Ordinary casts taken in plaster vary somewhat, owing to the shrinkage of
the plaster; but it has hitherto not been possible to regulate this
so as to produce any desired change and yet preserve the proportions.
Hoeger, of Gmuend, has, however, recently devised an ingenious method
for making copies in any material, either reduced or enlarged, without
distortion.
The original is first surrounded with a case or frame of sheet metal or
other suitable material, and a negative cast is taken with some elastic
material, if there are undercuts; the inventor uses agar-agar. The usual
negative or mould having been obtained as usual, he prepares a gelatine
mass resembling the hektograph mass, by soaking the gelatine first, then
melting it and adding enough of any inorganic powdered substance to give
it some stability. This is poured into the mould, which is previously
moistened with glycerine to prevent adhesion. When cold, the gelatine
cast is taken from the mould, and is, of course, the same size as the
original. If the copy is to be reduced, this gelatine cast is put in
strong alcohol and left entirely covered with it. It then begins to
shrink and contract with the greatest uniformity. When the desired
reduction has taken place, the cast is removed from its bath. From
this reduced copy a cast is taken as usual. As there is a limit to the
shrinkage of the gelatine cast, when a considerable reduction is desired
the operation is repeated by making a plaster mould from the reduced
copy, and from this a second gelatine cast is taken and likewise
immersed in alcohol and shrunk. It is claimed that even when repeated
there is no sacrifice of the sharpness of the original.
When the copy is to be enlarged instead of reduced, the gelatine cast is
put in a cold water bath, instead of alcohol. After it has swollen as
much as it will, the plaster mould is made as before. For enlarging,
the mould could also be made of some slightly soluble mass, and then by
filling it with water the cavity would grow larger, but it would not
give so sharp a copy.
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