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SUNLIGHT ON GLASS. 



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By THOMAS GAFFIELD. 






[From the American Journal of Science and Arts, September and 

November, 1867.] 



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NEW HAVEN: 

printed by tuttle, Morehouse * taylor, 

1867. 



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[From the Amb&ican Journal of Soiinox and Arts, Sxfy. and Nov., 186^.] 



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ACTION OF 'SUNLIGHT ON GLASS. 



By THOMAS GAFFIELD. 



The ffreat attention now giyen to all the phenomena connected 
with light and heat may awaken some interest in the experi-' 
ments in which I have been engaged for the past four years on 
the subject named at the head of this article^ Perhaps I cannot 
better commence my essay, than by quoting from the ** Proceed- 
ings of the Natural History Society," (vol. ix, p. 847) an account 
given before that Society, of my experiments in 1863, and after 
I had been engaged in them only a few months. 

" He believed that his experiments in connection with the 
subject were original as to their method and their extent, al- 
though it had long been observed in Europe that colorless or 
light-colored plate-glass had turned to a purple hue by exposure 
to intense sunlight. One case^ is cited of a change to a gold 
color; and one experiment recorded by Dr. Faraday ^f some 
forty years ago, proving that a light purple changed to a darker 
hue after eight month^s exposure. 

" Other experiments are on record showing the action of glass 
of different colors as media in the transmission of light and of 
heat ; but none, with the above exception, showing the effect 
produced on the glass itself. 

" An experience of some twenty years in the window-glass 
business had only presented a few isolated cases of supposed 
change of color from this cause, which were attributed to some 
obvious defect in an article of inferior manufacture ; but, within 

* Joarnal of Society of Arts for Feb. 15, 1864. 

f Dr. Faraday's Chemical Researches. London, 1859, p. 142. 



2 T, Gaffield on the action of Sunlight on Glass. 

a short time, he had heard of the change of color in an article of 
superior manufacture, in a quantity of white plate glass, of which 
some lights had been broken out of a window in which they 
had been exposed to the sun. 

" This fact coming to his knowledge led him to try an experi- 
ment with several specimens of plate, crown, and sheet glass, 
during the month of July last; which proved that a month's 
exposure to a hot sun would change the best white French plate 
and all white sheet glass, such as is used for photographs and 
engravings, to a color containing more or less of a yellow hue. 
The dark green and dark blue or bluish green did not experience 
any change ; but any hue which approached a white, whether 
bluish, greenish, or yellowish white, turned to a yellowish color. 

A second series of experiments, commenced in July, and con- 
tinued three months, on some thirty specimens from France, 
England, Belgium, Germany, and the united States, only con- 
firmed the results of the first ; and a daily examination at first, 
and afterward from week to week, and month to month, revealed 
the interesting fact, that, even after a single day's exposure to a 
July sun, the change of color will, in some instances of the 
lightest hues, commence. 

" So remarkable was the change in a week, aflfecting nearly all 
the light-colored glasses, that he commenced a third experiment 
on the 6th of August which should speak for itself. He then 
exhibited to the Society ten pieces of JFrench white plate-glass, 
four by two inches in size (all of which were cut from the same 
sheet), one of which showed the original colorless glass, and the 
others exhibiting the change of hue towards yellow, after ex- 
posure respectively of one, two, and four days ; one, two, and 
three weeks ; one, two, and three months. 

" The changes in the first four days were slight ; but the last 
specimens were so yellow as to exhibit a contrast very marked, 
and excited the interest of all the members present. That the 
color permeates the body of the glass, and is not confined to the 
surface, or produced by reflection therefrom, has been conclu- 
sively proved by grinding off about one-sixteenth of an inch 
from both surfaces and the four edges of a duplicate exposed 
specimen, which, after repolishing, still exhibited the same 
yellow color. * 

" The glasses exposed were all what are called colorless win- 
dow-glasses, although they varied in tinge and hue from the 
whitest French plate to the darkest green English sheet-glass. 

" An experiment for four months, from July to November, on 
really colored glasses, red, green, yellow, blue, and purple, showed 
no change except in the purple, which became slightly darker. 

" The experiments were carried on upon a rough plate-glass 
roof, nearly horizontal, and which received the rays of the sun 



T. Gaffield on the action of Sunlight on Glass. 3 

during the greater part of the day. In all cases, strips corres- 
ponding to those exposed, and cut off from the same pieces, 
were placed in the dark, to be compared with the other speci- 
mens after exposure. 

" It will be noticed that the dark green, blue, and bluish green 
did not change. The color of the Belgian sheet (called German 
or French by glass-dealers in America), a yellowish or brownish 
green, did not change; and these were the only exceptions. 
All plate-glasses changed, except an inferior blue quality, and a 
superior crystal plate of a greenish color, made in Germany, 
and at the only factory which has not given up the use of pot- 
ash for soda-ash. 

" It is possible that a longer exposure of a year, or of years, 
might change every color in some degree. 

"His inquiries, since he instituted these experiments, have 
brought out some fine specimens of Belgian sheet-glass from a 
house built three years ago, which had changed in some instances 
to a golden and in others to the well-known purple hue. 

" It is his intention to pursue the experiments farther, with a 
view to ascertain the effects of sunlight during each month and 
season of the year ; and also whether exposure to heat, air, or 
moisture alone, out of the direct action of the sun's rays, will 
produce any corresponding change. 

" Mr. Gaffield does not propound any theory to explain these 
changes of color, which, under our sunny skies, probably take 
place much more rapidly than in the different and less clear 
atmosphere of England. 

'* Some writers point to the presence of oxyd of manganese 
in the original composition of window-glass, and some to the 
oxyd of iron, as a chief cause. 

