M. Brongniart has lately made many experiments on the subject of staining glass. The colours, however, are the same as we noticed. A green glass may be made by putting on one side of the glass a blue, and on the other a yellow. A black glass may be made by a mixture of blue with the oxides of manganese and iron. Painting on glass is an ancient art. When pieces of old painted glass are examined, they have always on one side a transparent red varnish burnt into them. The moderns, however, excel in this art.
Glass is not acted upon by the acids, except the fluoric or hydrofluoric. Hence the acid of Derbyshire spar, which is a fluate of lime, is used for etching on glass, in the same manner as nitric acid is, on copper. Fluoric acid, a compound of fluorine and hydrogen, is decomposed during this action, and is changed, by the union of its fluorine with silicon, into the silicated fluoric acid.
When a quantity of alkali is used just sufficient to fuse silica, glass is the result; but when the quantity is greater, as three or four to one, the fused mass is soluble in water, and then forms the silicated alkali, or liquor of flints. From this the silica is obtained in a pure state, by the addition of an acid.
Glass, when melted and dropped into water, assumes an oval form, with a slender projection, called a tail. This is called Prince Rupert's drop. If a small part of this tail be broken off, the whole bursts into powder, with a kind of explosion. The Bologna, or philosophical phial, is a small cylindrical vessel of glass, rounded at the bottom, but open at the upper end. It is made thick at the bottom, so as not to be easily broken; but if a pebble be dropped into it, it immediately cracks, and the whole falls into pieces. In both these, (the drop and the bottle,) the glass is unannealed. When the external part of glass is suddenly cooled, the inner part is kept, as it were, contracted. Now annealing, the process of tempering glass in an oven, renders the glass uniformly alike, and capable of sustaining the variations of temperature, without breaking. By a crack or fissure, the internal parts which remained in a state of tension, endeavour to recover the full state of expansion, and consequently the glass is rent asunder.
Sec. XLVIII. Glue and Isinglass.
Both glue and isinglass are animal products. They are used in fire-works, but always in the state of solution, as vehicles to mix up compositions in order to make them unite, and to preserve them from falling to powder. The quantity, however, is never large, or either would destroy the effect. The proportions are generally prescribed. A solution of glue is employed in the old process for refining saltpetre. See [Nitre.] In making priming paste, isinglass dissolved in brandy is sometimes used.
Glue and isinglass owe their adhesive quality to the presence of gelatin; the most remarkable property of which is, that it unites with, and precipitates the tanning principle from its solution in water. For this reason, the use of oak bark and other astringent substances, in the tanning of leather, is obvious, the gelatin of the hide or skin, uniting with the tannin and forming tanned leather. Gelatin exists in bones, muscles, tendons, ligaments, membranes and skins. Skins, especially those of old animals, furnish the best and strongest glue.
For the preparation of glue, the parings and offals of hides, pelts, and the hoofs and ears of horses, oxen, calves, sheep, &c. are first digested in lime-water to clean them; then steeped in fresh water, which is suffered to run off; and being previously inclosed in a strong linen bag, are boiled in a copper cauldron with pure water. The impurities are removed as they rise. To the solution, alum, or finely powdered lime, is added. It is then strained through baskets and allowed to settle; after which, the clear fluid is again boiled. When it becomes thick, or of a proper consistence, it is poured into moulds or frames, when it concretes into jelly. It is cut into pieces by a spade, and then into thin slices by means of wire, and finally dried on coarse net-work.
The goodness of glue is known by its brittleness, and equal degree of transparency, without black spots. It swells up in cold water, and becomes gelatinous, but does not dissolve. It is a mark of want of strength, when glue dissolves in cold water.
Size is also a gelatinous substance, and is colourless and transparent. Eel skins, vellum, parchment, &c. are used in its preparation. They are treated in the same manner as hides. Isinglass, or fish glue, is a finer kind of gelatin, obtained from the air bladder and sounds of different kinds of fish of the accipenser genus; as the sturio stellatus, huso ruthenses, &c. The bladder, when taken from the fish, is washed and stripped of its exterior membrane, and then cut lengthwise and formed into rolls, or cut into strips. Isinglass dissolves in water with more difficulty than glue. A coarser kind of fish glue is made from sea wolves, porpoises, sharks, cuttle fish, the sturgeon, &c. The head, tail, fins, &c. are boiled in water, and the solution evaporated. Isinglass is used for a variety of purposes, as the making of court plaster and size, the clarification of liquors, &c.