IMPROVEMENTS IN AND RELATING TO THE MANUFACTURE AND PRESERVATION OF ARTICLES OF GYPSUM, STUCCO OR THE LIKE
Year: 1884

page_text: PROVISIONAL SPECIFICATION.
I, MAXIMILIAN DENNSTEDT, Doctor.of philosophy, of Tzschecheln, Nieder, Lausitz,
Prussia, German Empire do hereby declare the nature of my invention for
IMPROVEMENTS IN AND RELATING TO THE MANUFACTURE AND PRESERVATION OF
ARTICLES OF GYPSUM, STUCCO OR THE LIKE" to be as follows:-
"
Numerous attempts have heretofore been made to harden works of art, and
architectural or other objects consisting of soft stone, terra-cotta, stucco and the
like in order to preserve the same.
All the methods or processes, heretofore employed, however, involve too many
manipulations and an enormous cost and have not, therefore, attained any practical
importance.
These methods are designed to impregnate tho said objects with various chemical
substances, that in part enter into insoluble combinations which are deposited in
the pores of the mass.
It has for instance been proposed to impregnate such objects alternately with
lime-water or baryta-water and with soluble potash glass (Kaliwasserglas) or
solutions of sulphates, in order to fill the pores of the material gradually with
insoluble silicates or sulphates.
•
Although this process is correct in principle, insoluble secondary combinations
also invariably occur which decrease the effect considerably and attempts have
therefore continually been made to render such combinations as harmless as possible
by resorting to special manipulations. To this end these soluble portions are either
lixiviated with water, or the alkaline portion of the same is converted by the action
of hydrofluo-silicic acid into the corresponding insoluble silico fluoride of the
alkali (Kiesel-fluor-Alkali) and the remaining portion (mostly carbonic acid or
chlorine) is, as far as possible, expelled by burning.
Apart from the doubt as to whether or not any soluble portions are left in
the objects thus treated, this process involves a great deal too much work and
expense.
These defects are entirely obviated by the present invention which is chiefly
applicable to objects consisting of gypsum, stucco and terra-cotta and is based
upon the admission, or introduction, in a free state, of the acids requisite to form
Prine A73

page_text: the insoluble combinations into the said objects which are either previously or
subsequently, soaked with a hot, and completely saturated basic solution, so that
the formation of soluble accessory products is thoroughly provided against from
the outset and all further treatment of the object is dispensed with.
I employ as such free acids, more especially carbonic acid, sulphuric acid, silicic
anbydride and chromic acid, whilst solutions of strontium hydrate or, because of its
cheapness, barium hydrate, are made use of as bases.
In order that the material shall be completely soaked from the commencement,
the basic solution is applied only in a hot and therefore completely saturated
state.
The method or process is as follows, that is to say :-The object is first carefully
dried and then a hot solution of saturated barium hydrate is repeatedly spread
over the same until it will not take up any more thereof. The spreading of the
solution is best executed by means of a brush made of a glass rod surrounded with
glass-wool. In the partial cooling of the solution when it is spread, some crystals
of caustic baryta separate upon the surface of the object. These can be taken off
with a soft brush, or washed away with water.
Immediately after the object has been soaked with the hot, saturated solution, it
is placed in an atmosphere containing a considerable quantity of carbonic acid and
is dried therein. For this purpose receptacles can be employed into which carbonic
acid is either introduced from the outside, or which are themselves provided with
an apparatus for generating carbonic acid. As the solution of the barium hydrate
is, from the commencement, in a hot state, the formation of insoluble barium
carbonate occurs very rapidly and completely and the separation of crystals of
caustic baryta upon the surface is almost entirely obviated;
If it is desired to obtain an especially tight dense or weatherproof surface, the
operation can be repeated one or more times, the objects being completely dried
between each operation.