"Some writers have peculiar theories about the different 
classes of the sun's rays. Some may think the change referred 
to, a molecular or chemical one ; and. others, wiser than the rest, 
refrain from any explanation, waiting for a larger multiplication 
of experiments, and a greater accumulation of facts, before 
educing any satisfactory law of nature which governs these 
curious and interesting phenomena. 

" Mr. G. makes no pretentions to any discoveries, unless it be 
to the very rapid change in glass observed in our climate in 
July, but only gives the result of his experiments, in the hope 
that the great interest now manifested in the subjects of light 
and heat may lead others to examine the matter, to repeat the 
same experiments in other countries, and to give the world the 
result of their researches, and enable the learned and scientific 
men of the age to- explain this remarkable power and action of 
the sun's rays. 

" It should be remembered that he submitted his specimens to 



4 T. Gaffield on the action of Sunlight on Glass. 

the most severe tests by placing them where they received re- 
flected as well as transmitted light and heat. The change in 
glass, when glazed in the windows of our dwellings and stores, 
is so much more gradual, that it very rarely attracts the atten- 
tion of observers, except in the marked variation from white to 
purple." 

In accordance with the intention above expressed, I have con- 
tinued my experiments on this interesting subject, and under 
different heads will now give some account of their method and 
results. 

My first experiment was with pieces of glass four by six 
inches, placed in a sash six by sixty inches long, in the grooves 
of which the specimens were placed, the sash being fastened 
together by wooden pins, and placed on a nearly horizontal 
rough plate-glass roof, which received the direct rays of the sun 
during the greater part of the day. In my next experiment, I 
placed some oT the specimens directly on the rough plate-glass. 
These were carried on in summer and autumn. It was neces- 
sary to make a different arrangement for winter, when the 
fall of snow and the formation of ice might interfere with the 
full success of the experiments. I concluded that the best size 
for specimens was four by two inches, and I made some wooden 
boxes about f inches deep, 4 J inches wide, and of a length to 
fit the sills of windows facing the south, in the upper story of 
a Boston house. One of the windows was three-sided, look- 
ing east, south, and west. In these boxes, painted white, my 
specimens were placed in a nearly horizontal position, side by 
side, and, (after the loss of a few pieces, blown awav by hurri- 
canes and squalls), were secured in their positions oy cords of 
twine or slight copper wire fastened at each end. I have ar- 
ranged boxes on the roof in front of this window also, some 
being of greater depth in which to place pieces of plate and 
rough plate about one inch in thickness. All of these boxes 
are provided with covers, which are placed over the glass, and 
fastened by buttons, on the occurrence at any time of a snow 
storm. At all other times, the glass is exposed. Holes in 
proper places in the boxes allow the rain which falls to pass off 
easily. Every piece of glass is carefully marked by a diamond 
in an upper corner with its name, and if necessary, with some 
abbreviation descriptive of the experiment to which it belongs. 
This precaution is necessary to prevent mistakes, when a com- 
parison is to be made of several kinds and colors, of exposed 
and unexposed specimens. In all cases, where an experiment 
is to be made with any description of glass, the pieces (4x2 
inches in size) must all be cut from the same sheet, as there is 
frecjuently a slight difference in the shade of sheets from the 



r. Oaffield on the action of Sunlight on Glass. 5 

same factory, arising from the difl&culty of having the materials 
in their manufacture, and the circumstances attending the melt- 
ing, blowing or casting precisely the same. Two pieces or more 
should be laid aside to show the original color, and to compare 
with the others, and thus to show the changes produced by ex- 
posure to sunlight for days, weeks, months or years. At the 
commencement of an experiment, say for instance, the exposure 
of white plate-glass for from one to twelve mont^, a neat 
paper box is provided, 4J inches long, 2-J- inches deep and wide 
enough to hold fourteen pieces, (two of the original color, and 
twelve exposed specimens) and a description of the contents is 
marked on its cover on an adhesive label. At the end of each 
month, a piece is withdrawn from exposure, carefully cleaned, 
and marked either with a diamond or by an adhesive label, and 
placed in the box. Ab "order is heaven's first law," it is pecu- 
liarly necessary in all observations upon the actions of heaven's 
brightest luminary. By observing the above directions, much 
time will be saved, and at the conclusion of an experiment, 
everything is in shape for exhibition to friends at home, or 
students at a lecture room. 

Of course a perfect arrangement' could only be made when a 
perfectly flat roof or platform in an open field could be provided, 
and the sunlight could act with full force during every hour and 
minute of the day. But mine was sufficiently near this point to 
show very interesting results. 

In one of my earliest experiments, I kept a record of the 
changes going on in the various kinds of glass, at first from day 
to day, and afterwards from week to week, and month to month. 
The following is a specimen : 



Kinds of glass. 


Original color 
before exposure. 


Color after one 
day's exposure. 


Here were inserted 
other columns to 
show effect of ex- 
posure for 2 and 
4 days, for 1,2 and 
8 weeks, for 1 and 
2 months. 


Color at end Qf 
exp't (3 mo«.) 


French white plate, 
English crown glass. 
French white sheet. 
Belgian sheet. 
American sheet. 


• 







I might give the names of all the different kinds of glass 
which I have exposed to sunlight, but I refrain because for the 
reason given previously, I have found different specimens from 
the same manufactory of plate, of crown, and of sheet-glass, 
sometimes to differ in shade, sometimes in result of exposure, 
and sometimes in both. 

I will, however, give below, (not naming the particular man- 
ufactories), a general description of my experiments in 1863, 
when an exposure of thirty-three specimens for a few months in 
summer and autumn showed the following results. 



