This is a digital copy of a book that was preserved for generations on library shelves before it was carefully scanned by Google as part of a project
to make the world's books discoverable online.
It has survived long enough for the copyright to expire and the book to enter the public domain. A public domain book is one that was never subject
to copyright or whose legal copyright term has expired. Whether a book is in the public domain may vary country to country. Public domain books
are our gateways to the past, representing a wealth of history, culture and knowledge that's often difficult to discover.
Marks, notations and other maiginalia present in the original volume will appear in this file - a reminder of this book's long journey from the
publisher to a library and finally to you.
Google is proud to partner with libraries to digitize public domain materials and make them widely accessible. Public domain books belong to the
public and we are merely their custodians. Nevertheless, this work is expensive, so in order to keep providing tliis resource, we liave taken steps to
prevent abuse by commercial parties, including placing technical restrictions on automated querying.
We also ask that you:
+ Make non-commercial use of the files We designed Google Book Search for use by individuals, and we request that you use these files for
personal, non-commercial purposes.
+ Refrain fivm automated querying Do not send automated queries of any sort to Google's system: If you are conducting research on machine
translation, optical character recognition or other areas where access to a large amount of text is helpful, please contact us. We encourage the
use of public domain materials for these purposes and may be able to help.
+ Maintain attributionTht GoogXt "watermark" you see on each file is essential for in forming people about this project and helping them find
additional materials through Google Book Search. Please do not remove it.
+ Keep it legal Whatever your use, remember that you are responsible for ensuring that what you are doing is legal. Do not assume that just
because we believe a book is in the public domain for users in the United States, that the work is also in the public domain for users in other
countries. Whether a book is still in copyright varies from country to country, and we can't offer guidance on whether any specific use of
any specific book is allowed. Please do not assume that a book's appearance in Google Book Search means it can be used in any manner
anywhere in the world. Copyright infringement liabili^ can be quite severe.
About Google Book Search
Google's mission is to organize the world's information and to make it universally accessible and useful. Google Book Search helps readers
discover the world's books while helping authors and publishers reach new audiences. You can search through the full text of this book on the web
at |http: //books .google .com/I
, . ;.'-i5'
- ' •■ .i:lt
rc-c; ,> y-L Ci.--6 .
%• ' -
SUNLIGHT ON GLASS.
By THOMAS GAFFIELD.
[From the American Journal of Science and Arts, September and
printed by tuttle, Morehouse * taylor,
[From the Amb&ican Journal of Soiinox and Arts, Sxfy. and Nov., 186^.]
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
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
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.
Color after one
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
Color at end Qf
exp't (3 mo«.)
French white plate,
English crown glass.
French white 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.
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.
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.
The time at whicli
i change of color was first observed.
Color after exposure.
Yellow or yellowish green.
Deeper yellow tinge.
Yellowish green tinge.
Table of Glasses not changed in three months.
Kinds of glass.
Original color of glass.
1 kind of German crystal plate.
** English rough plate.
" English crown.
" American "
" " sheet.
Dark bluish green.
" English "
" Belgian «
Yellowish or brownish green.
« French "
(( « (I (f
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.
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
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.
Kind of glass.
Color before exposure.
Color after exposure.
French white plate.
Qerman crystal plate.
Light purpush color.
Deep " «
Light blnish white.
American crystal sheet.
Light yellowish green.
i< (• it
Lighter bluish white.
*• ordinary "
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
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
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-
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) :
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-
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
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
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
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
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
" 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
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.
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
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
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
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.
— ^ 7