, while others are limited in their choice. Owing to their less exacting moist- ure requirements, wood-destroying fungi are able to live on heartwood as well as sapwood. The fruiting bodies, usually quite large, are found on the surface in the form of brackets, crusts, or mushrooms or toadstools. They are not developed until the hyphze have been at work for some time; consequently, the presence of fruiting bodies indicates serious decay. Two types of wood-destroying fungi may be recognized, (1) those mainly attacking the heartwood, rarely the sapwood, of standing living trees, and (2) those principally confining their activities to the manufactured product, such as sawed lumber, crossties, and. poles. The former type may continue their work of destruction after the tree has been cut down and worked up into lumber. The latter, attacking the manufactured product, usually invade the sapwood first, since it is far richer in stored food, generally has a higher moisture content than the heartwood, and is not so inherently re- sistant to decay. Fungi causing this type of decay are often very abundant in yards where the lumber is closely piled on damp earth, with little or no aeration under the piles, and much accumulated wood débris scattered throughout the yard. Unfortunately, such conditions are all too prevalent in mill yards. Sanitary yards both at the mills and the factories are badly needed. Humphrey (28) gives a complete account of the life history and habits of these fungi. the damage caused by them, and methods for their control. CONDITIONS AFFECTING DECAY. All conditions which favor sap stains are equally favorable to wood-destroying fungi. Furthermore, the latter can attack wood with a lower moisture content, so the fact that wood does not sap- — stain is no indication that fungi causing decay may not be present. The discolorations caused by the latter in sapwood are not so pro- nounced as sap- -stain; consequently, they are much harder to detect. Moisture in wood. —Dry lumber will not decay. The most efficient method to prevent decay is to air-dry or kiln-dry lumber immediately and then keep it dry by proper methods of storage. Placing dry lumber in the open, exposed to rain, or in damp sheds can not be too strongly condemned. If the lumber becomes moist again, it is just ie oe DECAYS AND DISCOLORATIONS IN AIRPLANE WOODS. or as liable to ony as before. To be sure, kiln drying is much better than air drying, since the high temperatures employed i in the former process are probably fatal to the hyphe of some decay-producing fungi, while under the latter conditions the fungi may merely re- main dormant until suitable moisture conditions are again restored. However, since wood-destroying fungi are common around and in ards and wood-working factories, the chances are that kiln-dried umber will be reinfected, and if it becomes moist again decay will begin. Shipping green or even partially air-dried lumber on long voy- ages through tropical seas in the hold of a vessel offers a chance for a heavy loss through decay. The close humid air of the ship’s hold becomes a perfect forcing chamber for wood-destroying fungi when warm latitudes are reached. Shipments of Douglas fir leav- ing the Pacific coast perfectly sound have contained a considerable percentage of decayed lumber when unloaded at a South African port (36, p. 36). Indirect reports indicate that the same condition resulted during the World War in some shipments of Sitka spruce ped to Europe through the Panama Canal and the Mediterranean ea Durability of wood.—Resistance to decay, or as it is termed “durability,” is a factor that should no longer be neglected in selecting woods for airplane construction. Airplanes are being more and more exposed to unfavorable weather conditions as their use extends, conditions which in some instances are highly favorable to decay. Furthermore, certain conditions created by the construc- tion of an airplane promote decay. For example, in the interior of the wings the relative humidity may be much higher than that of the surrounding air, and there is often considerable condensation of moisture. In addition, the temperature is slightly higher. All pee. factors are favorable to the development of wood- destroying ngi. Within any species durability increases with the increase in specific gravity. Consequently, the fact that only wood with high specific gravity is used for aircraft not only increases strength but serves to increase durability. However, it is well known that differ- ent species vary widely in their durability. Unfortunately, spruce is not at all durable. Neither are basswood and birch. Douglas fir is fairly durable, as is also white oak. But the cedars are remarkable for their inherent durability, and among these Port Orford cedar compares favorably with spruce in all its streneth properties and is only slightly heavier. Consequently, this wood can not be too highly recommended for use in aircraft where resistance to decay must be considered. Sapwood must not be used under such cir- cumstances, for no matter what the species is it decays easily. Contrary to existing belief, the resin conan: of ie is of slight importance in relation to durability (74, p. 153-154; 75, p. 66-68). Resin itself has no poisonous effect, on the aia. “of fungous hyphe, and its only beneficial effect in increasing durability is its waterproofing action on wood. This is so slight, however, if the normal resin content of softwoods is considered, as to