among artificers, a tenacious viscid matter, which serves as a cement to bind or connect things together.
Glues are of different kinds, according to the various uses they are designed for, as the common glue, glove glue, and parchment glue; whereof the two last are more properly called size.
Hamel du Monceau has written one of the best works on the subject of glue. According to this author, glue was at first principally prepared from the membranous, tendinous, and cartilaginous parts of animals, and after being dried, they were melted into tablets. It is certain, however, that every animal substance containing jelly, may be used in the manufacture of glue; and, according to Du Hamel himself, a strong, but black-coloured glue may be obtained from bones and hart-horn, after they are dissolved in Papin's digester. Of the truth of this fact Papin himself likewise assures us, for he prepared a jelly from bones, and even from ivory, by which he glued together some pieces of broken glass; and subsequent experiments made by other chemists have confirmed his assertion.
To the information contained on this subject in the works of Papin, Spielman has added many valuable remarks. He not only extracted glue from bones, but also from all the solid parts of animals, by boiling alone, as well as from the teeth of the sea horse, the wild boar, the wood-loufe, and the viper.
The glue manufactured in Europe is of different kinds; but that which is made in England is esteemed the best. Its colour is of a brownish red. The Flanders glue is considered as of an inferior quality to that made in England, while the glue manufactured in France is not so good as either. The reason assigned for this difference of quality is, that bones and fines are made use of by the Flemish and French in the manufacture of this article, while the English employ skins, which yield a much stronger glue. Dr Lewis informs us that the English steep and wash the cuttings of the hides in water, then boil them in fresh water till the liquor becomes of a proper consistence; after which they strain it through baskets, allow it to settle, then expose it to further evaporation, and pour it into flat moulds, where it unites. When thoroughly cooled, it is converted into solid cakes, which are cut into pieces, and dried on a kind of net.
Grenet for many years turned his attention to the manufacturing of glue. Having made a number of experiments on every substance formerly employed for this purpose, he found that bones afford the most abundant quantity of glue, and yield it with facility. Having deprived them of the fat they contain, he procured a jelly by simply boiling them, which, when dried, and thus changed into glue, he found superior to that which was prepared in France, and nearly equal to the best glue of commerce.
From the experiments of Parmentier, it appears that six pounds of button-makers rappings yielded a pound of excellent glue, not inferior to that which is manufactured in England. The glue which he obtained from the filings of ivory was equally as good, but more highly coloured. The filings of horn yielded none of this substance.
To obtain glue as colourless as possible, a very small quantity of water should be employed for extracting the jelly, by which means it may be concentrated without long evaporation, as exposure to heat has always a greater or less influence on the colour in proportion to the time. The whiteness and transparency of the Flanders glue are said to originate from an adherence to this plan.
In their consistence, colour, taste, smell, and solubility, glues are found to differ from each other. Some glues will dissolve by agitation in cold water, while others are only soluble at the point of ebullition. It is generally admitted that the best glue is transparent, of a brownish yellow colour, and having neither taste nor smell. It is perfectly soluble in water, forming a viscous fluid, which, when dry, preserves its tenacity and transparency in every part, and has more solidity, colour, and viscosity, in proportion to the age and strength of the animal from which it is produced.
For the following account of the manufacture of glue, we are indebted to Mr John Clemen of Newcastle. "The improvement (he observes) of any manufacture depends upon its easy access to men of science, and a prudential theorist can never be better employed than in attempting to reduce to regularity or to system the manufactures that may fall under his attention. In conformity to the first principle, I made some notes whilst visiting a glue manufacturer a few years ago in Southwark, and those, interwoven with the remarks on that subject of some chemists of the first respectability, I take the liberty of fending you: at the same time I must beg of you, or your correspondents, that where it may be corrected in any manner, it may be done, and I shall feel myself obliged by the attention.
"Glue is an inspissated jelly, made of the parings of hides or horns of any kind, the pelts obtained from furriers, and the hoofs and ears of horses, oxen, calves, sheep, &c. quantities of all which are imported in addition to the home supply, by many of the great manufacturers of this article: these are first digested in lime water, to cleanse them as far as it can from the greasle or dirt they may have contracted; they are then steeped in clean water, taking care to stir them well from time to time; afterwards they are laid in a heap, and the superabundant water pressed out; then they are boiled in a large brass caldron with clean water, skimming off the dirt as it rises, and further cleansed by putting in, after the whole is dissolved, a little melted alum or lime finely powdered, which, by their detergent properties, still further purge it: the skimming is continued for some time, when the mass is strained through baskets, and suffered to settle, that the remaining impurities, if any, may subside; it is then poured gently into the kettle again, and further evaporated by boiling a second time, and skimming, until it becomes of a clear but darkish brown colour: when it is thought to be strong enough (which is known either by the length of time a certain quantity of water and materials have boiled, or by its appearance during ebullition), it is poured into frames or moulds of about fix feet long, one broad, and two deep, where it hardens gradually as the heat decreases: out of these troughs or receivers it is cut, when cold, by a spade, into square pieces or cakes, and each of these placed within a fort of wooden box, open in three divisions to the back; in this the glue, as yet soft, is taken to a table by women, where they divide it into three pieces (A) with an instrument not unlike a bow, having a brass wire for its string; with this they stand behind the box and cut by its openings, from front to back: the pieces thus cut are taken out into the open air, and dried on a kind of coarse net work, fastened in moveable sheds of about four feet square, which are fastened in rows in the gluemaker's field (every one of which contains four or five rows of net work); when perfectly dry and hard, it is fit for sale.
