Take pulverized glass, 10 parts; powdered fluorspar, 20 parts; soluble silicate of soda, 60 parts. Both glass and fluorspar must be in the finest possible condition, which is best done by shaking each, in fine powder, with water, allowing the coarser particles to deposit, and then to pour off the remainder which holds the finest particles in suspension. The mixture must be made very rapidly, by quick stirring, and when thoroughly mixed must be at once applied. This is said to yield an excellent cement.
Glue is undoubtedly the most important cement used in the arts. Good glue is hard, clear (not necessarily light-colored, however,) and free from bad taste and smell. Glue which is easily dissolved in cold water is not strong. Good glue merely swells in cold water and must be heated to the boiling point before it will dissolve thoroughly.
Good glue requires more water than poor, consequently you cannot dissolve six pounds of good glue in the same quantity of water you can six pounds of poor. The best glue, which is clear and red, will require from one-half to more than double the water that is required with poor glue, and the quality of which can be discovered by breaking a piece. If good, it will break hard and tough, and when broker will be irregular on the broken edge. If poor, it will break comparatively easy, leaving a smooth, straight edge.
In dissolving glue, it is best to weigh the glue, and weigh or measure the water. If not done there is a liability of getting more glue than the water can properly dissolve. It is a good plan, when once the quantity of water that any sample of glue will take up lias been ascertained, to put the glue and water together at least six hours before heat is applied, and if it is not soft enough then, let it remain longer in soak, for there is no danger of good glue remaining in pure water, even for forty-eight hours.
From careful experiments with dry glue immersed for twenty-four hours in water at 60° Fah., and thereby transformed into a jelly, it was found that the finest ordinary glue, or that made from white bones, absorbs twelve times its weight of water in twenty-four hours; from dark bones, the glue absorbs nine times its weight of water; while the ordinary glue made from animal refuse, absorbs but three to five times its weight of water.
Glue, being an animal substance, it must be kept sweet; to do this it is necessary to keep it cool after it is once dissolved, and not in use. In all cases keep the glue-kettle clean and sweet, by cleansing it often.
Great care must be taken not to burn it, and, therefore, it should always be prepared in a water bath.
Carpenters should remember that fresh glue dries more readily than that which has been once or twice melted.
The advantage of frozen glue is that it can be made up at once, on account of its being so porous. Frozen glue of same grade is as strong as if dried.