be practically negligible. If wood is rendered more durable through a sufficient increase in its resin content to have a decided waterproofing effect, 32 BULLETIN 1128, U. S. DEPARTMENT OF AGRICULTURE. it is usually completely resin soaked or contains pitch streaks which make it unsuitable for painting or contact with fabric coverings. INCIPIENT DECAY. It is a simple matter to recognize well-advanced rot or typical decay. Here the changes in the wood structure due to the longer action of the wood-destroying fungus are so profound as to be very plainly apparent, but the earlier stages of decay, termed incipient decay, immature decay, or advance rot, are often far from easy to detect (6,7). In some cases detection is practically impossible with- out a microscopical examination of the wood. — Specific gravity is not a reliable index of decay. It has been sug- gested that decay in any piece of wood will be immediately reflected in a lowering of the specific gravity. But this can not be detected unless the specific gravity of the piece was known before decay com- menced, a manifest impossibility in most cases. Incipient decay does not cause a sufficient reduction in the specific gravity to bring the heavier pieces of wood below the minimum set for the species. The writer has tested pieces of yellow birch, white ash, and Douglas fir with conspicuous incipient decay and found the specific gravity of the affected pieces to be from 0.05 to 0.2 higher than the minimum per- missible. The same condition will exist in all species. Douglas fir with pronounced white cellulose pockets characteristic of the final stage of red-rot or conk-rot has been found in some cases to have a higher specific gravity than the minimum of 0.45. Of course, when sound such wood had a high specific gravity. Wood is weakened by incipient decay, the degree depending on the stage of the decay and somewhat on the species of fungus at work. Furthermore, if infected material] is merely air dried the hyphe may remain dormant, ready to continue their work of destruction again if suitable conditions arise. The chalky quinine fungus (Pomes laricis (Jacq.) Murr.) , which normally causes decay in the heartwood _ of various coniferous trees, either living or dead, has been found causing decay in the roof timbers of cotton weave sheds (5). Un- doubtedly this originated from timbers containing incipient decay of this species placed in the roofs at the time they were built, where the high temperature and humidity which prevails in such sheds soon resulted in renewed activity of the fungous hyphe and their spread to adjoining sound timbers. The rose-colored Fomes (/omes roseus (Alb. and Schw.) Cke.), which is common on dead trees and is some- times found on living trees in the coniferous forests of the Pacific Northwest, has been found to be very destructive to timbers in base- — ments with high humidity and poor ventilation in the Northeastern © States (26, p. 28). As a general rule, infected wood must not be used. , It is extremely doubtful whether incipient decay in one of the laminations of ply wood can be considered an important defect. In the first place, the reduction in strength would be negligible. © Furthermore, there would be but little danger of the fungus ever resuming its activities, because the high degree of heat and humidity to which the ply wood is subjected during various stages of its manu- facture must kill the vegetating hyphe. However, this does not prevent reinfection and subsequent damage if conditions for decay DECAYS AND DISCOLORATIONS IN AIRPLANE WOODS. Bo again become suitable. Laminations with incipient decay should not be used in propellers. In this place the reduction in strength need not be so carefully considered as the variation from the normal shrinking and swelling that would result. Unequal and particularly unusual strains and stresses must be avoided above all things in propellers. Incipient decay usually appears as a discoloration, in some cases pronounced, in others so faint as to be practically invisible. Most of this decay in airplane lumber was actually in the tree when it was cut or in the logs when they left the woods. It is rare that any serious effort is made in the woods or at the mills to cut out incipient decay. When the logs are bucked and sawed the typical decay is usually trimmed off, leavi ing the less apparent incipient decay in the lumber. After sawing, the upper grades of lumber, which include airplane stock, are usually handled carefully enough at the larger mills to prevent further damage. When decay commences in a living tree, it spreads upward in the heartwood if the infection entered at the butt, or in both directions if it occurred higher on the trunk. Very rarely do the decays in the heartwood of living trees attack the sapwood. Beyond the typical decay, that is, where the wood is decidedly rotted, extend the incipi- ent stages of decay, which become less and less apparent as the dis- tance from the typical decay increases. Finally, the incipient decay ends entirely. The wood beyond is then sound. The incipient decay rarely ends abruptly or evenly, but usually fades out in one or more irregular streaks, which may be short or long. It usually extends only 3 or 4 feet longitudinally beyond the typical decay, but with certain wood-destroying fungi on some hosts the incipient decay may extend 15 feet or more in advance of the typical decay. Fur- thermore, the latter is always bounded radially by incipient decay, and