# Popular Science Monthly/Volume 5/September 1874/Correspondence

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CURIOUS ACTION OF MUCILAGE ON GLASS.
THE article on "The Action of Sunlight on Glass," published in The Popular Science Monthly for May, has elicited from Dr. F. Hollick, of this city, inquiries concerning the large plate-glass window of 104 Broadway, which is very much disfigured. Dr. Hollick says: "A 'Notice' was written on a piece of common brown paper, and pasted on the inside of the window with ordinary mucilage. On removing this 'Notice,' it was observed that the mucilage did not come off clean. Water, alcohol, and various other solvents, were employed, but all to no purpose, the glass remaining dim wherever the mucilage had been applied; the fact was, as was evident on inspection, that the surface of the glass was corroded, as if it had been acted upon by fluoric acid; and, what is more singular still, this corrosion has been extending ever since, till it now covers a large space. There seems to be a process of disintegration, or molecular change going on, which bids fair to destroy, in time, the whole pane. Now, the question comes, What is the nature of this change, and how was it started? The paper was of the ordinary brown wrapping kind, and the mucilage such as is in common use, and which has no action upon the bottle which contains it."
In reply to this inquiry, Mr. Thomas Gaffield, of Boston, writes:
To the Editor of the Popular Science Monthly.
Sir: A few days ago I received from a gentleman in New York some pieces of a large pane of plate-glass, taken from a window on Broadway, upon which had been affixed, with common mucilage, a notice written upon brown paper. When this notice was removed in about forty-eight hours, it was found that the portion of the surface covered by the mucilage was roughened, and presented an appearance of little hollows, or pits, from which the glass had been actually torn away by the washing or tearing off of the brown paper.
My informant says that some workmen passing by noticed the injured glass, and went in to examine it, stating that they had removed two other panes for the same defect. At the same time, experiments on other glass with similar paper and mucilage had led to no similar results, showing that all glasses are not so affected. My correspondent is surprised at what he has noticed, and desires an explanation of the cause. I will give my humble suggestions on the matter, and let others with more scientific knowledge enjoy the same privilege.
Let me at first, however, give a brief account of a few similar but very rare occurrences; for, though my New York friend names a fact not often noticed in the books, yet it is another illustration of the old saying, that "there is nothing new under the sun."
While spending some time, in 1862, in looking over the "Transactions" of the English and French scientific associations for one or two past centuries, I found the following very interesting item in the "Histoire de l'Académie," for 1708, page 22:
"EXFOLIATED GLASS.
"A person having applied to a piece of glass, about six inches square, a paste of Spanish white and glue size, placed the whole in the sun, during the great heat of summer. The paste, which was turned toward the sun, having been heated, rolled itself up, so that, in its movement, its under side was raised upward. But, what was more singular, this surface raised with it, and carried away, a layer of the glass. This layer made on the paste a species of varnish, as of porcelain, the thickness not exceeding one-half a line. It was astonishing that the adherence of the paste to the glass was so strong; and equally so, that it should be able to detach from the glass so considerable a sheet. It had been blown, and apparently they had replunged the pipe, with which it was blown, in the crucible at different times, which had given it several layers, which, however, were not apparent, because they were exactly ​applied one upon another. It is to Geoffrey that we owe this observation."
This was the only item of the kind which I ever found in the old books, and I had my doubts of its accuracy, until I read, in the London Photographic News of July 17, 1868, the following article:
"CURIOUS EFFECT OF GELATINE UPON GLASS.
"A correspondent sends us the following account of a curious result:
"Having, for experimental purposes, poured a thick solution of gelatine upon a number of glass plates, three of them were set aside upon a shelf for some months; and one day, upon looking at them, I found that, in all three cases, the gelatine had separated from the glass, bringing away the whole surface of the glass plates in shivers, which firmly adhered to the gelatine. The surface of the glass was left full of ruts, like water-worn stones. I suppose it to be caused by the strong contraction of the gelatine, and its firm hold upon the glass."
I wrote a short notice of these two similar facts for the Philadelphia Photographer, of November, 1868.
Singularly enough, just after this date, while experimenting in making my "photographic self-prints from Nature" (an account of which I have sent you in my little pamphlet), I noticed a similar phenomenon.
You will recollect that I place leaves and ferns upon glass with mucilage, and print their forms upon sensitive paper by exposure to the sunlight. After the ferns are dried up, I clean the glass for further use. In washing one of these glasses, it was impossible to make the surface perfectly clean. On a close examination, I found that, in removing the ferns and mucilage, the latter had taken off a portion of the glass, so that I could distinctly observe, on the crowded surface, the outlines of an anchor (which was the figure produced), and the forms of some of the individual ferns. I have this curious specimen—not of plate, but of sheet glass—in my cabinet, and will show it to you or any of your correspondents who may call on me.
