A colorless cement for joining sheets of mica is prepared as follows:—Clear gelatine softened by soaking it in a little cold water, and the excess of water pressed out by gently squeezing it in a cloth. It is then heated over a water bath until it begins to melt, and just enough hot proof spirit (not in excess) stirred in to make it fluid. To each pint of this solution is gradually added, while stirring, one-fourth ounce of sal-ammoniac and one and one-third ounces of gum mastic, previously dissolved in four ounces of rectified spirits. It must be warmed to liquefy it for use, and kept in stoppered bottles when not required. This cement, when properly prepared, resists cold water.
A Cement That Will Resist the Damp.
A cement that will resist the damp, but will not adhere if the surface is greasy, is made by boiling together 2 parts shellac, 1 part borax, and 16 parts water.
To Make Glue Waterproof.
The best substance is bichromate of potash. Add about one part of it, first dissolved in water, to every thirty or forty parts of glue; but you must keep the mixture in the dark, as light makes it insoluble. When you have glued your substances together, expose the joint to the light, and every part of the glue thus exposed will become insoluble, and therefore waterproof. If the substances glued together are translucent like paper, all will become waterproof; if opaque like wood, only the exposed edges will become so, but they also protect the interior—not exposed parts—against the penetration of moisture.
Two Glue Recipes.
A glue ready for use is made by adding to any quantity of glue, common whisky, instead of water. Put both together in a bottle, cork it tight and set it for three or four days, when it will be fit for use without the application of heat. Glue thus prepared will keep for years, and is at all times fit for use, except in very cold weather, when it should be set in warm water before using. To obviate the difficulty of the stopper getting tight by the glue drying in the mouth of the vessel, use a tin vessel with the cover fitting tight on the outside to prevent the escape of the spirit by evaporization. A strong solution of isinglass made in the same manner is an excellent cement for leather.
A valuable glue is made by an admixture with common glue of one part of acid chromate of lime in solution to five parts of gelatine. The glue made in this manner, after exposure, is insoluble in water, and can be used for mending glass objects likely to be exposed to hot water. It can also be made available for waterproofing articles such as sails or awnings, but for flexible fabrics it is not suitable. A few immersions will be found sufficient to render the article impervious to wet. It is necessary that fractured articles should be exposed to the light after being mended, and then warm water will have no effect on them, the chromate of lime being better than the more generally used bichromate of potash.
Postage Stamp Mucilage.
Postage stamp mucilage can be made by dissolving an ounce of dextrine in five ounces of hot water, and adding one ounce of acetic acid and one ounce of alcohol. The dextrine should be dissolved in water in a glue pot, or some similar vessel, which will prevent burning. The quantities in this recipe may be varied by taking any required weights in the proportions mentioned. Dr. Phin says that dextrine mixed with water makes a good label mucilage if a drop or two of glycerine be added to it. Too much glycerine will prevent the mucilage drying; with too little it will be likely to crack.