Popular Science Monthly/Volume 5/May 1874/Action of Sunlight on Glass


IN a quiet street at the "West End" of Boston, there stands a house, the window-sills and roof of which, for more than ten years, have been covered with hundreds of pieces of glass, exposed to the full force of the sun's rays during the whole or greater portion of every day, only being protected by covers in the event of snow-storms. The results of these experiments, instituted to show a change produced in the color of glass by the actinic rays of the sun, have been discussed in this country and in Europe, but as yet the cause of this remarkable phenomenon has not been fully explained.

As early as 1825, Prof. Faraday noticed a change of color in glass containing oxide of manganese, due to the action of solar light, and it was thought that advantage might in some cases be taken of this action for the removal of color in glass. Prof. Faraday found that glass of a pale color, or even colorless at first, became pink by long exposure to solar rays, while portions of the same glass, not so exposed, were apparently unaltered. This effect he attributed to the solar light acting upon the manganese. In 1839 Splittgerber recorded the following interesting fact in Poggendorf's Annalen, published in Berlin: "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-pane for more than twenty years, and on which was an inscription 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 could again be clearly seen. The parts which had been under the letters remained white, while the remainder of the plate had assumed a violet tint, in consequence of the manganese it contained, a coloring which permeates the whole mass, as the grinding of the surface proved. The uncovered part of the plate, especially when laid upon a white background, shows the clearly-readable characters."

From the above, it will be seen that the power of the sun's rays to change the color of glass has been publicly announced for at least a half-century; but it does not appear that elaborate and systematic experiments upon this subject were instituted prior to those referred to in the opening sentence of this article. These were begun in 1863, by Mr. Thomas Gaffield, a window-glass merchant of Boston, who has made an enthusiastic study of many matters pertaining to glass, and whose collection of authorities on this and kindred subjects is probably not equaled by any private, and by very few public, collections in existence. These experiments now cover a period of eleven years, and embrace some eighty different kinds of glass, of English, French, German, Belgian, and American manufacture, including specimens of rough and polished plate, crown, and sheet window-glass; flint and crown optical glass; opal and ground glass; colored pot-metal (i. e., glass colored in the pot during the process of melting); flashed and stained glass of various colors; and glass-ware and glass in the rough metal. The experiments have been conducted with pieces of glass usually four by two inches, of which several hundred specimens have been exposed, showing the effect of sunlight in producing a change of color by exposure, from one day in summer to several years. The changes produced in the colorless glasses are from white to yellow, from greenish to yellowish green, from brownish yellow to purple, from greenish white to bluish white, and from bluish white to a darker blue.

Mr. Gaffield's plan of procedure has been to cut a number of pieces of the size mentioned above, from the same sheet of glass, the number depending upon the nature of the experiment to be made. Suppose that white plate-glass is to be tested by exposure from one to twelve months: fourteen pieces, precisely alike, are cut from the same plate; two are carefully put away in a neat box, from which the light is excluded, and twelve are exposed to the sunlight. At the end of each month one of the exposed pieces is withdrawn from the light, carefully marked with a diamond to show the length of time exposed, and placed in the small dark box. At the end of the year, therefore, the collection embraces fourteen pieces—two of the original color, and twelve showing the effects of exposure from one to twelve months. Other collections may be made to show the daily, weekly, and yearly progress of the sun's rays in changing the color of the glass.

Mr. Gaffield found that, in the time required to produce a change, different specimens of glass presented widely different qualities, the change being much more easily effected in some than in others. In some specimens a marked change of color was observed in a few days; others, after resisting the powerful influence of the solar rays for years, were finally overcome, and made to assume a new color. Several kinds, in which no perceptible change took place in three months, were very sensibly affected by an exposure of a year. But in almost all the change took place.

"It is very interesting," says Mr. Gaffield, "to witness any one of these series of specimens, showing, as in one of white plate, a gradual change, 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, until it becomes a dark yellow or gold color; and, in some Belgian sheet specimens, a gradual change, commencing in a few weeks in summer, from brownish yellow to deeper yellow, yellowish pink, pink, dark pink, purple, and deep purple."

One interesting experiment was carried on for one year with nine different kinds of glasses, representing plate, crown, and cylinder glass, the manufacture of both hemispheres, and almost every shade and color of what are known as colorless glasses. The results were as follows:

KIND OF GLASS. Color before Exposure. Color after Exposure.
French white plate Bluish white Yellowish.
German crystal plate. Light green. Bluish tinge.
English plate "" Yellowish green.
English crown "" Light purple.
Belgian sheet Brownish yellow. Deep"
English sheet Dark green. Brownish green.
American crystal sheet Light bluish white. Purplish white.
""" Lighter "" Light yellowish green.
"ordinary. Bluish green. No change.

These colors appear from an observation of the glass edgewise, when is seen 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 when a white curtain or paper is placed behind it.