There are numerous very interesting thoughts and queries suggested by the various and yet similar incidents referred to above. In making sheet window-glass, the workman makes three, and, for very thick glass, four gatherings upon his blowpipe, creating, as suggested, three or four layers in the finished pane of glass, although not visible to the naked eye. Some workmen reheat the glass after the last gathering, in order, by what is called "burning over," to make the heated ball more uniform and homogeneous. The glass is then more easily and perfectly annealed, and more easily and safely cut by the diamond. The sheet-glass, named in the curious incidents related above, was probably of a kind not "burnt over" and perfectly homogeneous, and, for this reason, more easily disintegrated by the strong adhesive and contracting power of the gelatine and mucilage, overcoming the cohesion of the atoms and layers of the glass.
While crown and sheet glass have an original fine surface, that of plate-glass is softer and more easily affected, because it is an artificial one, which has been subjected to the three successive operations of the grinding, smoothing, and polishing machines.
The above explanation supposes mechanical action only. But, it is possible, a chemical action took place also, especially in the plate-glass, in the formation of some acid, by the fermentation of the gelatine or mucilage, when under the influence of sunlight or the atmosphere. The glasses all contained alkali, in the form of soda or potash, and perhaps some uncombined alkali, which might have formed a chemical combination with the acid of the mucilage, and so corroded or disintegrated the surface of the glass. The effect observed was, undoubtedly, the result of both mechanical and chemical action.
I found, on inquiry of several dealers in chemicals, that mucilage frequently contains acetic acid or alum, to prevent the formation of mould. In such cases, the acetic acid might easily form a chemical attachment, under the warming influence of the sun's rays, for some of the constituents of the glass, creating acetate of soda, of potash, or of iron.
Alum (which is a compound of sulphate of alumina and sulphate of potash), under the same influence, might be subject. ​to the chemical interchange called isomorphism, giving up its potash for some of the soda in the glass. These new compounds, if soluble in water, would be removed in washing off the mucilage, or, if insoluble, would be torn away when pulling off the paper attached by the adhesive gum.
In view of these facts, it would be well not to run the risk of possible injury to any of our valuable windows by affixing notices upon them with mucilage.
My correspondent thinks he noticed a continued or increasing corrosion, or disintegration, of the surface of the plate-glass after the removal of the mucilage. On examination of one of the pieces sent to me, I found a discoloration, occasioned by what we call "rust," in a part not affected by the mucilage, and this was probably what our friend observed.
There are glasses sometimes made with an excess of uncombined alkali which effloresces upon the surface, sometimes before it is placed in the window, and sometimes at a later period. This, of course, causes a series of infinitesimal holes or furrows in the glass, whose surface by the reflection and refraction of the rays of light presents, like the mother-of-pearl, all the colors of the rainbow when held in a certain position, and at the right angle for such effect. This is one kind of what is called stained or rusty glass. In other specimens the disintegration goes beyond the infinitesimal character and is plainly visible, making the glass appear as if fire-cracked, and in its ultimate effect producing the appearance, on one or both surfaces, of ground glass.
In one of these stages of rust or disintegration I find a piece of the glass referred to by my correspondent.
In reference to the matter of rust on glass, for the comfort of my readers and of my New York friends who are dealers in window-glass (which was my business for many years), let me say that this difficulty or defect is not an every-day trouble, but only one of the incidents or curiosities of glass-making. A good glass-maker knows how to avoid it, and a good glass-dealer can, in many instances, remove the first appearance of rust or stain on plate or sheet glass by a skillful and nimble use of dilute hydrofluoric acid and rouge or oxide of iron.
Glass-making and glass-dealing have their troubles, and this of occasional rust on glass is one. But all other kinds of business have their trials, and they all have their points of interest and satisfaction. In the study necessary to remove troubles and avoid mistakes, comes half the pleasure of life. If we had no obstacle in the path of our business, we ourselves would rust out.
In closing my article, let me add that I give my opinions only as humble suggestions, after such examination as I have been able to give to the subjects referred to.
In the light of new experiments, and the progress of scientific research by those who make it the business of life, the theories of to-day may be set aside by the revelations of to-morrow. If "an undevout astronomer is mad," so must be an undevout chemist, or student in any domain of natural science, who will not humbly bow before the wise and wonderful workings of the Great Maker of the universe. We cannot fully understand or explain them now. We can only look "through a glass darkly." We can see and enjoy the light of the sun, but, though great and leaned volumes be written, who can fully explain all the laws and all the wonders of light, one of the gentlest and most ethereal, and yet one of the most interesting and powerful elements of the universe? We must be content to labor and to wait.