(1) In chemical laboratories it is customary to put a coating of clay on glass vessels that are to be exposed to a temperature that would soften or melt the glass, or where they are liable to be broken by draughts of air. Sometimes cows' hair or asbestos is mixed with the clay to strengthen it. Although this mass is cheap, it is liable to fine checks and cracks, or it scales off, which frequently causes the glass to break. The disadvantages referred to can be entirely overcome by mixing up the materials with a little glycerine. This cheaply and easily prepared mass is thus rendered very easy to apply, always retains its desired softness, and never cracks nor checks. (Gewerbe - blatt.).
Sulphuric ether . •
(2) I have recently been using, with better results, a mixture of infusorial earth and water - glass, which, if properly applied, will last for weeks, and hence is not expensive, while it protects and strengthens the vessel to such an extent that I have, for the sake of experiment, heated thick but cracked retorts that were protected in this way to 400° or 500° C. (932° F.) when exhausted almost to a vacuum, and yet they did not break or collapse. It is important to make this mixture so that it shall form a soft and somewhat elastic, but not liquid, paste. A mixture of 1 part by weight of infusorial earth with 4 or 4 1/2 of water - glass will fulfil this end approximately; the exact proportions cannot be given, because commercial water - glass differs in strength, and the infusorial silica is not always dry. The part of the vessel to be protected is covered 1/5 to 2/5 in. thick, and dried at not too high a temperature; it is better to dry in a drying closet or on a support over the stove. If the temperature is too high at first, it will cause air bubbles in the mass, and it is not so good then. It can be dried by swinging it back and forth over a flame, the bubbles being prevented by pressing them out.
If a crack appears, it is plastered over with more of the mixture, and allowed to dry again. If some parts of the vessel are to remain transparent, they may be protected by water - glass alone, by applying several thin coats, and letting each dry before putting on the next. The same mass can be used to cover gas retorts, furnaces, stoves, and walls, just as well as for glass and porcelain utensils. (Schaal.)