For the manufacture of cements the use of pure odorless glycerine is not required, the yellow crude article, which is much cheaper, answering all purposes. The principal point is to use very highly concentrated glycerine, as otherwise the cements prepared with it solidify very slowly and besides do not possess a proper degree of hardness and solidity.
It is of especial importance to have the lead oxide free from water. To accomplish this, heat it thoroughly and mix it with the glycerine while still hot. Cement thus prepared solidifies very quickly, and can be used for many purposes. It is an excellent material for quickly joining the stones of submarine works.
Glycerine and litharge cement. Moisten elutriated litharge with glycerine so that a thin homogeneous paste is formed. This cement is adapted for uniting the joints of steam pipes, cementing wood, glass, porcelain, and also glass upon metal, etc. It solidifies to a very hard mass in a quarter to three-quarters of an hour. Before applying the cement coat the surfaces to be joined with pure glycerine.
LIME CEMENTS.
Quick lime, slaked lime and chalk are used for this purpose. Quick lime, which is obtained by burning limestone, combines gradually with the fats to insoluble lime soaps. Slaked lime, which consists of a combination of lime with water, acts in the same manner.
For the preparation of cements the lime is slaked by placing it in a dish and pouring as much water over it as it will absorb. Good lime, technically called fat lime, should eagerly combine with water, evolving much heat, swelling greatly, and crumbling to a light white powder.
Quick lime exposed to the air until, by the absorption of moisture and carbonic acid, it is converted into a powder is called air-slaked.
Cements prepared with quick lime will, as a rule, solidify more quickly than those prepared with air-slaked lime.
Chalk is a carbonate of lime consisting of the shells of microscopic animals, and can be readily pulverized and elutriated. In the latter state it is known as whiting. For the preparation of entirely white cements the use of pure white lime or chalk is absolutely necessary. Yellow or reddish lime contains oxide of iron, and furnishes cements of the same tinge.
Cement for glass. Litharge 30 parts, quick lime 20, linseed-oil varnish 5.