""
When sulphuric acid or chromic acid is employed the procedure is reversed, that
is to say, the objects are at first brushed over with one of these acids; they are
then dried at about 40° Celsius, and after this they are soaked with the hot
saturated solution of barium hydrate or of strontium hydrate.
The acids in this case are applied in about the following degrees of dilution :
Chromic acid from 5 to 6 grammes to 100 cubic centimeters of water.
Sulphuric acid from 2 to 3 cubic centimeters to 100 cubic centimeters of
water.
In this case also an exceedingly tight, dense or weatherproof surface can be
effected by repeated treatment with acid and a hot solution of barium hydrate.
When silicic anhydride is, employed, it is added to the mass before moulding, and
after the drying or burning, the barium hydrate solution is used, as above stated.
As, however, the combination of the silicic acid with the baryta does not take place
rapidly, the barium hydrate can in this case be added in a solid state to the mass
prior to moulding, and moreover in its stead the cheaper calcium hydrate can if
desired be employed.
The treatment above described with reference to the employment of carbonic
acid, sulphuric acid, or chromic acid can be dispensed with and an increased
tightness or density extending equally throughout the object, be obtained by.
mixing the mass which is used in the production thereof, for instance calcined
gypsum, directly in its preparation with solutions of metallic salts, such as metallic
sulphates, which with barium hydroxide enter into such a reaction that insoluble
barium salts are formed. If, then, these objects are, after the drying, soaked with
the hot solution of barium hydrate; a decomposition of the metallic salts takes
place throughout the entire mass, and besides, the insoluble barium salt which
forms, filling all the pores, a separation of the metallic oxides or metallic hydroxides
occurs and these, being insoluble, conduce materially to the preservation of the
objects;

page_text: Moreover substances containing silicic anhydride and barium hydrato or calcium
hydrate can be treated after preparation with the solutions of metallic salts above
named.
In this case the silicic anhydride can be used in the form of finely pulverized
glass.
It is also to be noticed that, with the employment of free sulphuric acid as above
stated, by adding a little sugared water to the solution of barium hydrate, a deep
black colour of the objects can be obtained with facility. The solution contains
100 cubic centimeters of water and about 5 to 10 grammes of sugar, and as much
hydrate as with gentle heating of the water will dissolve in it; the solution is then
spread in a hot state upon the object, which is then slowly dried so that a
separation of crystals of caustic baryta is avoided as much as possible; it is
afterwards brushed repeatedly with the application of dilute sulphuric acid
(10 cubic centimeters of acid to 100 cubic centimeters of water) and dried at
from 70° to.80° Celsius. The sulphuric acid gradually causes the decomposition of
the sugar, and the formation of carbon, thereby giving the black colour to the
object. In lieu of the saccharates, other organic substances which carbonize with
sulphuric acid can be used, such for instance as tartaric, citric and oxalic acid.
They are in this case best employed directly in the form of their barium salts.
All the objects of gypsum, stucco; or terra-cotta, treated in the above described
manner, can, without causing the. least injury, be washed off with water and
therefore permit of being cleaned more easily and thoroughly than the sensitive
material heretofore used.
.

page_text: I, MAXIMILIAN DENNSTEDT, of Tzscheoheln-Niederlausitz, Prussia, Germany, do
hereby declare the nature of my invention for "IMPROVEMENTS IN AND RELATING
TO THE MANUFACTURE AND PRESERVATION OF ARTICLES OF GYPSUM, STUCCO, OR
THE LIKE" 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 a process of increasing the density of articles or objects
made of gypsum, terra-cotta, stucco, soft stone and other like substances, which
articles are more or less frangible, liable to injury, and cannot be cleansed without
destroying the finer and sharper lines thereof.
The object of the invention is to increase the density or homogeneity of such
articles, harden the same so as to render them less liable to injury, adapt them to
be readily cleansed by washing with water, and, if desired, to impart to such articles
or objects an intensely black color.