this boundary is often irregular. Boards sawed from diseased trees may contain all stages of decay or incipient decay, occupying part or all of the board. The fact that the fungi causing decay in standing trees may continue their work of destruction in logs in the woods, or even in sawed lumber if conditions are favorable, indicates the necessity for having logs removed from the woods, sawed, and the lumber dried with reasonable promptness. When lumber is green the discolorations indicating incipient decay are more intense than when the wood has seasoned for some time. During the drying process the discolorations fade in varying de- grees. Furthermore, if a new discoloration appears within one or two weeks after the lumber comes from the saw it is practically certain that it is not caused by one of the wood-destroying fungi attacking the piled lumber, since the latter work more slowly. A sap-staining fungus or a chemical reaction is the most likely agent in such a case. Incipient decay should be detected and eliminated before the lumber is worked into individual parts. If the entire piece is not defective the sound portion can be sawed out and utilized. In marking a piece for cutting, however, it must be remembered that decay extends more rapidly with the grain in a tree or piece of wood than it does across the grain; thus, to be perfectly safe, an allow- ance of 2 feet should be made in the direction of the grain beyond 34 BULLETIN 1128,-U. S. DEPARTMENT OF AGRICULTURE. the last visible evidence of incipient decay, while across the grain an allowance of 2 to 3 inches will suffice. TYPES OF DECAY IN LIVING SOFTWOOD TREES. One of the most common decays in airplane lumber is that caused by the ring-scale fungus (Zrametes pint (Brot.) Fr.) in the heart- wood of living trees. It may occur in practically any species of soft- wood, but is very common in Douglas fir, spruce, and pine. The de- cay, known under various common names, such as red-rot, red-heart, conk-rot, white honeycomb rot, pecky wood-rot, and ring-scale rot, is readily recognizable in its typical stage by the fact that the heart- wood is honeycombed with small white pits in which the wood is reduced to a soft fibrous mass of cellulose (in a chemical sense cotton is practically pure cellulose), these pits being separated by firm and apparently sound wood. Plate II shows typical decay in Douglas fir. While the typical decay is closely similar in appearance in various species of wood, there is considerable difference in the incipient de- cay. In Douglas fir as a general rule it appears as a pronounced reddish purple or olive-purple discoloration, gradually tapering and becoming fainter until it is lost entirely. The color is often most pronounced in the outermost heartwood just where it joins the sap- wood. In some cases it appears brownish against the red or yellow heartwood. At the lower limits of the incipient decay, where it be- gins to merge into typical decay, a close scrutiny will usually reveal faint indications of the cellulose pits. Vertically the discoloration may extend 10 feet or more in advance of the cellulose pits, but radially this is limited to 2 or 3 inches. The discolorations described are often bounded by a narrow zone of pronounced red color. Plate IIT shows discoloration in Douglas fir with the formation of cellu- lose pits beginning. Inrare instances the first indication of the decay may be the tiny golden white spots or streaks which indicate the initial stage in the formation of cellulose pits. In this case the discoloration is probably too faint to be recognized, and material of this kind is quite easily overlooked. : In white and red spruce (54, p. 32) this incipient decay first ap- pears as a change in color from the pale yellowish or reddish brown of the normal heartwood to a light purplish gray, which deepens to a reddish brown, with the gray forming the outer boundary of the reddish brown discolored portions. Next, the cellulose pits ap- pear, visible at first as very tiny black lines following the grain of the wood, but soon revealing their true nature. The discoloration is not so pronounced as in Douglas fir. In Sitka spruce the tiny black lines preceding the cellulose pits are not found. The yellow pines first show the decay by a pronounced pink color which rapidly gives way to a red-brown; hence the names red-rot and red-heart. During this stage the wood is hard and firm. Then ~ the white pits develop, although in some cases they appear so spar- ingly that they are readily overlooked. In certain woods there is little or no discoloration with this incipi- ~ ent decay. This is true with incense cedar, Port Orford cedar, and western red cedar, and is probably the same with other cedars. The first indication of the diseased condition of the wood is the appear- DECAYS AND DISCOLORATIONS IN AIRPLANE WOODS. 