6 



T. Gaffield on the action of Sunlight on Glass. 



The first table shows that twenty -four kinds were found to 
be easily affected, and exhibits the time at which the change in 
color was first observed. The second table shows nine kinds 
which did not change in color during the same time. 

Table of glasses easily changed. 



Kinds of Glass. 


Original color before exposure. 


6 of French white plate. 


White, with light bluish green tinge. 


2 ** German « " 


White " " " " *' 


1 " English white plate. 


White « " « 


6 « " plate. 


Yello)^i8h green. 


1 " Belgian rough plate. 


White, with slight green tinge. 


3 *' French white sheet. 


** with light bluish green tinge. 


1 « Belgian " « 


" " " yellowi^ green tinge. 


4 " English " " 


One light yellowi^ green and three light blu- 




ish green tinge. 


2 " American" ** 


White, with light blue green tinge. 


24 





The time at whicli 


i change of color was first observed. 




Days. 1 


Weeks. | 


Months. 


Color after exposure. 


1 

1 

1 


2 


4 
4 


1 


2 

1 


3 


1 


2 


3 




Yellow or yellowish green. 


1 


















t( ti 


3 


4 


1 




1 










Deeper yellow tinge. 
Yellowish green tinge. 


1 


















« (( 




1 




1 


1 




1 






(< (( 


1 
8 


6 


6 








1 






K (( 


1 


3 





Table of Glasses not changed in three months. 



Kinds of glass. 


Original color of glass. 


1 kind of German crystal plate. 


Light green. 


1 " 


« plate. 


Dark blue. 


1 ** 


** English rough plate. 


Dark green. 


1 ** 


" English crown. 


Light green. 


1 " 


" American " 


Bluish green. 


1 " 


" " sheet. 


Dark bluish green. 


1 " 


" English " 


Dark green. 


1 u 


" Belgian « 


Yellowish or brownish green. 


1 '* 


« French " 


(( « (I (f 


9 




- 



Subsequent experiments with five of these kinds, (all which 
I could conveniently obtain), showed that an exposure of a year, 
or even less, would change all but an ordinary kind of American 
sheet, which was of a dark bluish green color. 

The experiments which speak for themselves are the most 
fiatisfactory ones, that is to say, where one has not only the 



T. Gaffield on tlie action of Sunlight on Glass. 7 

record made at the end of each month, but a piece of glass 
taken in and laid aside at the same time to show the actual color 
produced, and the truth of the records. It is very interesting 
to witness any one of these series of specimens, showing, as in 
one of white plate, a gradual changje, commencing in a day 
or a few days in summer, from greenish or bluish white to a 
yellowish white, or light yellow, a deep and deeper yellow, un- 
til it becomes a dark yellow or a gold color ; and in some Bel- 
gian sheet specimens, a gradual change, commencing in a few 
weeks in summer, from brownish yellow to deeper yellow, yel- 
lowish pink, pink, dark pink, purple and deep purple. 

There are several kinds of glass in which no perceptible 
change took place in three months, which were very sensibly 
affected by an exposure of a year. 

JExperimenis. — I have given a general account of my first 
experiments in 1868, and a portion of the tables kept in my 
journal at that time. I might have given names and results in 
full and shown the actual effects and shades of color produced 
by exposure for a few months, on some thirty kinds of glass. 
But in my case, as in many novel and original investigations, 
the results of first experiments, and the theories based upon 
them, were modified by subsequent ones. I supposed that many 
kinds of glass not changed in three months would not change 
at all : that all which changed would take a yellowish color, 
unless by exposure of many years: that no color but some 
shade of yellow or pink would ever be produced in any kind by 
exposure to sunlight. 

The experiments of 1864 and the two following years, proved 
to me that nearly evei'y kind of window glass I had exposed, 
could be changed in one year ; that a rose or pink color, (or 
some tint approaching them) could be produced in various kinds 
in a few months ; and that some kinds of greenish white glass 
would, after exposure, assume a bluish, tint or bluish white. 

It may seem singular for one who has been a glass dealer and 
manufacturer like ourselves, thus to advertise what may be 
called a defect in his own wares. It might seem unkind to 
other manufacturers to expose the defects of their productions, 
literally before the light of day. But my scruples have been all 
removed, when I have noticed in a late communication of Pelouze 
(see Comptes Kendus, Jan. 14, 1867) the following statement : 
" I do not believe that there exists in commerce a single species 
of glass that does not change its shade in the sunlight." As all 
manufacturers are in the same category, it will do no harm for 
me to repeat what Pelouze says, as the result of my experiments, 
and to affirm that a longer or shorter exposure to the direct 
action of the sun's rays will probably change in some degree the 
color of all or nearly all kinds of window glass. 



8 



2\ Gaffield on the action of Sunlight on Glass. 



I subjoin an account of an experiment carried on for one year 
with nine different kinds of glasses. These represent plate, 
crown and cylinder glass, the manufactures of both hemispneres 
and almost every shade and color of what are called colorless 
glasses. 

I name only the nationalities, and not the particular manu- 
factories of the glasses in any of my tables. I have this infor- 
mation recorded in my journal, and shall be happy to commu- 
nicate it to any who may desire it. 

Memorandum of nine different kinds of glau exposed from Jan. 12, 1866, to Jan. 

12, 1867. 



Kind of glass. 


Color before exposure. 


Color after exposure. 


French white plate. 


Bluish white. 


Yellowish color. 


Qerman crystal plate. 


Light green. 


Bluish tinge. 


English plate. 


«( « 


Yellowish green. 
Light purpush color. 


English crown. 


(( u 


Belgian sheet. 


Brownish yellow. 


Deep " « 


English sheet. 


Dark rreen. 
Light blnish white. 


Brownish green. 


American crystal sheet. 


Purplish wbite. 
Light yellowish green. 


i< (• it 


Lighter bluish white. 


