Improvements relating to the Treatment of Gypsum for Building, Statuary, and other Purposes.
Year: 1890

page_text: Date of Application, 17th Oct., 1890-Accepted, 14th Feb., 1891
COMPLETE SPECIFICATION.
Improvements relating to the Treatment of Gypsum for Building,
Statuary, and other Purposes.
I, GEORGE FREDERICK REDFERN, of the firm of G. F. Redfern & Co., the
General Patent Office, No. 4, South Street, Finsbury, in the County of Middlesex,
Patent Agents, do hereby declare the nature of this invention (a communication
from abroad by Werner Heller, of 44, III, Zimmerstrasse, Berlin, in the Empire
of Germany, Merchant) 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 for the purpose of rendering
it suitable for building, statuary and other purposes.
The process hitherto in use for treating gypsum by burning with alum, to enable
it to resist the influences of the atmosphere is ineffective.
Endeavours have been made to treat so-called burnt or anhydrous gypsum with
borax (borate of soda) this, however, while delaying the hardening, according to
the strength of the borax-solution used, has the effect of preventing the hardening
entirely when large quantities of borax are used. It is therefore impossible to
obtain gypsum by treatment with borax capable of resisting atmospheric
influences with certainty.
Borate of potassium acts in quite a different way, as a solution of it of from
5 to 20% will cause the hardening of the gypsum and at the same time prevent it
from hardening too quickly. This quality of delaying the hardening is desirable
as the various metallic salts for colouring the gypsum are used in a concentrated
solution. The delay in the hardening is proportionate to the strength of the
solution of borate of potassium employed. The borate of potassium enters into
chemical combination with the sulphate of calcium and other sulphates of metallic
salts or precipitates, whereby an insoluble crystalline combination is produced
which cannot be obtained when borate of sodium employed, as it is impossible
for sodium to form insoluble combinations or double combinations with sulphate of
calcium.
The process forming the subject of the present invention, for treating anhydrous
gypsum with borate of potassium for the purpose of regulating the hardening can
be utilized for the production of an artificial marble which is similar in appearances
to the natural marble but excels the latter in weather resisting qualities.
The gypsum to be employed should be chosen from the older formations, care
being taken to avoid that which contains carbonate of lime and which is anhydrous,
as well as all fibrous gypsum, as otherwise the gypsum crumbles under the
treatment.
The selected gypsum, either in its natural condition as blocks or suitably shaped,
is submitted to heat according to its formation until the water of crystallization
contained therein is reduced to from 10 to 20% of its original quantity, whereby
the blocks or the objects lose their crystalline nature. This, however, is restored to
the said blocks or objects by placing them, into a solution of about 5 to 20% of
borate of potassium which prevent the untimely hardening of the gypsum and
produces under the succeeding treatment with metallic or rock-salts a hard,
crystalline insoluble product.
A suitable solution of borate of potassium can be prepared by neutraliz.ng
solution of about 10% of boracic acid with carbonate of potassium.
a
The gypsum blocks or objects treated as previously stated are now. ready for the
succeeding process of colouring and hardening, which consists. practically in a
second heating of the said blocks or objects to about 80 to 100° C., so as to expel
the superfluous free water after which the hardening and colouring is produced
1

page_text: by placing the gypsum into a solution of metallic salts which, in connection with
the sulphate of calcium, form an insoluble or nearly insoluble combination.
Soda salts, chlorides and nitrates are unsuitable. The following salts have been
found suitable-Sulphate of potash, sulphate of alumina, sulphate of iron and
misy. The latter may also be in combination with yellow or red ferrocyanide of
potassium or prussiate of potash, or with the chromate or bichromate of potash,
sulphate of zinc, salts of manganese and sometimes even the salts of copper.
The objects previously treated with borate of potassium and heated as before
described become when dipped in a solution of sulphate of potassium and sulphate
of aluminium similar in colour to that of natural marble, while coloured marble is
obtained by dipping the objects successively in different solutions containing some
of the before named salts. After having been treated in the above manner the
blocks or objects harden under great heat.
As an example illustrating the colouring of the gypsum after its treatment with
borate of potassium solution I will describe the process for producing a brown
colour.
In order to produce the brown colour the objects are first treated with borate of
potassium as before described and then immersed in a 10% solution of sulphate of
protoxyde of iron (a small quantity of sulphuric acid is added to the solution to
prevent decomposition). After saturation the object is taken out of the bath and
after being dried is immersed in a 5% to 10% solution (according to the shade to
be obtained) of bichromate of potassium until it becomes completely saturated.
According to the shade of colour desired the object may be immersed alternately
in both of the said solutions and under certain circumstances heated to 80° after
which it is immersed in a 2% alum solution (potassa alum neutralicum) where all
the free colouring matter is precipitated.
The object is now taken out of the bath and dried by exposing it to the air
for 3 to 4 days according to its dimensions and during which time the chemical
process, that is to say, the crystallization or in other words the combination of the
sulphate of calcium with the impregnated matter (metal oxides) becomes
completed.
The above described colouring process is the same with respect to all other
metallic salts for obtaining other colours.
Where the objects have to be exposed to the influence of the atmosphere it is
preferable to submit them once more for about one hour to a temperature of
between 75° and 80° C. and then to place them while still hot into hot fossil wax
(ozokerit). Other liquefied carburets of hydrogen, such as paraffine may, however,
be employed. Instead of placing the objects into the hot liquid they may be
introduced into a solution containing the above, and the said liquid carburet
solution may be replaced by liquefied resins or a solution of the same.
The product now obtained is indifferent to atmospheric influences and therefore
for architectural and art purposes not only equals natural marble, but owing to
its greater power of resistance against atmospheric influences replaces it with
advantage.
Having now particularly described and ascertained the nature of the said inven-
tion and in what manner the same is to be performed, I declare that what I
claim is :-
1. The process of preparing anhydrous gypsum for treatment with metallic and
rock-salts, by placing it into a solution of borate of potassium, whereby an
insoluble mass which can be coloured is obtained, substantially as described.
2. Heating the blocks or objects treated in the manner indicated in the preceding
claiming clause and then introducing the same into a solution of such metallic salts
as will form an insoluble or nearly insoluble combination with the sulphate of
calcium, such as sulphate of potash, sulphate of alumina, sulphate of iron or misy,
yellow or red prussiate of potash (or ferrocyanide of potassium) chromate or
bichromate of potash, sulphate of zinc and salts of manganese and copper...

page_text: 3. The process of rendering the blocks or objects produced as referred to in
claim 2, weather-resisting, by introducing them while in a hot state into a bath of
fossil wax or other liquefied carburet of hydrogen such as paraffine, which latter
may be replaced by liquefied resins or solutions containing the same, substantially
as described.
3. The process of rendering the blocks or objects produced as referred to in
claim 2, weather-resisting, by introducing them while in a hot state into a bath of
fossil wax or other liquefied carburet of hydrogen such as paraffine, which latter
may be replaced by liquefied resins or solutions containing the same, substantially
as described.
4. The use of blocks or other objects of gypsum treated as hereinbefore
described.
ed the 17th day of October 1890.
G. F. REDFERN & Co.,
4, South Street, Finsbury, London, Agents for the Applicant.