35 ance of cellulose pits. Hence, the purplish red color commonly found in the heartwood of incense cedar (see p. 16) and western red cedar need not be mistaken for decay. As yet very little is known in regard to the reduction in strength due to incipient decay caused by the ring-scale fungus. However, it seems probable that such reduction is slight until the appearance ‘of the white cellulose pits; but it is to be remembered that pieces with discoloration contain hy phe which may again attack the wood, if suitable conditions arise. Consequently, stock with any stage of this decay should not be used. The chalky quinine fungus causes a pronounced decay in the heart- wood of many softwoods. The typical decay is a brownish red friable crumbly mass, often with conspicuous mycelium felts filling the cracks. This is shown in Plate IV. The incipient decay is very difficult to detect, as a rule. Even when the wood has been severely weakened the extremely faint brownish discoloration is not discern- ible to any but the most expert eye. However, the incipient stage of this decay in western yellow pine appears as a red-brown or pro- nounced brown discoloration in the pale-lemon to light. orange- brown heartwood. ‘The discoloration is not uniform over the entire affected portion, but may occur on the radial or tangential face in broad bands of varying intensity or even intermingled with narrow bands of the normal light-colored heartwood. In cross section the infected wood presents a mottled appearance. The horizontal limits of the discoloration are bounded by a narrow band of pronounced pink or red. At the upper limits of the incipient decay the discolora- tion becomes fainter until it finally disappears. The discolored wood seems to be hard, firm, and strong, but in reality it is seriously weak- ened. Plate V illustrates this condition. The typical decay caused by the sulphur fungus (Polyporus sul- fureus (Bul.) Fr.) is very similar to the foregoing. However, it is not confined to softwoods. It is common only in the true firs among the softwoods, but is very prevalent among the hardwoods, particu- larly the oaks. The heartwood of living and dead trees is affected. The incipient decay is difficult to detect, being first indicated by a faint brownish discoloration. The velvet-top fungus (Polyporus schweinitzii Fr.) also causes a reddish brown friable rot, which is, however, confined to the butt and roots of the tree. The mycelium felts are very fine and inconspicuous. Only softwoods are affected. Normally the incipient decay is very difficult to detect. It first becomes evident i in Sitka spruce ® as pale- yellow to lemon-yellow streaks or spires extending longitudinally beyond the light yellowish to reddish brown discoloration which characterizes the more visible incipient decay. In the latter stage a softening of the wood is apparent. In Douglas fir the incipient decay is first evident as a faint yellowing or browning of the normal heart- wood. This or an exactly similar. decay in western red cedar is first indicated by a decided deepening in the color of the normal brownish heartwood. The discolored zone often extends horizontally for sev- eral inches around the typical decay and for a foot or more in ad- vance of it. The discoloration may be confused with the normal ©The description of the decay in this species caused by poLpare schweinitzii is based on notes furnished to the writer through the courtesy of E. E. Hubert. 36 BULLETIN 1128, U. S. DEPARTMENT OF AGRICULTURE. darker colored bands of heartwood which are found in some trees, but such bands are confined to a definite group of annual rings. Redwood is subject to a brown friable decay which is not con- fined to the butt of the tree. The fungus causing this is unknown (57). The first indication of the incipient decay is a very faint light brownish discoloration on the light-cherry to deep reddish brown heartwood. This is most readily detected on the tangential face in the summer wood. When the brownish discoloration is plainly apparent, the decay has progressed so far that the affected wood feels softer than the normal to the thumb-nail. The typical decay is dark brown in color, very soft, and easily crumbled. Thin crustlike mycelium felts occur along the sides of the cracks. These reddish brown or brown friable decays which are so difficult to detect in their incipient stages, particularly in woods with a pro- nounced reddish or brownish heartwood, reduce the strength of ‘the wood far more seriously than incipient decays of the red-rot type; in fact, the wood may be weakened before the incipient decay is visible. Consequently, in cutting out such decays from lumber it is advisable to leave a margin of safety of at least 2 feet in a longi- tudinal direction beyond the last visible evidence of the incipient stage. Incense cedar is very commonly decayed by the incense-cedar dry- rot fungus (Polyporus amarus Hedge.). The typical decay con- sists of vertically elongated pockets, varying in length from half an inch to about a foot, which are filled with a brown friable mass, and the line of demarcation between the sound and decayed wood is very sharp. In some of these pockets small cobweblike or feltlike masses of white mycelium occur. The pockets are separated from each other by what appears to be sound wood, although in some eases streaks of straw-colored or brownish wood may extend verti- cally between two pockets. This is especially noticeable between young pockets. The pockets of incipient decay are at first firm and very faintly yellowish brown. This color deepens slightly, and the wood becomes somewhat soft. The incipient decay extends but a short distance vertically in advance of the typical decay, and a _ distance of 2 feet beyond the last visible evidence will usually exclude all decay. The incipient decay is only faintly apparent, occurring as it does in pockets with the color in the very earliest stages differ- ing but slightly, when at all, from the normal wood. The fact that an occasional pocket may be found several feet in advance of the main body of decay makes this decay an exceedingly dangerous one. The wood, even in an incipient pocket is decidely weakened (al- though the intervening wood is apparently not affected), and this makes a weak spot that is hard to detect. Such cases are fortunately not common, and the fact that most incense-cedar stands are so badly decayed will probably preclude this species from any extensive use for airplane construction. Other woods are subject to similar decays. That found occasionally in western red cedar may be caused by the — same fungus, while “peckiness” of bald cypress (Laxodium dis- — tichum (linn.) Rich.) results (33) from the work of a different organism (omes geotropus Cke.). . One of the most striking discolorations indicating decay and at _ the same time one of the most serious incipient decays is that caused DECAYS AND DISCOLORATIONS IN AIRPLANE WOODS. 