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Gypsum is combined with many substances to form cements, some of them of great and peculiar value.

Iron pulverised is the basis of a great number of very durable and strongly resistant cements.

Alum may be included among the bases, as it is very important in several compositions, forming a powerful chemical aid. It is excellent as aiding resistance to both moisture and heat. For an exhaustive work on alum consult Die Fabrikation des Alauns, &c., by Frederic Junemann, which should be carefully studied by all who work in cements.

There is a very great number of “indifferent” or minor aids to these, such as sugar, milk, honey, spirits of wine, water, ochre, galbanum, tannin, ammonia, feldspar, plumbago, sulphur, vinegar, salt, zinc (white), umber, bismuth, tin, cadmium, clay, ashes, &c., which are essential in certain combinations.

Dextrine, the gum of flour or starch, or Leiokom, much resembles gum-arabic, but is more brittle. Its adhesiveness depends somewhat on the manner in which it is dissolved. “It is,” says Lehner, “prepared by heating starch which has been moistened with nitric acid; also by warming paste with very much diluted sulphuric acid.”