"That is thought the best glue which swells considerably without melting, by three or four days immersion in cold water, and recovers its former dimensions and properties by drying. Glue that has got froth, or that looks thick and black, may be melted over again and refined, with a sufficient quantity added of fresh to overcome any injury it may have sustained; but it is generally put into the kettle after what is in it has been purged in the second boiling. To know good from bad glue, it is necessary for the purchaser to hold it between his eye and the light, and if it appears of a strong dark brown colour, and free from cloudy or black spots, the article is good."
A glue that is colourless and of superior quality, is obtained from the skins of eels, and known by the name of fisee. It is even procured from vellum, parchment, and some of the white species of leather; but for common purposes this is by far too expensive, and is only made use of in those cases of delicate workmanship where glue would be too gross. The skins of the rabbit, hare, and cat, are made use of in the manufacturing of fisee, by those who are employed in gilding gold, polishing, and painting, in various colours.
From the experiments of Mr Hatchett it appears, that membrane yields different quantities of gelatine, the solutions of which evaporated to dryness, afforded him an opportunity of observing the different degrees of viscosity and tenacity of mucilage, fisee, and glue. He also found that the more viscid glues are obtained with greater difficulty than such as are less so. When a cake of glue has been steeped three or four days in cold water, it is considered of the best quality, if it swell much without being dissolved, and if, when taken out, it recovers its original figure and hardness by drying.
On comparing the skins of different animals, Mr Hatchett found, that such as were most flexible more readily yielded their gelatine, and that produced from the skin of the rhinoceros was by far the most viscid of any. The true skin of any animal was most affected by long boiling; but the hide of the rhinoceros was the most insoluble.
He found that hair was not so much affected as skin; but the cartilages of the joints, when boiled long in water, were as perfectly soluble as the cutis, which is not the case with the other cartilages, as they afford little or no gelatine. The horns of the ox, ram, and goat, are very different from those of the stag; and the small quantity of gelatine they are found to contain, is produced more gradually, and with greater difficulty.
According to Hatchett, the effects of diluted nitric acid on the substances commonly employed in the manufacturing of glue, were exactly analogous to those of boiling water, and were always most powerful on those substances which contained the greatest quantity of gelatine. Almost all animal substances are convertible either into glue or soap, with this additional advantage, that those parts of them which would not be employed in making the one, are the most proper in the manufacture of the other.
Another fine species of glue, known by the name of ifinglefs, is the produce of certain fish, very common in the Russian seas, found on entering the rivers Wolga, Lyak, Don, and Danube. In Mucovy it is prepared of the flurgeon and the sturled, which yield the most beautiful ifinglefs. The fish from fresh water are esteemed the best, as they afford an ifinglefs more flexible and transparent than any other.
When the bladder is extracted, it is washed in water to free it from the blood, if any adheres to it, but not otherwise. It is then cut longitudinally, and the outer membrane taken off, the colour of which is brown, while the other membrane is so fine and white as to be
(A) When the women, by mistake, cut only two, that which is double the size is called a bishop, and thrown into the kettle again. with difficulty separated from the fish. They are formed into rolls of the size of the finger, with the fine membrane in the middle, and hung in the air to dry by degrees. Good ifinglaf is white, perfectly dry, semitransparent, and without smell. It is soluble in water with a gentle heat, but is easily dissolved in alcohol, in which it differs essentially from common glue. That which is made from different parts of sea wolves, sea cows, sharks, and whales, is employed in the clarifying of different wines and other liquors. Ifinglaf is of all shades of colour, from pure transparency to black; but such as are large and yellow are reckoned the worst. They are opaque, and their smell is disagreeable.
From 500 grains of ifinglaf Mr Hatchett obtained 56 grains of coal, from which 1/2 grain of earthy residuum were obtained by reducing it to ashes. Of consequence there were only 54.5 grains of pure coal, and the remaining 1.5 he found to be phosphate of soda, with an extremely small proportion of phosphate of lime.