IMPROVEMENTS IN TAWING OR CONVERTING HIDES OR SKINS INTO LEATHER
Year: 1880

page_text: LETTERS PATENT to Frank Wirth, of the Firm of Wirth and Company, Patent
Solicitors, at Frankfort-on-the-Main, in the Empire of Germany, for an
Invention of "IMPROVEMENTS IN TAWING OR CONVERTING HIDES OR SKINS
INTO LEATHER." A communication from Christian Heinzerling, Doctor of
Philosophy, of Frankfort, in the Empire of Germany.
PROVISIONAL SPECIFICATION left by the said Frank Wirth at the Office
of the Commissioners of Patents on the 31st August 1880.
FRANK WIRTH, of the Firm of Wirth and Company, Patent Solicitors, at Frankfort
on the Main, in the Empire of Germany. "IMPROVEMENTS IN TAWING OR
CONVERTING HIDES OR SKINS INTO LEATHER." A communication from Christian
Heinzerling, Doctor of Philosophy, of Frankfort, in the Empire of Germany.
This Invention relates to the process of tanning, for which I have obtained
Letters Patent, dated the Twenty third day of December, 1878, No. 5247, a
communication (as in this case) from Dr. Christian Heinzerling, of Biedenkopf, and
consists in additions to and improvements in the aforesaid patented process.
It is found on experimenting that there are many chrome and aluminous
compounds other than those named in the aforesaid Patent Specification that can
be used with effect. Thus chromic acid, chromate of ammonium (acid or neutral),
chromate of aluminium, chromate of iron, chromate of manganese, and other
soluble chromates or chromic acid salts, can be used in lieu of chromate of
potassium; similarly acetate of chromium, sulphate of chromium, chloride of
chromium (Cr, Cl) and like (soluble) salts of chromium will be found available.
These can be used in connection with or without aluminium salts or precipitated
hydrate of alumina (obtained by precipitation from a solution of aluminium salt by
means of a carbonate of an alkali or a caustic alkali), and also with or without the
addition of chloride of sodium or chloride of potassium (and or), such metallic salts
or oxides as sulphate of copper, sulphate of manganese, sulphate, chloride, or borate
of zinc, of protoxide or sesquioxide of iron, an oxide of manganese, or tungstate of
sodium or potassium (Wo O, Na or Wo O, K). The borate of zinc, though an
expensive salt, is mentioned, as it is useful for improving the colour. With almost
any of these salts I may use the ordinary tanning materials containing tannic acid,
preferably if the hides have been for a certain time tanned in the above mentioned
liquors containing the chrome compounds they can be finished in solutions
containing tannic acid.
The mode of operation is as follows:-The hides are unhaired in the usual
manner, and then put first into a weak and then into a stronger solution of chromic

page_text: acid or soluble (acid or neutral) chromate, with or without the addition of a soluble
aluminium salt or oxide, and with or without the addition of chloride of sodium
or chloride of potassium. A favourite mixture in Germany consists of free chromic
acid or neutral or acid chromate of ammonium and chromate of aluminium, to
which are added soluble aluminium salts, soluble modification of hydrate of
alumina and chloride of sodium in varying proportions to suit the quality of the
leather operated upon. During the cold season the liquors in the pits may be
moderately heated up to thirty degrees Celsius.
The hides are left in this solution, with or without the addition of metallic salts
useful for improving the colour or tanning, for from ten to twenty days, according
to size, texture, and thickness of the hides, until they are perfectly converted.
Their progress and complete conversion may be easily tested by cutting off a piece
and seeing if the tanning liquor has thoroughly penetrated it. After this they are
put into a solution of, say, from three to eight per cent. of a soluble carbonate,
such as carbonate of sodium or potassium, or a solution of soluble soap, say, three
to six per cent., in order to fix superficially the tanning materials. In some cases
it is advantageous to use instead of the above bath a soluble barium salt, especially
when alum or other sulphates have been largely used.
Instead of treating the hides with carbonate of soda or soap solution, as before
mentioned, in some cases it may be advantageous to use albuminous matter, either
dissolved in acid or alkalies or in the solid state, finely divided, as it is used in the
kid leather manufactories. The hides are either plunged for a certain time till the
matter has entered superficially if a solution is used, or fouled" (foulé) or tramped
in a "fouling" or tramping ton for an hour, in order to facilitate the mechanical
entering of the albuminous matter.
<<
The hides intended for upper or belt leather are now dried till they are merely
damp and then stretched; those for sole leather are made quite dry. They are
then put into a solution of paraffin, stearine, colophony, or the like, or vegetable or
animal wax, or such stuff as that formed from treating rape or linseed oil with ten
to eighteen per cent. of chloride of sulphur, or the solid soluble residues which are
obtained in petroleum and paraffin refineries, or varnish and birch tar oil; the
above solution is heated from twenty six to thirty eight degrees Celsius in a water
bath before and whilst the hides are in. The hides are kept in this bath at from
twenty eight to thirty degrees Celsius for from one to three days, according to size,
texture, and thickness. Upper and belt leather can be greased a second time, if
required, with the usual substances or with vaseline. The above said stuffs may
also be used in the melted state if they are fusible at a low temperature.
After a certain quantity of hides have been tanned it will be found advantageous
instead of adding soluble aluminous salts to add a quantity (proportional to the
acids set free) of basic aluminium compounds got by incomplete precipitation of
aluminium salts with carbonate of sodium or ammonium, or of hydrate of
alumina.
The tanning liquor should be analysed from time to time and the mixture kept
up to standard chemical composition.
It is found that light has a very useful action on the skins which have been
treated successively with the chromate bath and the paraffin or equivalent material.
Insoluble compounds of chromic acid and the paraffin or equivalent are probably
formed. If chromates or free chromic acid have been used as tanning materials a
partial reduction of these and an oxidation of the paraffin appears to take place.
The skins, therefore, after having been treated in the paraffin bath, should be
exposed to diffused light for a few weeks to two months. The action and reaction
of the light upon chrome compounds and paraffin (and the like stuffs) when used
for impregnating in the manner described makes the leather nearly impermeable
and gives a great resistance to the action of water upon it.