37 by the Indian paint fungus (£'chinodontium tinctorium EK. and E.). This is found on the true firs in the western United States, being especially prevalent and severe on white fir (37). It is also exceed- ingly serious on western hemlock (77). In white fir the first indications of this decay on a radial or tangen- tial section are light-brown or golden tan spots or larger areas of dis- coloration in the light-colored heartwood, which may be accompanied by small but clearly distinct radial burrows, resembling somewhat very shallow insect burrows without the deposit of excrement. These burrows are not easily detected in cross section. Next, rusty red- dish streaks appear following the grain. Throughout this stage. the wood appears firm and strong, but in reality is so greatly weak- ened that boards may separate along the annual rings when dried. The discoloration intensifies, the wood becomes soft, showing a de- _ cided tendency to separate along the spring wood in the annual rings, and finally the typical stage is reached, in which the wood is brown, with pronounced rusty, reddish streaks and becomes fibrous and stringy. Hence, the name stringy brown-rot is applied to the decay. The incipient decay usually extends from 2 to 6 feet beyond the typical decay. Plate VI shows the incipient decay. In western hemlock the incipient decay 1s much harder to detect, because the initial discoloration above described so closely approxi- _mates the pale-brown, slightly tinged with red, color of the normal heartwood. The wood first assumes a faint yellowish color, which is sometimes intensified by the presence of small, hardly discernible brownish areas. These areas later develop into the typical decay. The extension of the incipient decay beyond the typical decay varies from 1 to 5 feet. For the sake of safety 2 feet should be added be- yond the last recognizable yellowish discoloration in order to elimi- nate all incipient decay. : TYPES OF DECAY IN LIVING HARDWOOD TREES. Hardwood trees are subject to very serious decays. One of the most important from our standpoint is the white heartwood rot (58) so commonly found in commercial white-ash stock, caused by the ash Fomes (fomes fraxinophilus (Pk.) Sacc.). This fungus attacks the heartwood of living trees and produces a very characteristic rot. On cross section the first indication of the decay is a light brownish discoloration, often difficult to distinguish from the normal grayish brown or reddish brown heartwood. This discoloration is most ap- parent in the broad bands of summer wood. Next, there is a bleach- _ ing of the spring wood, during which it turns to a straw color, and then small white spots or specks appear. On the radial (edge- _ grain) and tangential (slash-grain) faces these appear as small whit- ish spots, streaks, or blotches, usually following the grain, but some _ may be at right angles to it if the decay follows a medullary ray. _ The whitish color becomes more marked, until the entire spring wood is affected and appears disintegrated. Then the fibers fall apart. The summer wood passes through the same process, but much more _ slowly, thus during the earlier stages of the typical decay causing a banded appearance. The completely rotted wood is whitish or straw _ colored, very soft, and spongy, readily absorbing water. A section 38 BULLETIN 1128, U. S. DEPARTMENT OF AGRICULTURE. from a white-ash longeron with this incipient decay is illustrated in Plate I, right side. Apparently mycelium does not occur in the brown discolored wood in advance of the white spots. It would seem that the wood is not weakened until the white spots are found, and the wood with the brown discoloration alone need not be rejected. It is an excellent hint for close scrutiny of an affected piece, however. The incipient decay is somewhat obscured in rough lumber, but is usually readily apparent on smooth surfaces. This stage does not extend many feet beyond the typical decay, and on long boards the latter will most . likely also occur. Once the presence of the typical decay is ascer- tained it is a relatively simple matter to determine the limits of the incipient stage. Areas in which the wood failed to change color upon transition from heartwood to sapwood (see p. 16) can be differentiated from the initial stages of white-rot by nee larger size, by the straw-yel- low color as opposed to the whitish of the decay, by the sharp line between the two colors, and by the fact that the spots are much larger, without becoming soft and spongy, than would be the case with the decay. Sweet birch and yellow birch are subject to a white heart-rot (32) which, although very similar to the foregoing, is