Among other experiments made by Mr. Gaffield, two may be noticed as of peculiar interest, and as suggesting a process of producing very delicate designs and pictorial effects on glass. In the first, an inscription was made on a piece of Belgian sheet-glass, in part with gold and silver leaf, and in part with black and white paint. The gold and silver leaf were washed off, but the letters painted in black and white remained. After an exposure of nearly two years the surface of the glass was cleaned, when the clearly-marked words of the inscription appeared in the original color of the glass, while the surrounding portions were changed by the action of the sunlight to a purple color. By the second experiment the gradually-increasing effect of sunlight on glass may be shown by exposing to the solar rays a piece of easily-changed glass. Take a piece about twenty inches long by four wide, and at each end cover a strip about four by two inches with black paint. At the end of one month, two months, and at biennial periods thereafter, paint an additional similar strip in black, until the entire piece is painted. Then, upon removing the paint, there will appear a single piece of glass presenting the original color, and all the gradations of color and hue presented by exposure from one to thirty-six months.

In 1825 Faraday thought that only glass containing oxide of manganese was subject to this change of color. In 1867 M. Pelouze did not "believe that there exists in commerce a single species of glass that does not change its shade in the sunlight." The results of Mr. Gaffield's experiments have led him 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," and that the phenomenon is not limited to glass containing oxide of manganese. It should be observed that Mr. Gaffield's statement is limited to the ordinary window-glass, although embracing many different kinds of that class.

Specimens of flint and of colored glass have also been subjected to the test, but, with one or two exceptions, without exhibiting a change of color. An experiment, continued for five years, with red, yellow, green, blue, and purple pot-metal, i. e., glass colored in the pot, produced no change in any case except the purple. Still, this does not prove that changes may not be effected by longer exposure. Subsequently, Mr. Gaffield experimented with pot-metals, not of the primary colors, but of the intermediate ones, which most nearly approach those produced in colorless glass by sunlight exposure. In every specimen of the brownish, yellowish, and rose or purple colors thus exposed, a change in color or shade was produced in a short time. A change was also observed in the colorless body of some of the specimens of flashed and stained glass.

As pot-metal colors of this class were used in the early-painted windows, it is pertinently asked whether these experiments may not throw some light upon the many interesting questions relating to the alleged superiority of the old cathedral-glass.

"The fact of coloration," says Mr. Gaffield, "or change of color or shade by sunlight being established, must we not transfer some of our praise for the old artists in glass to the wonderful pencil of the brightest luminary of the heavens, which, during the centuries, has noiselessly but unceasingly been at work, deepening and mellowing the colors of all the windows of the venerable cathedrals of the world? We do not see to-day the glass as it was when it came from the artist's studio or the glass-factory. The dust of ages has accumulated upon its surface, the corroding tooth of Time has eaten into it; but how often has the wonderful alchemy of the sunlight done more in penetrating beneath the surface, and changing the shade or color of the entire body of the glass!"

The cause of the interesting phenomena above stated has not yet been explained, although not a few theories have been advanced to account for it. The change in color has been variously attributed to the presence of oxide of iron, to arsenic, to sulphur, and to oxide of manganese, in the constituent materials of the glass. But the true solution remains yet to be given. That the effect is not due to heat, but solely to the actinic rays of the sun, is shown by the fact that no change of color is produced in the glass when exposed to heat; while, on the contrary, after the discoloration has been produced by solar light, the colors thus acquired disappear under the action of heat, and the glass assumes its normal color. This process may be repeated indefinitely, the change of color being produced by solar light, and the original color restored by heat. It has also been shown that the effect is not produced by air or moisture.

Prominent among other interesting facts shown by these experiments is the varying effect of sunlight during each season, and each month of the year. The comparative actinic power of the rays during each month is shown, at the end of the year, by the comparative depth of yellow or purple color produced. The actinic effect increases from January to July, and decreases after the latter month; the greatest effect is observed in the summer, and the least in winter; in the spring and autumn it is about equal, being midway between that of winter and summer.

The comparative power of different kinds of glass to transmit the actinic rays was also tested. Of the colored glasses it was found that blue transmitted the most, purple less, and red and orange the least; which shows the propriety of the preference given by photographers to blue glass for skylights, because it transmits the blue rays, which exert the most active power. Mr. Gaffield's observations have not been confined to the glasses exposed on his own roof and window-sills; but he has been earnest in collecting, from various sources, specimens of window-glass that have been exposed for greatly-varying periods. By observing the portion of the pane exposed and that protected by the sash and putty, a comparison is afforded between the original and the acquired color of the glass. Among the gathered specimens was one of crown-glass, set in a church in Lexington, Mass., in 1794, from which the windows were removed in 1846, and afterward used as covers for hot-beds. The original color, ascertained by removing the putty from the edges, was a light green, and that produced by seventy-three years' exposure, a purple. Mr. Gaffield's efforts have also been directed toward examining the old cathedral-glass of Europe, where such observation is practicable. He still continues with great enthusiasm the experiments begun eleven years ago, and carefully records the results of his observations on a well-known phenomenon, "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 exact 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."