The age long progress in the development and perfection of
typographical printing surfaces, from the period of Xylographic blocks
on through the successive inventions of individual movable cast-metal
type, stereotyping and electrotyping, by both the wax and lead-molding
processes, reaches its culmination in _Aluminotypes_.
Briefly, it is a method of casting printing plates of aluminum alloy
in molds made from a composition of plaster-of-paris. In its essential
points it is a modern adaptation of the process credited to William
Ged of Edinburgh in 1730 and afterwards modified and improved in the
early 19th century by Earl Stanhope of England.
In practice, the original to be duplicated is placed on a
molding-slab. A molding frame is set upon the slab and enclosing the
original. A special kind of oil is then sprayed on the face of the
original. This is to facilitate the release of the plaster mold so
that it will not "tear" when it is ready to be lifted off the original
after solidifying, and at the same time to retain the sharpness of the
mold.
The molding medium of plaster composition in a semi-liquefied state is
then poured on to the original in the molding frame. The surplus
plaster is scraped off flush with the top of the molding frame.
After the plaster matrix in its molding frame has set sufficiently it
is released by means of cams from the working pattern on the
molding-slab.
The plaster matrix is then placed in a drying oven, through which a
forced draft of hot air is kept circulating at high pressure. The
thorough drying of the mold takes approximately ninety minutes.
When the plaster mold has become sufficiently dried, a round hole is
cut through the bottom of the matrix in an offset of the molding
frame. This hole is the gate through which the molten aluminum is
forced. The mold is then securely locked upright in a specially
designed casting machine.
The Aluminotype is cast by pressure and not by pouring as in the case
of stereotypes, which depend entirely upon gravity. Fused aluminum
alloy is poured into a hopper on the casting machine. A piston
operated by the agency of compressed air forces the aluminum evenly
into all parts of the plaster matrix.
When the cast is completed the molding frame is taken from the casting
machine and the Aluminotype removed from its plaster-of-paris matrix.
_AUTHORITIES_:
"An Outline of the History of Printing," by R. A. Peddie.
"Typographical Printing Surfaces," by L. A. Legros.
"Manual of Electro-Metallurgy," by Napier.
"The Encyclopedia Brittanica."
"Electrotyping and Stereotyping" Typographical Technical Series, Vol. XV.
The Rapid Electrotype Company.