Ivory is obtained from the tusk of the elephant, and although material nearly resembling it may be obtained from other animals, yet the true ivory stands unequalled as a material for ornamental turning and carving. It is not so brittle as bone, neither does it splinter so much when broken, and as it is entirely free from the vessels or pores which permeate all bone, the finished articles have a much more solid and even appearance. Although distinctly fibrous it cannot be torn up in filaments like bone or divided into thin leaves, except by the saw. It is in all respects the most suitable material for ornamental turning, as it is capable of receiving the most delicate lines and of being cut in the most slender proportions. But while it is thus valuable as a material for ornamental work, it is useless for any article requiring accuracy in its dimensions - such for example as the scales of draughtsmen and the graduated arcs of instruments for niea-suring angles. Owing to the great alterations which it sustains under slight atmospheric changes it cannot be relied upon, and has been condemned officially by the survey commissioners of almost all countries.
It is imagined by some that ivory may be softened so as to admit of being moulded like horn or tortoise shell. Its different analysis contradicts this expectation; thick pieces suffer no change in boiling water, thin pieces become a little more flexible, and thin shavings give off their jelly, which substance is occasionally prepared from them. It is true that the caustic alkali will act upon ivory as well as upon most animal substances, yet it only does so by decomposing it. Ivory, when exposed to the alkalies, first becomes unctuoias or saponaceous on its outer surface, then soft, if in thin plates, and it may be ultimately dissolved provided the alkali be concentrated; but it does not in any such case resume its first condition.
As a material to be worked by the mechanic, ivory stands midway between wood and brass, and is turned and cut by tools having more obtuse angles than those employed for wood, and yet sharper than those used for brass. It may be driven at a fair speed in the lathe, and is easily sawed by any saw having fine teeth.
The tools used for cutting and turning ivory should have their edges very finely finished on an oil stone so that they may cut smoothly and cleanly.
Turned works with plain surfaces may in general be left so smooth from the tool as to require but very little polishing, a point always aimed at with superior workmen by the employment of sharp tools. In the polishing of turned works very-fine glass paper or emery paper is first used, and it is rendered still finer and smoother by rubbing two pieces together face to face; secondly, whiting and water as thick as cream is then applied on wash leather, linen, or cotton rag, which should be thin that the fingers may the more readily feel and avoid the keen fillets and edges of the ivory work, that would be rounded by excessive polishing; thirdly, the work is washed with clean water, applied by the same or another rag; fourthly, it is rubbed with a clean, dry cloth until all the moisture is absorbed, and, lastly, a very minute quantity of oil or tallow is put on the rag to give a gloss.
Scarcely any of the oil remains behind, and the apprehension of its being absorbed by the ivory and disposing it to turn yellow may be discarded; indeed the quantity of oil used is quite insignificant, and its main purpose is to keep the surface of the ivory slightly lubricated, so that the rag may not hang to it and wear it into rings or groovy marks. Putty powder is sometimes used for polishing ivory work, but it is more expensive and scarcely better suited than whiting, which is sufficiently hard for the purpose.
The polishing of irregular surfaces is generally done with a moderately hard nail brush, supplied with whiting and water, and lightly applied in all directions, to penetrate every interstice; after a period the work is brushed with plain water and a clean brush, to remove every vestige of the whiting. The ivory is dried by wiping and pressing it with a clean Linen or cotton rag, and is afterwards allowed to dry in the air, or at a good distance from the fire; when dry a gloss is given with a clean brush on which a minute drop of oil is first applied.
It is better to do too little polishing at first, so as to need a repetition of the process rather than by injudicious activity to round and obliterate all the delicate points and edges of the works, upon the preservation of which their beauty mainly depends.