Various attempts have been made to increase the homogeneity of objects of art
and other articles or ornaments made of materials of the nature of those enumerated
above, with a view to harden such articles or objects and render them less frangible
and less liable to abrasion and injury. The methods heretofore employed have,
however, not only the great disadvantage of being very complicated and at the
same time very costly, but the objects aimed at are only partially attained. These
methods consist, essentially, in impregnating the objects with chemicals that form
in part insoluble combinations which fill up the pores of the material. It has.for
instance been proposed to impregnate the objects alternately with lime-water or
baryta-water and silicate of potash or solutions of sulphates, in order to gradually
fill up the pores of the material with the resultant insoluble silicate or sulphate.
Although this method is correct in principle yet soluble bye-products or combi-
nations are produced which greatly affect the result sought, and which can only be
counteracted by special treatment or manipulation. This has been effected by
lixiviation in water or by conversion of the alkaline portion thereof by means of
hydrofluo-silicic acid into the corresponding silicio fluoride of alkali, any remaining
portion (principally carbonic acid or chlorine) being then removed by baking or
..calcining. Irrespective of the danger of an imperfect elimination of the soluble
elements from the article or object treated, the process itself is too complex for
practical application and too costly. In all the processes proposed these difficulties
and disadvantages have been encountered, and chiefly for the reason that cold
solutions of lime or baryta were employed.
By my improved process all these disadvantages are overcome, and my invention.
consists, essentially and broadly, in the use of hot saturated solutions (that is to
say, highly-concentrated solutions) of barium hydrate or strontium hydrate, alter-
nately with such free acids or metallic salts thereof as will prevent the formation of
soluble bye-products, thereby rendering unnecessary all further treatment. The
free acids referred to are carbonic acid, sulphuric acid, anhydrous silicic acid,
chromic acid or phosphoric acid, all of which I have found to produce the desired
results.
In carrying out my invention I proceed as follows; that is to say. The object
to be treated is carefully dried and heated to about 80° or 100°. It is then washed

page_text: '
or painted with a hot saturated solution of barium hydrate until the material is
saturated, that is to say, until the said material will absorb no more of the solution.
In the process of washing or painting, an ordinary brush or a sponge cannot be
employed and I therefore employ a glass brush-that is to say, a brush made up of
threads of glass.
Inasmuch as it is very difficult, if not impossible, to make a brush of the proper
size that will be durable, I unite a number of small or thin glass brushes, placing
them in a row or in concentric circles in a suitable glass holder, and cementing the
same in position by means of a cement made of oyster-shells and soluble glass,
(silicate of potash). In this manner I can obtain either flat or round brushes of
any desired thickness. During the partial cooling of the solution, while applying
the same, isolated crystals of caustic baryta are formed on the surface of the article
or object treated, which may be readily removed with a soft brush. The object or
article saturated with the hot solution of barium hydrate is then exposed to an
atmosphere laden with carbonic acid and dried. To this end chambers may be used,
into which carbonic acid gas is introduced, or which are provided with means for
generating the gas.
.
If it is desired to obtain a particularly homogeneous or solid mass, the described
operation may be repeated one or more times.
When any other than carbonic acid of the acids above referred to is employed,
the operation is reversed that is to say, the object is first washed or painted with
the acid solution, then dried, and afterwards saturated with the hot solution of
barium hydrate or strontium hydrate. In this mode of procedure, the acid solution
should be sufficiently attenuated to leave always a portion of the barium or
strontium hydrate uncombined, which uncombined portion is afterward gradually
neutralized by atmospheric carbonic acid.
The mode of procedure last described may be greatly simplified and at the same
time better results obtained than by either of the processes set forth, by treating
the material from which the article is to be produced, the said material being mixed
with the free acid solution. In this case the article or object when made and after
drying is treated with a hot saturated solution of barium hydrate or .strontium
hydrate.
