Improvements in Treating Gypsum Casts.
Year: 1892

page_text: Date of Application, 14th Jan., 1892-Accepted, 20th Feb., 1892
COMPLETE SPECIFICATION.
Improvements in Treating Gypsum Casts.
:
I, ERNST WEBSKY, of Tannhausen, near Waldenburg, Silesia, in the German
Empire, Engineer and Manufacturer, do hereby declare the nature of this invention
and in what manner the same is to be performed to be particularly described and
ascertained in and by the following statement :—
This invention relates to the treatment of gypsum casts in such a manner that,
without detracting from their artistic appearance, they are rendered capable of
withstanding exposure to the weather and of being washed.
Amongst the many modes which have been proposed for accomplishing this end,
only one has been employed to any considerable extent. This consists in drying
and warming the casts, and then saturating them with melted stearin or paraffin.
Gypsum casts so treated have a surface somewhat resembling ivory, which, from an
artistic point of view, leaves much to be desired. While they can be easily cleaned
and are considerably harder than casts made of untreated gypsum, they have an
unpleasant glaze, which seriously detracts from their artistic appearance. Moreover,
the stearin or paraffin only penetrates a short way beneath the surface of the cast,
for which reason objects so treated offer but little resistance to the influence of
the weather.
By the following simple process, gypsum casts, at least as hard and durable as
those treated in the manner above described, capable of being washed and better
adapted to withstand the action of the weather, are obtainable. One effect of my
improved method of treatment is morcover to render gypsum translucent to a certain
depth, and to give the casts a stone-like appearance; thus causing them to have a
warm and artistically-beautiful tone, resembling that of material of quite a different
and more precious character. By careful rubbing with leather or flannel, the object
can be given a beautiful polish.
The process the subject of my invention is as follows:-The gypsum cast is
saturated with a "drying" oil in a warm place, and is then subjected to the action
of atmospheric air. The oxygen contained in the air causes the so-called
"drying" oil, which has completely penetrated the receptive material of the plaster
cast, to change into a translucent mass. Of the oils which can be used for this
purpose, the most suitable are bleached linseed oil, red poppy oil, hemp oil, nut oil,
castor oil and others of like nature.
'
In the oil employed, there may be dissolved under the influence of heat and
before application to the gypsum cast, a small quantity, say ten to twenty
per centum, of a clear rosin. I have found that the most suitable rosin is that
known as "Dammara rosin." The rosin increases the firmness and the durability
of the article treated.
The operation is carried out in the following way :-The carefully-dried objects,
heated to a temperature of from 170° to 200° Fahr., are completely immersed in
the oil, which is also heated to the same temperature, and are there left for from
eight to ten hours, the temperature being meanwhile maintained. At the end of
this time, they are taken out, and allowed to stand about twelve hours in the air in
a place free from dust. They are then again immersed in the hot oil for a period
of from two to three hours. On being removed, the oil adhering to their outer
surfaces is carefully removed, and they are then again exposed to the action of
atmospheric air as before. In the event of whitish spots appearing, they may be
easily obliterated by touching with a brush dipped in the oil."
As is well known, the "drying" oil can be made to dry more easily by boiling it
with metallic oxide, in which case the process would be accelerated. But I prefer
the unprepared oil, because it imparts an uniform and more beautifully-coloured
effect.

page_text: Having now particularly described and ascertained the nature of this inven-
tion and in what manner the same is to be performed, I claim :-
The herein-described method of treating gypsum casts with the object of
improving their artistic appearance and of rendering them capable of withstanding
exposure to the atmosphere and of being washed; the same consisting in
saturating the casts with bleached linseed or other drying oil, either alone or in
conjunction with rosin, and exposing them to the oxidizing influence of atmospheric
air substantially as herein described.
Dated this 14th day of January 1892.
G. G. M. HARDINGHAM,
191, Fleet Street, London, E.C., Chartered Patent Agent.