caused by a dif- ferent fungus, the false tinder fungus (Pomes igniarius (u.) Gill.). The first indication of the incipient decay is a brown discoloration, not very apparent against the reddish brown heartwood. Next, faintly paler streaks or spots appear, which finally become a yel- lowish white, strikingly apparent against the dark background. This stage is illustrated by Plate VII. In the center of these streaks small spots are found in which the yellowish white wood appears to have collapsed. Usually the long axis of these spots is parallel to the grain, but in some it may be at right angles to it. The wood up to this time appears firm and hard. Next the white streaks merge, the wood becomes soft, and finally the entire affected portion of the heartwood is reduced to a yellowish white fibrous mass composed principally of cellulose, the result of the delignification by the fun- gous hyphe. As in the white-rot of ash, hy phe are not found in the — brown discoloration. Hence, no reduction in the strength of the wood may be expected until the very first indications of the whitish streaks or spot, which may be found as much as 8 feet in advance of the typical decay. One of the most common decays (24) on oaks and es on cer- | tain poplars (Populus) is the heart-rot caused by the oak fungus (Polyporus dryophilus Berk.). The incipient decay of this whitish | piped rot in white oak has a water-soaked appearance in the unsea- — soned wood, but when dry the discoloration becomes hazel to tawny — in color. The discoloration may extend from 1 to 10 feet in advance of any other indication of the decay. The next stage of the decay, — - which is best seen on a radial face, is characterized by whitish spots — or streaks, usually following the medullary rays, which produce a | delignification process; that is, the lignin is removed from the wood, © leaving only the cellulose. In the final stages the decayed wood is © firm, with a white, stringy appearance, and the delignification is — mottled appearance of the wood. This mottling is the result of a practically complete. ae Ae ee DECAYS AND DISCOLORATIONS IN AIRPLANE WOODS. 39 A somewhat similar rot in oaks (34) is the honeycomb heart-rot (Stereum subpileatum B. and C.). As in the whitish piped rot, the first indication of this decay in white oak is a slight water- soaked appearance of the fresh heartwood, and when dry this “soak” be- comes a tawny color. Next, light- colored isolated areas appear in the tawny discolored wood, and pronounced delignification occurs. This is indicated by the appearance of very small. irregular whitish patches in the light-colored areas. These patches develop into small pits with their long axes parallel to the grain of the wood, and they increase in number until the affected wood is completely occupied. The pits are from one thirty-second to one-fourth of an inch wide by one-fourth to five-eighths of an inch long, and lined with cellulose fibers. At this stage the appearance of the decay is similar to the red-rot in softwoods previously described. Later the cellulose lining may disappear. The wood is “probably not weakened by this decay until the light-colored areas appear in the tawny discoloration. An incipient decay is sometimes encountered in African mahog- any, the cause of which is unknown to the writer. This decay ap- pears as light-yellow, brown, or merely lighter brown closely crowded spots or flecks on the reddish-brown heartwood. These flecks vary from one-sixteenth to one-quarter of an inch long and are several times longer than broad, the long axis corresponding with the direc- tion of the grain in the wood. Such wood is weakened. TYPES OF DECAY IN LOGS AND LUMBER. In addition to the wood-destroying fungi which normally attack living trees, and which may continue to decay the wood after the tree is cut, there are fungi which grow only or principally on wood in the form of logs or lumber. Owing to their destructiveness, some of these deserve more than passing mention. Although it is true that damage caused by such fungi is due to improper “handling of the timber during the course of manufacture and utilization, unfor- tunately such improper handling does occur and must be reckoned with. — Softwood logs and lumber.—One of the most important of these fungi is that which caMses dry-rot in stored logs or lumber and in timber in structures (22). The term “ dry- rot” is loosely applied to cover almost any type of decay, but it is correctly applicable only to the work of the dry-rot fungus (Jlerulius lacrymans ( Wulf.) Fr.). This decay is more common on coniferous woods than on hardwoods. The incipient decay appears as a yellow-brown dis- coloration not easy to detect. Wood with typical decay is yellow to brown in color, much shrunken and cracked, and is so badly disin- tegrated that it can be easily crushed to a powder. Both sapwood and heartwood are attacked. _ Another common decay on logs and sawed lumber, particularly on railroad ties, is the brown-rot “(62) caused by the brown Lenzites (Lenzites sepiaria (Wulf.) Fr.), which is practically confined to coniferous wood. The typical decay is brown, friable, and easily reducible to a powder. In the early stages of decay infected wood is darker in color than the normal. Sometimes the early spring wood of the annual rings may be completely decayed, while the 40. BULLETIN 1128, U. S. DEPARTMENT OF AGRICULTURE. summer wood is scarcely affected. In this condition the wood sep- arates readily along the annual rings. , Hardwood logs and lumber.