When a solution of anhydrous silicio acid is employed, it will be found best to
add the barium hydrate in a solid form to the mass. Instead of barium hydrate
calcium hydrate may be employed this being cheaper in view of the fact that the
combination of the silicic acid with the baryta proceeds very slowly.
Instead of the solutions of the free acids above referred to, solutions of their
metallic salts may be employed; and in this case it should be remarked that the
reaction of the metallic salts upon the barium hydroxide gives rise to the formation
of insoluble metallic oxides or hydroxides, besides the insoluble baryta salts. If the
object thus treated is, after drying, saturated with a hot solution of barium hydrate,
a décomposition of the metallic salts take place, the metallic oxides or hydroxides
separating from the insoluble baryta salt that fills the pores, and, as these oxides or
hydroxides are also insoluble, they assist materially in the preservation of the
5 article or object.
Articles or objects made from materials prepared with anhydrous silicic-acid and
barium hydrate or calcium hydrate, as above set forth, may also be treated with the
'described metallic salt solutions.
It will be observed that I have herein-above referred to the use of hot solutions
0 of barium hydrate or strontium hydrate only, for the reason that better results are
obtained than by the proposed method of treating objects of this class with cold
baryta water. In the latter case but a comparatively small proportion of the
barium hydrate-enters into the reaction, because the calcium hydrate is much less
soluble than the barium hydrate in the conversion of barium gypsum' saturated
5 with barium into sulphate of baryta and calcium oxide, so that but a small pro-
portion of sulphate of baryta and carbonate of lime results from the chemical
reaction, the burdening of the object being due almost exclusively to the barium

page_text: carbonate derived from the excess of barium hydrate. It is obvious, therefore, that
when cold baryta solutions are employed the more delicate and sharper lines of the
object are liable to be obliterated or destroyed during the immersion of the object,
by reason of the partial solubility of the calcium hydrate formed during said
immersion. This defect is entirely obviated by the use of hot solutions, because the
solubility of the calcium hydrate is at high temperatures greatly diminished. On
the other hand, the deposit of the insoluble elements when cold solutions are
employed is superficial only, the surface pores being completely closed when the
operation is repeated several times, consequently absorption ceases, so that any
attempt to further harden the surface by a repetition of the operation is fruitless.
If, on the contrary, the object is heated to 80° or 100° centigrade, and then treated
with a hot solution, the latter will, under atmospheric pressure, penetrate deeply
into the material, and, on cooling, the barium hydrate crystallizes within the pores,
and is there converted into barium carbonate, which conversion takes place very
rapidly when the object is subjected to the action of carbonic acid.
I have stated above that, if the material is mixed with a solution of anhydrous
silicic acid, the barium hydrate may be added in a dry or solid form. I would
state that this mode of proceeding may be reversed that is to say, the material may
be mixed up or prepared with a solution of barium hydrate and the anhydrous
silicic-acid added in a dry or solid state, and preferably in the form of finely divided
or. pulverized glass, and this tends to even increase the density and resistance or
strength of the object.
:
The strength or resistance as well as the density of the material may also be
increased by mixing with gypsum or other like substances, a substance which,
either by chemical action upon the barium hydrate or by its inherent properties,
will impart to the object formed of the compound the qualities sought. Thus, for
example, glass, cement, kaoline, or other like substances, either separately or mixed,
or combined with marble-dust, witherite, heavy spar, or analogous substances,. may
be added to the gypsum, and in proportion even as high as fifty per cent, without
affecting the property of the gypsum to reproduce the finest or most delicate forms
or designs.
It may be finally remarked that when free sulphuric acid is employed, as above
described, a jet or intensely black color may be imparted to the object by the
admixture of an organic substance-such as saccharine substance-with the barium
hydrate. This may be effected by adding from five to ten grams of sugar, for
instance, to every one hundred cubic centimeters of water, and as much barium
liydrate as the water will take up when slightly heated.
