CEMENTS. 
41 
passing them through a fine silk sieve, and a plaster cast is made 
in the usual way. It is first allowed to dry in the open air, 
and is then carefully heated in an oven ; the plaster cast, when 
thoroughly dry, is soaked for a quarter of an hour in a bath 
containing equal parts of white wax, spermaceti, and stearine, 
heated just a little beyond the melting point. The cast on 
removal is set on edge, that the superfluous composition may 
drain oft", and, before it cools, the surface is brushed with a brush 
like a sash tool, to remove any wax which may have settled 
in the crevices ; and finally, when the plaster is quite cold, its 
surface is polished by rubbing it with cotton wool. 
Seme casts, as will be seen, are in very high relief ; these are 
made in elastic moulds, — a composition of glue and treacle, 
which admits of being turned out from the “ under-cutting ” 
without injury. 
By subjecting plaster of Paris to certain methods of chemical 
treatment, it may be hardened to a considerable extent, and 
thus becoming much less liable to injury, its value as a cement 
is greatly increased. 
Keene’s Cement, according to the specification, is thus pre- 
pared : — Dissolving one pound of alum in a gallon of water, this 
solution is used for soaking 84 pounds of gypsum calcined, in 
small lumps. These lumps are then exposed for eight days to 
the air, and afterwards calcined at a dull red heat and then 
ground and sifted. 
The Parian cement is prepared by soaking the plaster in a 
solution of borax instead of one of alum. This is exempli bed 
in the cast of the “ Dying Gladiator ” and its base (No. 86), as 
well as in the coloured cement on the stairs, in which Derby- 
shire marbles, to be found in the collection, are imitated, for the 
purpose of showing to what extent the realisation of a natural 
stone can be secured in an artificial material. Martin’s cement is 
formed by combining pearl ash (bi-carbonate of potash) and 
alum with the plaster, hydrochloric acid being sometimes added 
to prevent an alkaline re-action. 
Scagliola differs from all these cements, in consisting of small 
fragments of marble and other ornamental substances, embedded 
in a base formed of a mixture of plaster of Paris and glue. 
This was the invention of Guido del Conte, an ingenious mason 
of Can, near Corregio, in Lombardy. Scagliola was much 
employed by the Florentines in some of their most elaborate 
works. 
Roman cement was at one time largely made from the 
septaria or “ turtle stones,” which occur abundantly in many 
beds of clay. Quantities of these cement-stones were formerly 
procured by dredging off the coast of Hampshire, off the Isle of 
Sheppey, and at Harwich ; the septaria being derived from the 
London clay. A septarium is simply a nodule of argillaceous 
limestone, often containing in its interior an organic substance 
serving as a nucleus around which the limestone aggregated. 
e 87039. D 