—Certain fungi (Polystictus versi- color (L.) Fr., Sterewm hirsutwm (Willd.) Pers., and others) cause a sap rot very difficult of detection in its incipient stage. The typical decay is very hght in weight, white in color, rather soft, and easily broken in the hands. But since the first indication of this decay is a faint whitening of the diseased wood and white is the normal color of most sapwoods, it is apparent that the initial stages may be readily overlooked. At the same time the wood is decidedly re- © duced in strength. The decay is most common on hardwoods, but also occurs to some extent on softwoods. Fortunately none of the fungi causing this white sap-rot attack living trees of the species which furnish airplane timber. Red-gum logs when left in the woods for any considerable time are subject to a very serious sap-rot (59) caused by the smoky Poly- porus (Polyporus adustus (Willd.) Fr.). The heartwood is com- paratively durable. Boards cut from diseased logs are very char- acteristic and striking in appearance. Normally, red-gum sapwood is a light yellowish white, commonly with a reddish tinge. The sap- wood in a decayed board has a number of various-colored streaks or lines irregularly distributed from the end of the board toward the middle. ‘These streaks are light orange at first, but in the more ad- vanced decay are a very light straw color (in fact, almost white) and are intermingled with lines and patches of bluish gray and the nor- mal-colored sapwood. Black zigzag lines may extend from the ends of the board for a distance of 2 inches or more parallel to the grain. The general consistency of sapwood with this incipient decay, which may extend 2 or 3 feet in advance of the typical decay, is firm and solid. Sapwood with the typical decay is badly broken down, being soft and pulpy and without firmness. This and other sap rots may be prevented by shortening the dry- ing period in the woods. Coating the ends with hot coal-tar creosote immediately after the logs are cut is also effective. Where possible, all freshly cut logs, particularly those cut during the spring and summer, when the rot develops best, should be peeled. Sap rots simi- lar to those found in the red gum are found in tupelo gum (Vyssa sylvatica Marsh) and in maple. DECAY IN FINISHED AIRPLANES. Little information about decay in finished airplanes is available. In the past there has been very small chance for airplanes to decay, because the completed machines rarely ever were stored, and their life in use was a relatively brief one; but since the conclusion of the World War immense quantities of airplane material have been placed in storage, and the average life of the machines has been materially increased by changes in construction. Under average conditions there should be practically no damage to finished airplanes by decay. When in use there is little danger from this source, owing to the fact that when not actually in flight the machines are properly housed. The wooden parts in the interior of the wings and around the en- gine are most susceptible. In these places there is an increased tem- perature and relative humidity. Keeping the machines in a dry : a 4 DECAYS AND DISCOLORATIONS IN AIRPLANE WOODS. 41 place when not in use will suffice in most climates. There is more danger in humid tropical or semitropical regions, particularly to seaplanes. Serious loss can easily result to machines through improper han- dling while being stored or shipped. Airplanes are usually knocked down for storing and shipping; that is, the machine is taken apart, and the individual assemblies, such as the wings, tail surfaces, and fuselage, are. handled separately. When shipped, these parts are carefully wrapped in heavy paper and packed in solid crates. If these crates are left out in the air, cracks open up between the boards, water may get in, and then the trouble commences. Once damp, it is almost impossible for the mass of paper wrappings to dry out un- less the crate is completely unpacked. Varnish or dope does not prevent the taking up of moisture, so that the wood soon attains a moisture content sufficient for the growth of molds and wood-destroy- ing fungi, while the other conditions within the crate, such as lack of air circulation with the resulting high humidity and the higher temperatures, are ideal for the development of these organisms. Even before the wood is decayed the elements of the ply wood are very likely to separate, owing to the action of moisture and molds on the giue. Even water-resistant glues can not permanently withstand such conditions. There is no cure for decay, once it has started. The damaged part can be replaced and further destruction prevented, but the con- stant aim should be not to let decay begin. Material should not be kept in packing cases any longer than is necessary. The practice of leaving packing cases containing airplanes or spare parts in the open for several months can not be too severely condemned. When put in storage, the parts should be removed from the cases and placed on racks, so that a complete circulation of air is possible around each unit or piece. The storage houses should be equipped with a forced-ventilation system, so that air of the proper humidity can be constantly circulated through the piles of material. The relative humidity should be maintained at 60 which will keep the wood at a moisture content of about 11 per cent, low enough to pre-. vent decay, mold, or