This solution is also applied in a hot state to the article or object, and when
saturated the latter is slowly dried, so as to avoid as much as possible the formation
of caustic baryta crystals. After drying the article or object is then washed with
a solution of sulphuric acid, (about ten cubic centimeters of the acid to about one
hundred cubic centimeters of water,) and again dried in a temperature of 70° to 80°
centigrade. The sulphuric acid gradually decomposes the sugar, and carbon is
separated, which imparts to the object the black color. Instead of saccharine
substances, other organic substances may be employed, which will be converted into
carbon by the sulphuric acid-as for instance, tartaric, acetic, and oxalic acid-in
which case they may be used in the form of their respective barium salts.
Any article or object made of gypsum, plaster-of-paris stucco, terra-cotta, or
other analogous substance, when treated as described, may be washed with water
without the least injury thereto, and may therefore be kept in a cleaner condition
than has been possible heretofore.
Having now particularly described and ascertained the nature of my said
invention, and in what manner the same is to be performed, I declare that what
I claim is
First. The process herein described of increasing the density of articles or objects
stucco terra-cotta and other analogous norous and frangiblo
made of ovnsum

page_text: materials, and adapting such articles or objects to be cleansed by washing with
water, consisting in treating the said articles or objects with a hot saturated barium
or strontium solution, and drying them in the presence of carbonic acid gas..
Second. The herein described process of increasing the density of articles or
objects made of gypsum; stucco, terra-cotta, and other analogous substances, and
adapting such articles or objects to be cleansed by washing with water, consisting
in mixing with the substance of which the said articles are formed finely pulverized
glass, cement, kaolin, or other like substances, either separately or in combination
with finely-pulverized marble, witherite, heavy spar, or other like substances, and in
treating the objects made therefrom with a hot saturated solution of a barium or
strontium hydrate, and drying the same in presence of carbonic-acid gas.
Third. The process herein described of increasing the density of articles or objects.
made of gypsum, stucco, terra-cotta, and other analogous substances, and adapting
such articles or objects to be cleansed by washing with water, consisting in mixing
with the substance from which the articles or objects are formed a solution of a free
acid or a solution of a metallic salt thereof that will in presence of a barium hydrate
or strontium hydrate form an insoluble metallic oxide or hydroxide besides the
insoluble barium or strontium salt, and treating the articles or objects formed from
such substance with a solution of barium hydrate or strontium hydrate.
Fourth. The process herein described of increasing the density of articles or
objects made of gypsum, stucco, terra-cotta, and other analogous substances, and
adapting such articles or objects to be cleansed by washing with water, consisting
in mixing with the substance from which the articles, or objects are formed finely
pulverized glass, cement, kaolin, or other like substances, either by themselves or
combined with finely-pulverized marble, witherite, heavy spar, or other liko sub-
stances, and a solution of a free acid or a metallic salt thereof that will in presence
of a barium hydrate or strontium hydrate form an insoluble metallic oxide or
hydroxide besides the insoluble barium or strontium salt, and treating the articles
or objects formed from such substance with a hot saturated solution of a barium
hydrate or strontium hydrate.
Fifth. The process herein described of increasing the density of articles or objects
made of gypsum, stucco, terra-cotta, and other analogous substances, and adapting
such articles or objects to be cleansed by washing with water, consisting in combining
anhydrous silicic-acid and either barium, calcium, or strontium hydrate, and treating
the finished articles or objects with a solution of these metallic salts, which form
with the alkaline earth or earths insoluble combinations.
Sixth. The process herein described of increasing the density of articles, or
objects made of gypsum, stucco, terra-cotta, and other analogous substances,
imparting to such articles or objects a black color, and adapting them to be
cleansed by washing with water, consisting in mixing sulphuric acid with the
substance from which the articles or objects are made, and treating such articles or
objects with a hot saturated solution of barium hydrate or strontium hydrate
combined with an organic substance convertible into carbon by the action of
sulphuric acid.