In reply to the question, What means there are of bleaching ivory which has become discolored? Holtzapffel, the great authority on such subjects, tells us that he regrets to be obliged to say that he is unacquainted with any of value. It is recommended in various popular works to scrub the ivory with Trent sand and water, and similar gritty materials; but these would only produce a sensible effect by the removal of the external surface of the material, which would be fatal to objects delicately carved by hand or with revolving cutting instruments applied to the lathe.
It is a well known fact that ivory suffers the least change of color when it is exposed to the light and closely covered with a glass shade. It assumes its most nearly white condition when the oil with which it is naturally combined is recently evaporated; and it is the custom in some thin works, such as the keys of pianofortes, to hasten this period, by placing them for a few hours in an oven heated in a very moderate degree, although the more immediate object is to cause the pieces to shrink before they are glued upon the wooden bodies of the keys. Some persons boil the transparent ivory in pearl-ash and water to whiten it; this appears to act by the superficial extraction of the oily matter as in bone, although it is very much better not to resort to the practice, which is principally employed to render that ivory which is partly opaque and partly transparent, of more nearly uniform appearance. It is more than probable, however, that the discoloration of ivory is due to the oil which it contains or has absorbed, and which becomes yellow and rancid, and every effort should be made to prevent oily or greasy bodies from coming in contact with ivory. Thus the keys of a pianoforte should be kept clean by carefully washing from the fingers the natural grease which all skin gives out. When ivory keys become very yellow they may be considerably whitened by allowing a paste of whiting, slightly moistened with potash, to lie on them for twenty-four hours. The potash extracts the oil which is absorbed by the chalk and may be thus removed.
It is a well known fact that most oils and resins may be bleached by exposure to sunlight. It is by this means that opticians render Canada balsam clear and transparent. It has been found that pieces of apparatus made of ivory, such as rules, etc., which have become yellow by age, may be bleached by dipping them in turpentine and exposing them to sunlight.
The fumes of sulphur, chloride of lime, etc., though fre-spjently recommended, are of no value as bleachers of ivory.