An Improved Process of and Compound for Tawing Hides and Skins.
Year: 1896

page_text: Date of Application, 27th June, 1896
Complete Specification Left, 1st Jan.. 1697-Accepted, 27th Feb., 1897
PROVISIONAL SPECIFICATION.
An Improved Process of and Compound for Tawing Hides and
Skins.
I, GEORGE WASHINGTON ADLER, residing at No. 1735, Bambrey Street, in
the City of Philadelphia, United States of America, Leather Manufacturer, do
hereby declare the nature of this invention to be as follows:-
My invention relates to the tawing of hides and skins in the manufacture of
leather of all kinds and whether of light or heavy texture; and is applicable to
the treatment of skins in all conditions, whether pickled, salted, dried or fresh.
It has for its object the production of tawed skins and hides froin which leather
can be made by the usual subsequent treatment, that will be practically insoluble
in water, strong and durable, and readily adapted in the case of kid leather, to
take colour and polish, and presenting in the finished article a soft and velvety
surface.
My invention consists in the improved compound and process hereinafter
mentioned; and is applicable to that stage in the art of making leather which
comprises the tawing proper, that is to say, after the skins and hides are
depilated by any of the usual methods commonly employed for the purpose, and
subsequently washed, pured or branned.
If pickled skins are to be used in the manufacture, they may be milled in salt
water, as usual, to remove the pickle, or they may be subjected in the pickled
condition to my tawing process and compound, depending somewhat upon the
character and purpose of the leather which it is desired to produce therefrom; but
so far as regards the application of my tawing process thereto, it is immaterial
whether the skins are milled in salt water or used in the pickled condition.
In carrying out my process I form a double sulphate of chromium and sodium,
by dissolving bichromate of potash (or other salt of chrome,) in sulphuric acid to
which water is added to dilute it. The solution is then reduced by any suitable
deoxodizing agent, for which purpose I have found sugar or alcohol to be con-
venient and effective. The solution is then precipitated, rendered neutral, and
combined with sodium, by adding carbonate of soda. The resultant double salt of
chromium and sodium is then split up by the addition to the solution of hydro-
chloric acid.
In compounding the solution I take a suitable quantity of bichromate of potash
and dissolve this in a concentrated solution of sulphuric acid and water,.say one
part acid and two parts by volume of water. The quantity of reducing agent
employed depends upon what material is used for the purpose, but the requisite
quantity is easily determined by noting the changing colour of the solution from a
yellow to a decided greenish colour.
Sufficient carbonate of soda is added to produce precipitation; which is easily
determined by inspection.
The quantity of hydrochloric acid required to split up the resultant double salt
of chromium and sodium, will vary somewhat, but I have found that 1 to 2 pounds
of acid to each gallon of the solution, will be sufficient for the purpose. The
resultant compound will probably best be represented by the formula

page_text: and the constituent double metallic salts will be in such union, chemically considered,
that they will each be severally capable of exerting their full effect as tawing
agents and each also capable of modifying the action of the other, i.e., the sodium
will modify the astringent effect of the chrome, and the latter will modify the
opening or swelling effect of salt in excess.
The skins are to be submitted to a bath of this compound diluted to an extent
varying according to the character of the skin to be acted on, the length of time of
immersion also varying for the same reasons. The tanner can easily determine
when the skin is completely tawed by cutting into the thickest part of the skin and
noting the penetration of the solution. The skins and hides treated by my
solution so prepared will be found to be insoluble and perfectly tawed, with a
remarkably fine grain and surface.
The process as compared with a double bath or two-step process will be found
much cheaper, more certain in operation and result produced, and will give a
better and more uniform quality of leather.
Any well-known modes of finishing the tawed skins for conversion into
merchantable leather may be employed.
Dated the 27th day of June 1896
D. YOUNG & Co.,
11 & 12, Southampton Buildings, London, W.C.,
Agents for the Applicant.
COMPLETE SPECIFICATION.
An Improved Process of and Compound for Tawing Hides and
Skins.
I, GEORGE WASHINGTON ADLER, residing at No. 1735 Bambrey Street, in
the City of Philadelphia, United States of America, Leather 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 :-
My invention relates to the tawing of hides and skins in the manufacture of
leather of all kinds and whether of light or heavy texture; and is applicable to
the treatment of skins in all conditions, whether pickled, salted, dried or fresh. It
has for its object the production of tawed skins and hides from which leather can
be made by the usual subsequent treatment, that will be practically insoluble in
water, strong and durable and readily adapted in the case of kid leather, to take
color and polish, and presenting in the finished article a soft and velvety surface,
and with a fine and close grain.
My invention consists in the improved compound and process hereinafter fully
described and claimed; and is applicable to that stage in the art of making leather
which comprises the tawing proper, that is to say, after the skins and hides are
depilated by any of the usual methods commonly employed for the purpose and
subsequently washed, pured or branned.
If pickled skins are to be used in the manufacture, they may be milled in salt
water, as usual, to remove the pickle, or they may be subjected in the pickled
condition to my tawing process and compound, depending somewhat upon the
character and purpose of the leather which it is desired to produce therefrom; but
so far as regards the application of my tawing process thereto, it is immaterial
whether the skins are milled in salt water or used in the pickled condition.
The principle involved in my process is the impregnation of the skin with a
.compound containing' normal chloride of chromium and sulphate of sodium, combined

