For an exhaustive work on the subject of not only mending stone-work, but also of making artificial stone and many cements, as well as combining and adapting to use paper, cellulose, sawdust and shavings, gypsum, chalk, glue, &c., including not only ancient but also the most recent recipes, consult Die Fabrikation künstlicher plastischer Massen, by Johannes Hofer; Leipzig, A. Hartleben, price 4s.
REPAIRING IVORY
Works of art in carved ivory or bone are very valuable when perfect, yet when broken or defective they may very often be purchased for a trifle. Yet the process of mending them or restoring the missing portions is not difficult.
The first thing to consider is the colour. When old ivory has only acquired a delicate hue, as of Naples yellow, this adds to its attractiveness; nor are the brownish shadows and marks which gather in the angles of the reliefs repulsive. These may be left untouched, and even imitated. But a great deal of old ivory becomes of blackish bistre, or of a dirty, spotted brown or neutral tint, which has nothing in common with artistic effect, and suggests, like old slums in cities, more that is repulsive than picturesque. To clean such pieces, dissolve rock-alum in rain-water till it is white or forms a full saturation. Boil this, and keep the ivory in the boiling solution for about an hour, taking it out from time to time and cleaning it with a soft brush. Then let it dry in a damp linen or muslin rag; it will then be cleaned.
Ivory is often bleached by the simple process of damping, or wiping it with water and then exposing it to the rays of the sun; which must, however, be frequently repeated. According to Lehner, the only perfect and certain process by which any ivory can be cleaned is to steep the article for some time in ether or benzole, in order to extract any fatty matter, then to wash it in water, and finally keep it in super-oxide of hydrogen (Wasserstoff, super-oxide) till it is bleached, after which wash again in water.
To supply Missing Portions.—Take ivory-dust, such as can be bought of every ivory-turner, sift it to an impalpable powder, or else levigate or grind it down under water as fine as flour in a mortar. Then combine this with gum arabic, in alum solution, or the silicate of potash. Egg-shells, levigated, may be substituted for the ivory-dust, and are even less likely to turn grey; and very fine white glue or gelatine of the clearest kind may be substituted for the gum-arabic.
Louis Edgar Andés, in his able work on Ivory, Horn, Mother-of-Pearl, and Tortoise-shell, explains a process much like that already described. According to him, take finely powdered bone (or ivory-dust), combine it with white of eggs, and the result will be an intensely hard substance, which can be turned or carved like ivory. To perfect this the mass should be subjected to a heat of from 50° to 60° centigrade, and then to strong pressure. Gelatine or best glue, with glycerine, is quite as good as the white of eggs, and it may to advantage be combined with the latter. Having very thoroughly mixed the composition, take the broken ivory article, repair the missing portions, and fill the cavities with the paste. Though not equal to celluloid as an imitation of new and fresh ivory, this cement is very much like old bone and ivory, and after a little experimenting the artistic amateur may succeed in so blending the binder or adhesive with the dust as to take casts which are almost perfect imitations of the originals. But let it be observed in this, as in everything, one must not expect perfect success at a first trial, as too many do.
When the paste is dry, smooth the surface with a sharp cutter, so as to remove any small projections, and then polish it, first with fine emery or tripoli, then with a burnisher, finally by hand.
If you have, for example, an old flat plate of ivory, like one of the fourteenth century now before me, which I bought for a mere trifle because it was broken, lay it in an exactly fitting box—a strip of tin in a square will answer—and fill in the vacancy. The missing ornament on the upper side can be carved, or even supplied from a hardened stamp or mould of rolled soft bread-crumb. This bread-crumb can be made very hard by admixture with a very little nitric acid and water. Imitation meerschaum pipes, which are rather like ivory or bone, are made from this composition by pressure.
I may here mention that this ivory or bone cement, which is little known, is admirably adapted to repair broken inlaying. There was in Florence, in the sixteenth century, an extensive manufacture of delicate bas-reliefs for small caskets from lime and rice, which greatly resembled bone or ivory. It was extremely durable, probably from being extremely well worked. Specimens of it bring a high price.