*• ordinary " 


Bluish green. 


No change. 



The colors named above are given from an observation of the 
glass edgewise, by which one can see a body of color two or 
four inches in depth, whereas the usual thickness of the glass 
varies from one-fourteenth to one-quarter of an inch, and shows 
its color easily only by placing a white curtain or paper behind it. 

I have tried several experiments showing the effect of sun- 
light during each month and each season of the year. At the 
end of the year, by the comparative depth of yellow or purple 
color produced in the various specimens, one can see the com- 
parative actinic power of the rays during each month and season. 
The results proved that the actinic effect increased from Jan- 
nary to July, and decreased after that month. The greatest 
effect during anv season was observed in the summer, the least 
in winter, and that in spring and autumn was about alike, and 
midway between that of summer and winter. 

Crystal or lead glass, and a piece of optical glass having prob* 
ably very little, if any, manganese, changed not in two years. 
Perhaps a longer exposure may produce some change. 

A rough piece of light colored window glass metal changed 
to a yellowish color in a year. 

Colored glasses after two or three years' exposure showed no 
perceptible change in any instance except a slight one in a sin- 
gle specimen of purple. Perhaps an exposure of many years 
may make a change in some other colors. 

I have made experiments with artificial heat on glass in vari' 
ous ways, from exposure to the heat of a cooking range oven 



T. Oaffield on the action of Sunlight on Glass. 9 

to that of a glass stainer'a kiln, without aay change of color in 
the common colorless window glasses, while the same or similar 
specimens exposed to sunlight have been nearly all changed in 
a few months. 

Specimens exposed in hot water for a mo&th, in doors and 
out of sunlight, experienced no change of color, while similar 
ones exposed during the same length of time in the bottom of 
a dish filled with two or three inches of water out of doors, and 
to the direct rays of the sun, experienced a decided change, 
though only about half as much as when directly exposed, out 
of the water. 

Being convinced that air, moisture, and artificial heat do not 
make any change of color, our experiments indicate that the 
change is effected by the actinic rays of the sun alone. 

This actinic effect is cut off in some degree by every medium, 
by water as stated above, and even by clear glass, as a specimen 
exposed inside of a window or under another piece of thin col- 
orless glass shows only about one-half as much change, as that 
exposed outside of the window or with no covering of glass 
over it. The amount cut off by colorless glass and by colored 
glass differs greatly with the difference of color. 

The comparative power of glass of different kinds to transmit 
the actinic rays I have tested, by placing underneath pieces of 
each kind, pieces of easily changing glass, (white plate or Bel- 
gian sheet glass,) exposing them one year, and noticing, at the 
end of that period, the comparative depth of the yellow or 
pink color to which the unaer pieces had changed. The re- 
sult of my experiments proved that the most easily transmis- 
sive of the colorless glasses were the English crown, French 
plate, two kinds of white crystal sheet made in Massachu- 
setts, (from the celebrated Berkshire white sand,) the New 
Jersey sheet glass, one kind of English plate, and one kind 
of Belgian sheet, and about in the order which I have named 
them. 

Of the colored glasses, the blue transmitted the most, the pur- 
ple less, the red and orange the least, the glasses under these two 
and the yellow and green showing little or no change. 

This last experiment proves the propriety of the preference 
given by photographers to blue glass for skylights, oecause it 
transmits the blue rays, which exert the most actinic power. 
But it may be added, that a colorless white glass, or bluish 
white, — if one which will not change by sunlight to a yellow 
or rose color, owing to the presence of manganese, or any other 
cause, — is equally good, as it will transmit all the rays, and 
among them, the actinic or blue ones. In proportion as any 
kind changes to a yellow or rose color, it will lose its power of 



10 T. Oajfield on the (xction of Sunlighi on Glass. 

transmission, and its value as photographic glass. I have seen 
specimens of the two kinds of white crystal sheet made in 
Massachasetts, before alluded to, which answered the demands 
of photographic artists. Of foreign glass, I have noticed a fine 
bluish white she&t, made lately without manganese, from a cer- 
tain excellent manu&ctory in Belgium, and one kind of English 
crown glass. 

Should plate glass be required, the most permanently endu- 
ring, or least likely to assume a yellow color, are a superior kind 
of white -plate, made by the French and Belgian Plate G-lass 
Companies, and an excellent quality of German crystal plate, 
made at a long established factory in Hanover. 

I desire to say here, however, that it is not the place where 
any glass is maae, which determines its good character, but the 
actual constituent materials and the superiority of its manu- 
facture. 

Manufacturers are frequently changing their mixture or " batch, " 
so that any results given with one set of samples might differ 
from those made with another set, from the same manufacturers. 
For this reason, in noticing any differences which may occur in 
experiments made by any of our readers, this fact should be 
considered as an explaining cause. 

I have seen specimens of glass from a factory which changed 
to a yellowish tinge in a few months, others which changed to a 

Eurplish hue, and still others from the same factory, which 
ardly changed at all. A difference in the mixture, (or batch, 
as it is termed), makes a difference in the tinge of the specimens 
from the same factory, both before and after exposure to sun- 
light. The chief points for photographers are to get glass made 
from as pure materials as possible, of as light a color as practi- 
cable, and free from oxyd of manganese. A glass like either 
of those named above, as most easily transmitting the actinic 
rays, might be good for one year or more, and then become very 
much injured for photographic effects, by the change of color to 
yellow or pink by sunlight. 

Any photographer can make these observations practical, by 
testing the action of sunlight for six months or a year, on all 
the specimens offered him for sale. And all manufacturers 
can make them practical, by making their glass of pure mate- 
rials, which will not have to be " doctored," to use the glass- 
makers' term for the use of manganese ; or by allowing the glass 
to assume its natural color, even if it be a little blue or green, 
rather than to run the risk of its subsequent change to yellow 
or purple by exposure to sunlight. 

