QUALITIES OF IVORY 231 



The resilience of ivory, its rebound when faUing against or 

 being impelled against a resisting object that suddenly checks 

 its course, is of an entirely different character from that of 

 rubber, for instance. It has not the long rebound of this latter 

 and so much lighter material, but its successive contacts and 

 rebounds are quite short and frequently repeated until it 

 finally settles to rest. The module of elasticity responds im- 

 mediately to impact, and this it is that makes ivory the very 

 best material for billiard balls, as the cue has no sooner 

 touched the ball than the latter springs away from the impact. 



The tendency of ivory to shrink under hygrometrical 

 influence induced the Tithe Commissioners on the Parlia- 

 mentary survey to reject the employment of ivory scales. 

 This material requires careful seasoning just as wood does, 

 and any sudden exposure to hot dry air should be guarded 

 against. The larger the pieces the greater the danger of 

 such shrinkage. As a general rule ivory ornaments should 

 not be placed on a mantelpiece beneath which there is a hot 

 fire.* 



Because of its fine and close-grained texture, its homo- 

 geneousness and adhesive hardness, its lack of brittleness, its 

 excellent wearing qualities and its absence of any tendency 

 to chip easily, ivory is a most excellent substance to en- 

 grave; its smooth, even grain and the coherence of its par- 

 ticles lend a fine surface for the engraver's art, and the 

 material lasts a long time. Its structure has the concentric 

 lines that resemble the growth lines of a tree. 



In China ivory has been utilized in various ways, the 

 dust being made up into " ivory jelly." It has also been 

 used for case hardening; ivory black as a colouring material 

 is another by-product of ivory working. However, in re- 

 cent times leather has largely replaced it both for case harden- 

 ing and for the production of ivory black. 



*Holtzapffel, op. cit.. Vol. I, p. 152 and note p. 47. 