page_text: SPECIFICATION in pursuance of the conditions of the Letters Patent filed by
the said Frank Wirth in the Great Seal Patent Office on the 28th February
1881.
FRANK WIRTH, of the Firm of Wirth and Company, Patent Solicitors, at Frankfort-
on-the-Main, in the Empire of Germany, "IMPROVEMENTS IN TAWING OR
CONVERTING HIDES OR SKINS INTO LEATHER." A communication from Christian
Heinzerling, Doctor of Philosophy, of Frankfort, in the Empire of Germany.
This Invention relates to the process of converting skins or hides into leather by
means of chromates. Hitherto this has always proved a failure, the chromic acid
hardening and eventually disintegrating the leather.
I have found however that if the hides after being subjected to the chromate
process are well permeated with stearine, paraffin, or their isomeric compounds,
and exposed to the action of light, the chromic acid appears to have an action
on the stearine compound, changing its constitution in the course of a month or
two, and effecting thereby a great improvement in the leather. The exact
chemical changes that go on in this process have not yet been ascertained, but
probably insoluble compounds of chromic acid and the stearine or equivalent
are formed. If free chromic acid or chromates have been used a partial reduction
of these materials and an oxidation of the paraffin appears to take place. The
action and reaction of the light upon chrome compounds and paraffin and the
like stuffs, when used for impregnating in the manner hereafter described, make the
leather nearly impermeable and give a great resistance to the action of water, so
that it no longer has a tendency to harden or disintegrate.
I have found too in experimenting that there are many chrome compounds
besides chromate of potash or magnesia that are effective. Thus chromic acid, the
other soluble chromates, especially those of ammonium, aluminium, iron, or the
sulphate, chloride, acetate, and other soluble acid chromium salts are all efficacious
for the purpose of converting skins or hides into leather.
I also find that the precipitation of soft white neutral mineral material in the
pores of the leather is very advantageous in preserving it, improving its color, and
making it impervious to moisture.
Further, that the addition of such salts or oxides as the sulphates of copper,
manganese, and zinc, chloride and borate of zinc, tungstate of sodium, or of
potassium (Wo O, Na or Wo O, K), sulphate or chloride of protoxide or sesquioxide
of iron, and oxide of manganese, improve the texture, durability, or color of the
leather.
I will now give the actual process in its most improved form as practised by me
in Germany.
The hides being unhaired in the usual manner, I put them first into a weak, and
after a few days into a stronger solution of a soluble chromate, or chromic acid, or
chromium salt as herein before described. To this solution I usually add a soluble
salt of aluminium or the soluble hydrated oxide which is formed when carbonate of
soda is added to a solution of sulphate of alumina or alum. The quantity of
aluminous matter in the solution depends on the texture of the hides, the density
of leather required, and the salts used; it varies therefor from nothing up to, say,
four parts by weight of Al. O, to one hundred of water. The quantity of chromium
salt or acid used also varies from one half to five parts of the salt or acid to one
hundred of water. In the first pit both the chrome and the aluminous materials
are weak, especially the latter, while in succeeding pits while the quantity of
chrome does not vary much, that of the aluminous matters varies widely, sole
leathers and other hard leathers requiring large quantities, whereas wash leather