In the Comptes Bendus for January 14th, 1867, Pelouze says, 
(and we believe he is the first and only writer who has made 
this observation) : 



i 



7. Gafield on the action of Sunlight on Glass. 11 

"Exposure to red heat decolorizes the glasses which have 
been made yellow by sunlight, or to speak more exactly, they 
retake the light green shade which they had before exposure. 
A second exposure to sunlight produces a second coloration, 
similar to the first, and a red heat makes it disappear again. 
These phenomena can be reproduced indefinitely. The glass 
preserves its transparency and does not give place to any striee 
or bubbles." He also says : 

"I possess specimens of glass rendered violet by sunlight. 
All present the property of being decolorized by heat. A tem- 
perature of 350 degrees is not sufficient. It is necessary to have 
that employed in the reheating of glass in general, and that is 
in the vicinity of red heat. The glass decolorized by heat when 
exposed to sunlight retakes the amethyst color which it acquired 
the first time, loses it anew when it is heated ; and these curious 
phenomena can be reproduced without cessation." 

In confirmation of this most interesting statement of Pelouze, 
I have exposed in a glass stainer's kiln, several specimens of 
glass which had been changed by the action of sunlight, some to 
a yellow and some to a purple color. The exposure to an ex- 
treme red heat made the glass assume, some a white, some a yel- 
lowish white, and some a green color, which were probably the 
original colors. These specimens were taken from windows 
where they had been exposed from a few years to more than 
half a century. Further experiments, which I have already 
commenced, will show whether we can reproduce the exact 
original colors by heat, after being changed Dy exposure to sun- 
light. 

We have in the same kiln exposed some dozen original and 
unexposed specimens of what are called colorless window glasses 
of different kinds and shades of color, and found them un- 
changed in the slightest degree by the action of great heat, while 
similar specimens have been changed in a few days, weeks or 
months, by the simple action of the sun's ray%. Fifteen speci- 
mens of really colored glasses, (red, green, yellow, &c.,) have 
been exposed in the same way without any change of color, ex- 
cept a very slight one in a few specimens which were burnt or 
over-heated. 

In PoggendorflTs Annalen, Berlin, of May 1st, 1889, is re- 
corded the following interesting fact by Ai Splittgerber : 

" I would mention a curious fact, in which the sunbeams have, 
if I may say so, done something in the art of penmanship ; not 
only on the surface, but by inscribing characters through the 
body of the glass ; and, though the matter is based upon causes 
well known by experience, yet there has probably never before 
been so striking an instance of their effect known. I am in 
possession of a plate of glass which was used as a window 



)2 T. ^^bifield ra <Ae actum af Sunlight on Glass. 

pane for xpope (ban twenty yeara^ and on whidh was an inscrip- 
lioa in gold letters. This inscription was taken off by grinding 
the plate on both sides, and polishing it so as to have a new 
surface. When the glass had been polished^ the inscription 
oould again be clearly seen. The parts which bad been under 
the letters remained white, while the remainder of the plate had 
i^umed a violet tint, in consequence of the manganese it con- 
tained, a coloring which permeates the whole mass, as the grind- 
ing of the surface proved. The uncovered part of the plate, 
especially when laid upon a white background show the clearly 
^peadable characters." 

The same or a similar instance is related by Dr. Herman Yogel 
in the Photographische Mittheilungen, Berlin, of Sept., 1866. 

Desiring to produce a similar result, we made an inscription 
o^ a piece of ^Igian sheet glass, in part with gold and silver 
leaf, and in part with black and white paint. The gold and 
silver leaf were sopn washed o£^ but the black and white painted 
letters remained, and being removed after an exposure of nearly 
two years, the words stood out in clear contrast and full propor- 
tions, the inscription being in the original color of the glass, and 
the surrounding portions having been changed by the action of 
the sunlight to a purple color. 

A very interesting experiment can be made, to show the grad- 
ually increasing effect of the sunlight on glass, by taking a piece 
of easily changing glass, say 4x20 inches, painting black a 
strip 4x2 inches at each end to preserve the original color, and 
then exposing the strip to sunlight At the endr of one, two, 
four, six, eight and ten months, one, two and three years respec- 
tively, cover with black paint a strip 4x2 inches, and at the 
end of three years remove all the paint, and you will have, in a 
single piece of glass, the original color and all the gradations of 
change effected by exposure from one to thirty-six months. I 
have made a similar one with Belgian sheet glass exposed nearly 
two years. It is one of those interesting experiments which 
speak for themselves, and defy suspicion or contradiction. 

I have made an experiment for one year with two kinds of 
eaaily changing gltuBS out of doors, and out of the direct rays of 
the sun, and found that they were both slightly affected, and 
changed toward a yellowish odor. I did not expect any change, 
but can, perhaps, properljr account for it, on the ground that it 
was the result of the action of diffused sunlight. It is barely 
possible that the sun may for a few minutes in some days of the 
year have cast some reflections when I was not present, in the 
dark corner in which I placed my specimens. 

It may be, that the action oi the sun's heat produced the 
slight effect noticed. If so, it would be an interesting confirma- 
(ion of Tyndall's experiments, and of his theory of the correla- 



T. Gaffield' m the a^^im of SmligH m CRass. IS 

ti^n ^f forc^a^ I do opt oopeideir mj^ si^gte ^peximi^t m en^ 

tirely conclusive, and shall make others, which will give mk mom 
materiail for prfop^r iheoFias and co»olusi<>i)a. 

