The principal points of importance for our purpose are the different degrees of hardness and brittleness and the melting-points of the various resins. While some possess but slight hardness, for instance elemi, others, such as copal and amber, excel in this respect and their brittleness and high melting-point.
To decrease the brittleness of resins, essential oils are sometimes added, or resinous cements are mixed with oil cements or a fat drying oil, or compounded with rubber cement.
Resinous cements are either softened by heating or entirely melted, or solutions of resins in volatile solvents are used, which, in evaporating, leave the resin behind.
The resinous cements possess great power of resistance, and are therefore well adapted for tightening water and gas pipes, but they have the disadvantage of not standing a high temperature and possessing a certain degree of brittleness which renders them unfit for the cementing of articles exposed to frequent shocks.
Many of these cements, especially those prepared with pitch or asphaltum, can be produced at a very low cost, and do excellent service for water-proofing vessels, water-reservoirs, brickwork, etc.
Rubber and gutta-percha cements. Caoutchouc, commonly called India rubber, or briefly rubber, is derived from the milky juices of certain tropical plants. It is distinguished by great elasticity and indifference to chemical agents.
Both these properties make it a valuable material for cement, and it is much used for this purpose either in the form of solution or as a constituent of other compositions. For cements which are to have a certain degree of elasticity combined with indifference toward chemical agents, it is absolutely indispensable, as no other known body possesses these properties in such a high degree.
The derivation of gutta percha is similar to that of rubber. At an ordinary temperature it forms solid and very tenacious masses, of a leather-like consistency, but at a somewhat higher temperature (below the boiling-point of water) it is converted into a very plastic, soft mass, which can be drawn into very fine threads, and rolled to very thin plates.
By itself or mixed with other substances it furnishes an excellent cement, possessing the valuable properties of tenacity and pliancy when exposed to shocks. As regards resistance to the action of water and chemical agents it is almost equal to rubber, and, for certain purposes, is frequently preferred to the latter.
Glue and starch cements. By itself, i. e., converted by boiling with water into a viscous mass which solidifies on cooling, glue cannot be classed with the cements; the same applies to paste, i. e., starch or flour swelled and boiled in water.