page_text: with an organic acid such as formic acid or acetic acid or their compounds, as the
principal constituent active agents in the tawing process, the same being in such
relation to each other that the chromium and sodium salts will each be severally,
capable of exerting its full tawing effect on the skin in the manner hereinafter
mentioned, and each also of controlling or rather of qualifying the separate action
of the other; and my invention further comprises broadly the method of forming
such a compound of chloride of chromium, sulphate of sodium, and an organic
acid, as hereinafter described.
In carrying out my process, a concentrated solution of the tawing liquor is
prepared as follows:-I first form a compound of sulphate of potassium or of sodium
with chromic acid, by dissolving a given quantity of bichromate of potash or of
soda in one-and-one-half times its quantity by weight of sulphuric acid, adding two
parts by volume of water to one of acid, and this chromic acid compound therein is
then to be reduced to a sesqui-oxide (Cr₂O₂) by an organic reducing agent, such as
sugar or alcohol, either of which I have found to be convenient for the purpose and
particularly effective by reason of the presence of formic or of acetic acid
respectively, produced therefrom in the compound.
To this resultant sesqui-oxide solution, is then to be added a solution of
carbonate of soda, whereby it is neutralized and sodium combined with it; the
resultant being sodium sulphate and formate or acetate, chromic hydrate and
chromium oxy-carbonate, the last two forming a precipitate; and this precipitate
is then to be dissolved in the presence of hydrochloric acid as hereinafter described,
giving normal chloride of chromium and sulphate of sodium with some sulphate of
chromium, some chloride of sodium, also some sulphate of potash (if bichromate of
potash instead of bichromate of soda has been employed in the initial composition),
and an organic acid, either formic or acetic-resulting from the decomposition of
. the particular organic reducing agent employed-combined with sodium as a formate
or an acetate.
The following proportions and method of procedure will produce the results
stated-Taking a given quantity of bichromate of potash or of soda, say
five (5) pounds, it is to be dissolved in fully one-and-a-half times that quantity
by weight (say 7 to 8 lbs.) of sulphuric acid, diluted with double the quantity by
volume of water that there is of acid used; and this chromic acid compound so
admixed with sodium or potassium sulphate, is then to be completely reduced to a
chromic oxide, the formation of which may be determined by the liquid assuming
a decided dark green color, for which purpose, the quantities named will require
about one pound of white sugar or one-and-one-half pints of alcohol, added slowly
to keep the temperature below the point of ebullition, and until effervescence ceases.
The solution is then to be rendered neutral and sodium sulphate formed therein, by
adding to the quantity named about twenty (20) pounds of carbonate of soda
dissolved in about seven-and-one-half (7) gallons of water, the sodium carbonate
solution being added slowly until effervescence ceases, and the mixture allowed to
stand several hours until precipitation of the contained chromium oxy-carbonate
is completed. The final step in the preparation of the compound consists in adding
hydrochloric acid to this solution containing the precipitate, in quantity enough
only to split up or decompose and completely dissolve all the precipitated chromium
compound the sodium compound being already in solution, and for this
purpose
about six to seven pounds by weight of such acid will be sufficient for the quantities
named of the other ingredients employed, and twelve (12) to twenty-four (24) hours
will be required for the purpose; but especial care must be taken that there shall
be no excess of hydrochloric acid used beyond the quantity necessary to accomplish
the object stated.
The solution thus compounded, of the ingredients and in the quantities named, will
be of about 20° to 23° of Baumè in density at about 60° Fahrenheit of temperature,
and will make ten (10) to twelve (12) gallons of the concentrated tawing liquor.
The solution is principally a normal chloride of chromium, with sulphate of sodium
and an organic acid compound such as a formate or acetate, though I cannot define