Th^ experiments which I have oarri^d oa for torn yeais em- 
brace oDe specimen of optical glass, a few kiods of flint glais» 
and glass wa,re ; sixteen kinds ai Freaeh, Belgiao^ GeFman and 
English plate glass^ fbar kinds of Amierican, Snglish, French 
and Belgian rough plate^ two of American and English erown 
glass, t^n kinds of American, Belgian, French and English 
white sheet glass^ ^r kind^ of American, Belgian an4 Eng- 
lish ordinary sheet glass, fifteen kinds and sbAdes of English 
colored glass, fou^r of opaque^ white eiianieUed and ground glass, 
and one piece of the rough metal of American sheet glass ; vb^ all, 
about sixty varieties. 

I have watched and recorded in some experiments, the resulta 
from day to day, in others from month to. month, and season to 
season. I have now commenced a series^ in which I may record 
results from year to year, for ten years or more. In these, it 
may be found that specimens of what are called eok)rlesa glasses 
changed to a yellow color bv exposure for a y^ar, may by much 
longer exposure be turned to a yellowish pink and a purpte. 
And som^ which have been entirely unaffected, may be affeeted 
by an exposure for ten or twenty years. Perhaps some of the 
colored glasses may show signs of a change of bue or shade. 

These new experiments include rough and polished pliite, 
crown, cylinder, ground, enamelled and colored glass. I have 
also begun to expose under several of these kinds of glasa^ pieces^ 
of easily changing glass, which I shall take in from year to 
year, these under pieces showing the power of these glasses 
above them to transmit the actinic rays. 

The most easily changing glasses are a certain kind of white 
plate, wbicb changes from a white to a yellowish color, and a 
certain kind of Belgian sheet, which the manufaeturerB used to 
make of a brownish yellow, (they now make it of a bluish or 
greenish hue, and it is not so easily changed,), which changes to 
a flesh color, or a pinkish hue. I have accordingly taken these 
two kinds, for my under glass experimeat& Under each of seve-> 
ral kinds, to be exposed from one to ten or twenty years, 
I have placed pieces 4x2 of the white pkte. I shall take 
in one piece at the end of the first, second, third, fourth, sixth 
and tenth year. These six lights will show the increased action 
of the transmitted rays from year to year. By comparing the 
different series with each other, one can perceive the comparative 
actinic power of each kind of glass, or rather, their compara* 
tive power of transmitting actinic rays. 

Another interesting under experiment is the foUowinff^ I have 
placed under one piece of each kind of glass exposeoj a piece 



14 T. Oaffield on the cu^tion of Sunlight on Olass. 

of easily changing glass, which I shall take in at the end of 
the year. 

At the beginning respectively of the second, third, fourth, 
sixth and tenth years, 1 shall place under the same piece, 
another strip of 4x2 inch glass, taking in each piece at the end 
of the year of its exposure. This series will show the diminish- 
ing or increasing power of the glasses under which they have 
been exposed, to transmit the actinic rays ; in other words, will 
show whether exposure to the sun increases or diminishes the 
actinic power of the glasses exposed, and renders them better or 
worse for photographic purposes. 

I have taken a piece of 4x18, of easily changing white plate, 
painted with black paint two inches of each end, to preserve 
the original color, and exposed the piece. At the end of the 
year, I shall paint over two inches more of the glass. At the 
end respectively of two, three, four, six, and ten years, I shall 

f)aint over two inches more. At the end of this time, or a 
onger term, I shall remove all the black paint, and on one light, 
I shall have all the grades of changed color and shade produced 
by their different lengths of exposure. I shall lay aside one 
piece of 4x18 white plate, taken from the same sheet with the 
exposed light, in order to compare the original with the changed 
specimen. 

I have painted and exposed, just in^the same manner as above 
described, a piece of 4x 16 of easily changing Belgian sheet glass. 

To show a speaking proof of the painting power of the sun- 
light, I have taken a piece of 4x6 Belgian sheet and covered 
it with a thin plate of brass, having the following letters cut out 
of it: T. G., Jan. 1, 1867. I have taken another piece 4x6 
Belgian, and stuck on with gum shellac the two letters T. G. 
After exposure of one year or more, the removal of the brass 
plate and letters will show in the former ease, rose or purple 
colored letters on a brownish yellow ground, and in the latter, 
brownish yellow letters on a rose or purple colored ground. 

A similar experiment as the above, I have commenced with 
two pieces of white plate, and the simple letters T. G., without the 
date. The result oi the experiment in a year or more, will be to 
show in one case, yellowish letters on a light colored ground, and 
in the other, light colored letters on a yellowish ground. 

I have thus given, as briefly as possible, and yet as fully as 
desirable, ah account of my past and present experiments. 
New ones are suggested from year to year. I trust that this in- 
teresting field for observation and experiment may be worked 
in other countries. There is ample room for research in the ap- 
plication of chemical knowledge, of qualitative, quantitative, 
and spectral analysis, and of photogenic tests, to discover the 



T. Gaffield on the action of Sunlight on Glass. 15 

exact action and causes of the interesting effects of the sun's 
rays, which have here been noticed. 