page_text: and thin uppers require none or very little. During the cold season the pits can
be moderately heated, say, to thirty degrees Celsius.
The hides are left in this chrome and aluminous solution, with or without the
addition of any of the above mentioned metallic salts useful for improving the
colour or for tanning, for from one to twenty days, according to size, texture, and
thickness of the hides, until they are perfectly converted. Their progress and
complete conversion may be easily tested by cutting off a piece and seeing if the
tanning liquor has thoroughly penetrated it. After this they are put into a
solution of, say, from three to eight per cent. of a soluble carbonate, such as
carbonate of sodium or potassium, or a solution of soluble soap, say, three to six
per cent., in order to fix superficially the tanning materials. In some cases it is
advantageous to use instead of the above bath a soluble barium salt, such as the
chloride, especially when alum or other sulphates have been largely used.
Instead of treating the hides with carbonate of sodium or soap solution as before
mentioned, in some cases it may be advantageous to use albuminous matter, such
as the serum of blood, either dissolved in acid or alkalies, or in the solid state
finely divided as it is used in the kid leather manufactories. The hides are either
immersed for a certain time till the matter has entered superficially, if a solution be
used, or "fouled" (foulé) or tramped in a "fouling" or tramping tun for an hour, in
order to facilitate the mechanical entering of the albuminous matter.
The hides intended for upper or belt leather are now dried till they are merely
damp and then stretched; those for sole leather are made quite dry. They are
then put into a solution of paraffin, stearine, colophony, or the like (dissolved in
any suitable solvent, such as benzol or photogen), or into vegetable or animal wax,
or such stuff as that formed from treating rape or linseed oil with ten to eighteen
per cent. of chloride of sulphur, or the solid soluble residues which are obtained in
petroleum and paraffin refineries, or varnish and birch tar oil; the above solution
is heated from twenty six to thirty eight degrees Celsius in a water bath before
and whilst the hides are in. The hides are kept in this bath at preferably from
twenty eight to thirty degrees Celsius, for from one to three days according to size,
texture, and thickness; they are then exposed to the light for from twenty five
days to sixty, until the stearine, paraffin, colophonium, or other hydro-carbon used
appears oxidized throughout. Upper and belt leather can be greased a second time
if required with the usual substances or with vaseline. The above described stuffs,
may also be used in the melted state if they are fusible at a low temperature.
After a certain quantity of hides have been tanned, it will be found advantageous
instead of adding soluble aluminous salts, to add a quantity (proportional to the
acids set free) of basic aluminium compounds got by incomplete precipitation of
aluminium salts with carbonate of sodium, or ammonium, or of hydrate of
alumina.
The tanning liquor should be analysed from time to time and the mixture kept.
up to standard chemical composition.
Having now described my Invention, what I claim is as follows:-
First. As a new article of manufacture, leather made from skins which have
been soaked in a chrome solution and then treated with stearine or an equivalent
compound as described, and afterwards submitted to the action of light before use,
as specified.
Second. As a new article of manufacture, leather made from skins which have
been treated with a chrome solution, an aluminous solution, a solution of soap,
chloride of barium, or other material having a decomposing action on the aluminous
solution (causing hydrate of alumina or barium sulphate or both to be deposited
in the pores of the skin), then treated with stearine or its chemically equivalent
compounds herein mentioned and exposed to light before use, as specified.
Third. The process of tanning leather by soaking it in a chrome solution, an
aluminous solution, a fixing solution acting upon the aluminous material so as
to form a precipitate in the pores of the leather, and then treating it with stearine,

page_text: or its chemical equivalent as described, and exposing it to light before use, as
specified.
Fourth. The process of converting skins into leather by soaking them in a
chrome solution and then treating them with stearine or the herein described
equivalent materials, and then exposing them to light before use, as specified.
Fifth. In the process of converting skins into leather by means of chromium
compounds, the employment of a soluble salt of alumina and of albuminous matter
successively as described.
Sixth. In the process of converting skins into leather by means of chromium
and aluminium compounds, treating the said skins with sulphate of copper, sulphate
of zinc, sulphate of manganese, chloride or borate of zinc, protoxide or sesquioxide of
iron, tungstate of sodium or potassium.
"
Seventh. The combination in the tanning of leather, of the "chromium process
and the tanning process, that is, converting a hide into leather by treating it with
a solution of a chrome compound and also with materials containing tannic acid,
and treating the so tanned leather with stearine or its chemical equivalent, as
described.
In witness whereof, I, the said Frank Wirth, have hereunto set my hand and
seal, this Twenty fourth day of February, in the year of our Lord One
thousand eight hundred and eighty one.