page_text: in chemical terms the exact character of the union of the several constituent elements..
It will probably best be represented by the formula
Cr₂(CHO), or Cr₂(C₂H₂O)6;
and a quantitative analysis of the concentrated compound prepared as hereinbefore
directed will show it to contain substantially the following:-Sodium sulphate
about 20 per cent.; sodium chloride and potassium chloride together about
7 per cent.; chromium chloride about 8 per cent. ; and of organic acids, in this
particular instance as a formate of sodium, about 2 per cent. It is unnecessary to
give all of the several reactions at each step in the formation of the compound, it
being sufficient to state that the chromium as a sulphate [Cr(SO4)3] neutralized
by the carbonate of soda (3 Na2CO3) will give a solution containing sodium sulphate
and also a precipitate of chromium compound as above described. When this is
acidified by the addition thereto of hydrochloric acid, the chromium precipitate
contained therein is dissolved and changed into chromium chloride as the con-
stituent chromic salt. The organic acid compound formed by the action of the
reducing agent in the prior step is carried forward without change into the final
resultant composition.
The constituent double salts of chromium and sodium, it will be found in practice
are, in this compound, in such union and relation to each other, and to the organic
acid combined therewith, chemically considered, that they will each be severally
capable of exerting their full effect as tawing agents (the sulphate salt of sodium
being a far superior and differently acting coadjutant of chromium salts for this
purpose than the chloride of sodium), and that each will modify the otherwise
undesirable action of the other when used separately or in excess, i.e. the sodium
sulphate will qualify the usual astringent effect of chromium salts, and the chloride
of chromium will qualify or reduce the usual opening or swelling effect of sodium
salt in excess; while at the same time the neutral chromium salt in this compound
will, on contact with the organic substance of the hide, give up the hydrochloric
acid with which it is combined and be precipitated as an oxide, the reaction taking
place on the fibre of the skin, producing fixation and insolubility of the coagulated
gelatinous tissues. Whether or not this peculiar action of the normal chromium
chloride (which proceeds without any deleterious effect on the skin and by what is
apparently a process of dialysis) is due to the use of the normal chloride of the
metallic salt in conjunction with the peculiar sodium salt employed, or solely to the
presence of the organic acid in the compound, it is at least certain that such organic
acid greatly aids and hastens the reaction stated, and produces a rapidity of insoluble
fixation that prevents any injury to the skin from the natural astringent action of
chromium salts, and results in the production of a remarkably fine and soft leather,
It will also be found that the skin will rapidly and completely take up and absorb
the tawing materials; the refuse liquor, after the tawing is completed, showing
almost a complete deprivation of the contained salts.
The skins are to be submitted to a bath of this compound diluted to an extent
varying according to the character of the skin to be acted on, the length of time of
immersion varying for the same reasons. Ordinarily this concentrated solution
should be diluted to the extent of three to four times its volume of water, in order to
reduce it to a hydrometi ic strength of 5 to 6 degrees Baumè. The tanner can easily
determine when the skin is completely tawed by cutting into the thickest part of the
skin and noting the penetration of the solution. The skins and hides treated by
my solution so prepared will he found to be practically insolubly and perfectly
tawed, with a remarkably fine grain and surface.
The process as compared with a double bath or two-step process will be found
much cheaper, more certain in operation and result produced, and will give a better
and more uniform quality of leather.

page_text: Any well known modes of finishing the tawed skins for conversion into merchant-
able leather may be employed.
Having now particularly described and ascertained the nature of my said inven-
tion and in what manner the same is to be performed, I declare that what I
claim is :-
1. As an improvement in the art of tawing hides and skins, the subjection of the
skin to the action of a bath containing normal chloride of chromium, sulphate of
sodium, and an organic acid compound, such as a formate or an acetate, admixed
substantially as set forth.
2. As an improvement in the art of tawing hides and skins, the submission thereof
to a solution derived from an admixture of sodium sulphate and chromic acid, the
said acid being theu reduced in the solution by an organic reducing agent, then
neutralized by adding an alkali, and the precipitated chromic compound decomposed
and dissolved in the presence of hydrochloric acid then added to the solution
containing the same, substantially as set forth.
3. The process of compounding a tawing liquor, which consists in dissolving
potassium or sodium bichromate in an excess of sulphuric acid diluted with two
parts by volume of water to one of acid, reducing the solution by sugar or its
alcoholic equivalent, then neutralizing and precipitating the same by the addition
of carbonate of soda, and finally dissolving the contained precipitate by exhibiting
hydrochloric acid thereto, substantially as set forth.
4. The improved compound for use in the tawing of hides and skins, prepared
substantially as herein before described.
Dated this 1st day of January 1897.
D. YOUNG & Co.,
11 & 12 Southampton Buildings, London, W.C.,
Agents for the Applicant.