Читать книгу A Manual of Mending and Repairing; With Diagrams онлайн

17 страница из 39

Neutral Substances, or Binding Materials.—Almost any substance not easily soluble in water, and many which are, from common dust or earth, or clay, sand, chalk, powdered egg-shells, sawdust, shell-powder, &c., when combined with certain adhesives, form cements. This is sometimes due to chemical combination, but more frequently to mechanical union. In the latter case the adhesive clinging to every separate grain has the more points of adhesion, just as a man by clinging with both hands to two posts is harder to remove than if he held by one.

Caseine or Cheese.—This in several forms, but chiefly of curd in combination with several substances, but mostly with lime or borax, forms a very valuable cement. It is also combined with strong lye and silicate of soda. It must not, however, be too much depended on as a resistant to water or heat.

Blood, generally of oxen or cows, combined with lime, alum, and coal ashes, forms a solid and durable cement.

Glycerine forms the basis, with plumbago, &c., of several cements. Like oil, it renders glue flexible and partly waterproof. For chemical details on this subject, vide Das Glycerin, by J. W. Koppe, Leipzig.