sap-stain. Circumstances will arise where planes are in use or while being _ shipped when it will be impossible to maintain proper conditions to prevent deterioration. In the warm climate and high humidity of tropical or semitropical regions in particular this will be true. It is advisable to have planes for use under such conditions constructed from a durable wood such as Port Orford cedar. Where this can not be done, methods should be employed to make the other species more durable. Wood may be moisture-proofed by the application of aluminum leaf. This not only prevents decay, since the wood is kept dry, but protects the glue joints from the action of moisture and mold. As a last resort, the wood could be treated with preservatives to prevent decay. These liquids are most effective when forced into the wood under pressure. Consequently the completed individual wood parts would have to be treated before assembly. Sodium - fluorid could be used on parts to be glued, while coal-tar creosote could be applied to the others. The most highly efficient of all, 42 BULLETIN 1128, U. S. DEPARTMENT OF AGRICULTURE. mercuric chlorid, is unfortunately a deadly poison, corrodes metal, and is very difficult to handle. The subject of preservative treat- ment is one about which little is known as applied to airplanes. Little information is available as to what fungi actually cause decay in finished airplanes or as to the types of decay found. Un- doubtedly the fungi most concerned are those commonly attacking the manufactured product, such as the dry-rot fungus, the brown Lenzites, or the rose-colored Fomes. Fungi decaying the heartwood of living trees are not commonly found. When they do appear, this is practically proof positive that the manufacturer used wood with incipient decay in the fabrication of the wooden parts. SUMMARY. Among the softwoods or conifers the most valuable for airplane construction are red, white, and Sitka spruce, the last being most important on account of its large size and the consequently greater proportion of clear lumber that can be obtained. A. splendid substi- tute for spruce, and its superior where durability must be consid- ered, is Port Orford cedar. However, the supply of this wood is limited. Douglas fir, which is much heavier than spruce and there- fore not so desirable, is also extensively used. In those parts of an airplane frame requiring great strength and toughness, hardwoods are used. White ash is best, but white oak, hard maple, and rock elm may be substituted. Hickory is principally used for tail skids. Black walnut and true mahogany are unsurpassed for propellers, but yellow birch, sweet birch, African mahogany, black cherry, hard maple, and white oak are acceptable substitutes. As the supply of timber diminishes in the future, a wider variety of woods will be acceptable for airplane construction. All wood is subject to defects, of which one of the most serious is decay; but other defects which reduce the strength of timber must be recognized. Among these can be mentioned spiral and diagonal erain, specific gravity that is too low or too high, brashness caused by excessive temperatures during steaming or kiln drying, com- pression failures, shakes, pitch pockets, and insect galleries. Decay in its incipient stage is often not readily recognized; but wood with incipient decay must not be used in airplane construction, since infected wood may be reduced in strength. Furthermore, the decay may continue if suitable conditions arise. The first indi- cation of decay is usually a discoloration of the infected wood, but not all discolorations result from decay. Marked discoloration of the wood, particularly the sapwood, usually accompanies pith-ray flecks and wounds made by lghtning and sapsuckers. Conditions favorable for decay also promote sap stains. These discolorations of the green sapwood of various softwoods and hardwoods occur in two ways: (1) By an oxidation of the organic compounds in the cells of the sapwood when exposed to the air and (2) by the attack of sap-staining fungi, the hyphe of which feed on the organic com- pounds in the cells of the sapwood without attacking the cell walls except to a negligible extent. The discolorations are confined to the sapwood as a rule, but occasionally the sap-staining fungi, may dis- color the heartwood slightly. For practical purposes wood so dis- colored is not reduced in strength. DECAYS AND DISCOLORATIONS IN AIRPLANE WOODS. 43 The discolorations resulting from incipient decay may be found in the sapwood or heartwood. Incipient decay extends for varying distances beyond the typical decay. In cutting out this defect it is advisable to leave a margin of safety of at least 2 feet in a longi- tudinal direction beyond the last visible evidences of the incipient decay, in order to remove all infected wood. This margin of safety is particularly important with brown or red-brown friable decays, since infected wood may be dangerously weakened by them while the incipient stage is still practically invisible. Many decays other than those described in this paper are found in living trees, in logs, and in manufactured timber, but the examples cited include both the most important decays and the principal types. For most purposes it is sufficient to recognize incipient decay as distinguished from other discolorations or defects without deter- mining the causal fungus. | | | | PLATE I. Bul. 1128, U. S. 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