Theories. — The interesting phenomena of which I have given 
an account, have given rise to many theories to account for their 
cause. Some attribute them to the presence of oxyd of iron, 
and some to oxyd of manganese. Exactly how the change 
takes place is a question on which writers differ, although it is 
my opinion that the precise explanation can only be given after 
a multiplication of experiments, and a thorough examination of 
exposed and unexposed specimens of glass by quantitative and 
qualitative analysis, and perhaps by spectral analysis and ob- 
servation of photogenic effects, or photogenic tests.* 

We will briefly state the part which the oxyds of iron and 
manganese play in glass making. In almost all kinds of 
window glass, and in some poorer qualities of flint glass, and 
glass ware, materials are used which are not perfectly and 
chemically pure. The sand, the carbonate or sulphate of soda, 
and the lime, one or all, contain slight impurities of iron, the 
protoxyd of which gives glass a green color. To correct this, 
after the batch is partially melted, a little oxyd of manganese, 
Q^iW^di glass-maher^s soap^\s put into the crucible or glass pot; 
some of the oxygen of the manganese flies off to the iron, and 
converts the protoxyd into peroxyd of iron. The peroxyd 
gives a yellowish color to the glass, and this, being complimen- 
tary to the natural pink of the manganese, is neutralized, and 
the glass is thereby made of a light color. When the sunlight 
acts upon glass thus made, the nice equilibrium between the 
oxygen of the iron and the manganese is disturbed, and some- 
times the yellow, and sometimes the pink or purple color is pro- 
duced. I have produced all shades oi the purples, running from 
pale lavender, mto the lilac, mulberry, flesh, amethyst, rose, 
violet, pink and deep purple. I have produced, or seen speci- 
mens, showing all shades of the yellow, from the brownish yel- 
low, up to the brightest gold color, and I have several series of 
specimens, in which the green has gradually changed into the 
yellow, and the yellow gradually run into the pink and purple. 

Pelouze, in an article in the Comptes Rendus, of Jan. 14th, 
1867, sets forth the following theory. 

"There is in glass colored yellow in sunlight some protoxyd 
of iron and sulphate of soda. Light provokes between these 
matters a reaction from which results peroxyd of iron and sul- 

* Since writing the above, by the kindness of Mr. John A. "Whipple, the distin- 
guished photographer of Boston, I have been enabled to show, by the comparative 
darkening of sensitive paper under several exposed and unexposed specimens, the 
effect of exposure to sunlight for one year. The loss of actinic power, or power 
to transmit the actinic rays, was in proportion to the change of color. This was 
in some varieties of glass quite perceptible, but in all will be more so after an ex- 
posure of several years. 



16 jH. Qiii0>eld on iks^ action of Sunlight on Ghss. 

phid of sodium. The heat brings about aa inverse reactioo 
and reproduces sulphate of soda and protoxyd of iron, From, 
thence comes the return of the glass to its primitive color. 
Analysis comes to the aid of this theory in demonstrating in 
glass rendered yellow by sunlight, the presence of ap infinitely 
feeble, but still very sensible proportion of a sulphid, whilst the 
reactions do not show the slightest trace in the same glass before 
their expoaure." 

" It may be asked, why glasses colored by the reduction of the 
sulphate, or the direct introduction of a sulphid into their mass, 
resist an equal or superior heat to that which produces the decol- 
oration of gla^ become yellow in sunlight. Here is the answer. 
In glass made yellow at a high temperature by the reduction of 
sulphate, the iron is found in a state of protoxyd, which cannot 
react in any degree upon the sulphids. Therefore the glass re- 
mains colored. In glass made yellow by sunlight, the iron is 
peroxyd and in consequence^ in a condition to change the sul- 

Ehid into sulphate, when we. expose the glass to the action of 
eat." 

In reference to glass made violet color by sunlight, he says, 
" The coloration seems to be due to the fact that the peroxyd of 
iron gives up a part of its oxygen to the protoxyd of mangaAese 
conformably to one of the two iollowing equations. 

Fe^O^ +MNO=2(FeO)+MN03 
or Fe»03-i-2(MNO)=2(FeO)-i-MN«0^ 

The reheating of glass, that is to say, the action of a tempei*- 
ature of red heat, produces an inverse reaction, which explains 
the decoloration. In which we have 

2(FeO)+Mn20 ==Fe203+2(MnO).'* 

Bontemps, in an interesting article in the Comptes Rendus of 
Feb. 4th, 1867, attributes the changes in color to the presence of 
oxyd of manganese. He also remarks that he thinks that the 
violet color occurs only in glass in which the silicates have a 
base of potash, and the yellow in cases where soda is used. I 
doubt the correctness of this opinion, as I am quite sure that I ' 

have several specimens colored violet, which contain nopota sh 
at all in their composition. 

I have also specimens which are colored both yellow and vio*- 
let in the same piece, the yellow portion in one case having been 
produced by a certain length of exposure, and then having been 
covered witn black paint, while the violet portion was produced 
by longer exposure- 

As before stated, I have noticed changes in what are called 
colorless glasses, from light colors approaching white, to yellow 
and pink or purple. I have noticed also a change in a few 
speeimens from a light green to a bluish shade. The former 



T. Gaffield on the action of Sunlight on Gfo^^^ ft 

may be accounted for hy the presence of manganese, a very mi 
nute proportion of which oxyd will have a sensible decolorizing 
effect in a crucible of meltea glass metal. Pelouze-s theory of 
the peroxydation of the iron may have some weight in determin- 
ation of the cause of the yellow or pink coloir by the action ot 
sunlight. But I know not how to account for the change of olie 
specimen each of plate, crown, and sheet glass in my possession, 
from a greenish white to a bluih tinge, not mingled with either 
yellow or purple. 

I have been pleased to find the interest in these experiments 
by photographers, who have long noticed that they could take 
better pictures under a newly glazed skylight, than under one 
which had long been exposed to sunlight. The cause of this 
change is, that the slightest yellow color interferes with the trans- 
mission of the actinic rays, and a very deep shade will cut them 
oflF in a very great degree. My experiments with glasses under 
other glasses proved which was best for photographers' use, in- 
formation which all can gain by exposure of the specimens of 
varicwis manufacturers which may be offered them. The iiiost 
pure glasses of light green, or bluish white color are the biest 
for photographers, and when I say pure glasses, I mean those 
most free from oxyd of iron or manganese, but especially of the 
latter, which, I think, is the cause of nearly all the changes 
which I have observed. Mr. J. W. Osborne, of New York, Uie 
gentleman who has done so much to bring the art of photo-lith- 
ography to perfection, and into practical use, writes as follows: 

" I believe your researches will prove of much practical im- 
portance, and I wish the glass manufacturers could be got to give 
serious attention to the subject. It takes but a very slight tinge 
of yellow to cut off twenty-five per cent, of the actinic rays, I 
am forced to work under glass, because of the protection from 
the wind and weather, but in doing so, I sacrifice in any case 
much of the light. I have to increase the time of exposure pro- 
portionately. With the best of glass, the loss from reflection 
and absorption is considerable. This may be fearfully increased 
by the color, and if that undergoes a continual change for the 
worse, the state of things is exceedingly unpleasant. I was 
warned by a photographic friend in England, to take care of a 
certain kind of cheap French glass, made for glass rooms, which 
turned yellow ; but I had no notion that the evil was so general 
as you appear to have found it. The subject is of such vital 
importance to photographers, that I intend drawing the attention 
of that friend to what you have done." 

1 am indebted to Mr. Osborne for bringing our researches to 
the notice of his friends at home and abroad, who have contrib- 
uted interesting articles and valuable information on the subject 
to the Philadelphia Photographer, (October, 1866,) the London 



16 T. Gaffield on the miction of Sunlight on Glass. 

Photographic News, (August 2, 1866,) the London Photographic 
Journal (Aug. 15, 1866,) and the Photographische Mittheilungen 
(Sept. 1866). 

It may seem singular that so long an experience in the window 
. glass business, haa not at an earlier period drawn my attention 
to the subject of this article. But my experience is not singular. 
In conversation with many glaziers and glass dealers, I have sel- 
dom found one who was aware of the great change of color ef- 
fected by sunlight. Few have supposed that specimens were to 
be found in any other windows in our city, than those on Bea- 
con street facing Boston Common and the south, and exposed to 
the full force of the sun's rays. But having my attention now 
particularly directed to the subject, I never pass a window 
without detecting where any considerable change has been effec- 
ted. I have found them in all portions of the city, and most 
generally in those positions which face the east and south. The 
color is most easily detected, when the glass has been ground or 
enamelled, or where a white window curtain forms a good con- 
trasting back ground. Many people suppose that the very ^dis- 
tinctly marked purple plate glass in Beacon st. was imported of 
this color, and that it is now no longer made ; and hence the 
reason why the windows looked like checkerboards, when bro- 
ken lights were replaced by those of the usual light greenish color 
of plate glass. 

On this point I have convinced many who had held contrary 
views, by showing pieces from which the putty on the edges has 
been removed, and displaying beneath the original color. When 
the putty covered t^e glass, the sun's rays could not reach it, 
and the color was unchanged. Such specimens are quite inter- 
esting. I have had many conversations with old glaziers, dealers 
and consumers of glass, and also with those who stain and enam- 
el it. I have thus gathered specimens of various kinds and 
colors, and of differing lengths of exposure from old windows. 
The oldest specimen was one of crown glass set in a church in 
Lexington, Massachusetts, in 1794, from which the windows were 
removed in 1846, and since used as covers for hot-beds. The 
original color, found by removing the putty from the edges, was 
a light green, and the present, after seventy-three years expo- 
sure, is a purple color. I have never yet met any one who has 
seen glass in original imported packages, of the purple color 
made by exposure to sunlight, and until I do, I shall adhere to 
my opinion, that all purple or rose colored glass which is seen 
in our city windows, was made so by said exposure. I have 
verv fortunately found an octogenarian, who has furnished me 
with some glass which was imported, he thinks from some part 
of Germany, which is of a light green or yellowish green color. 
The glass was imported more than thirty years ago. Much of 



T, Gaffield on the action of Sunlight on Glass, 19 

that which has been set in his windows facing the south is now 
purple. An experiment with the original glass, commenced this 
summer, showed a perceptible change in color in one day, and 
in two weeks, the change toward purple was so marked that I 
have no doubt that this color will be distinctly visible in less 
than a year. If two years were occupied in the erection of the 
Beacon street houses, or any others in which the plate glass 
purpled by sunlight is found (as I am informed was the fact in 
some cases), then the result of the single experiment named 
above is a sufficient reason for the mistaken belief of many oc- 
cupants and owners to-day, that the glass was purple when im- 
ported. It was probably changed from yellowish green to pur- 
ple before the houses were finished, and the owners had taken 
possession. 

The action of sunlight which I have spoken of in this article, 
must not be confounded with that called " rust " or " stain," 
which is occasioned by exposure to the weather, and manifests 
itself in two ways; first, by a disintegration and roughening of 
the surface, sometimes producing all the effects of ground glass ; 
and secondly, by an apparent formation of an infinitesimal coating 
of oxyd on the surface, on which the play of light gives all the 
colors of the rainbow, as with the action of light on the infinites- 
imal grooves of the surface of mother-of pearl. This is simply 
surface action, whereas the action of sunlight permeates the 
whole body of the glass, wherever the rays directly strike it. 

The writer of this' article, (compiled in the midst of the busy 
duties of mercantile and official life,) makes no pretension to 
accurate scientific knowledge, but gives the results of his obser- 
vations and methodical experiments on a well known phenom- 
enon, in the hopes that they may add some mite to the sum of 
human knowledge, and may stimulate and aid those who are 
better versed in scientific studies, to ascertain the causes and ex- 
act operations of this interesting power of the sun's rays to paint 
the products of art, as they do so beautifully and wonderfully 
the works of nature on the mountain, in the forest and field. 

I Boston, Mass., July, 1867. 





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