Improvements in Tanning Hides and Skins.
Year: 1896

page_text: Date of Application, 24th Sept., 1896
Complete Specification Left, 28th May, 1897-Accepted, 3rd July, 1897
PROVISIONAL SPECIFICATION.
Improvements in Tanning Hides and Skins.
I, WILLIAM CLARK of the Firm of A. M. & Wm. Clark of 53, Chancery Lane in the
County of Middlesex, Fellow of the Chartered Institute of Patent Agents; do hereby
declare the nature of this Invention (a communication to me by the Firm of
C. H. Boehringer Sohn of Niederingelheim-on-the-Rhine, Germany, Manufacturers ;)
to be as follows:-
. In former times tannin was the material exclusively used in the manufacture
of leather, but of recent years the practice of filling skins with. mineral oxides (for
example chromium oxides) has become more and more common.
In manufacturing chrome tanned leather the chromium oxide is formed within the
skins by charging them first with chromium salt (say chromium chloride) and
depositing the oxide of chromium by immersion in an alkaline bath; or the skins are
treated with salts of chromium-trioxide and these salts are subsequently reduced
in a second bath containing a reducing medium such for example as sodium
thiosulphate or sulphuretted hydrogen. The fixing of the oxide of chromium in the
skins may however be effected much more easily and even in a single bath if the
skins are treated with a. mixture of trioxide of chromium or its salts and lactic acid
or salts thereof.
Very good leather may be made in a comparatively short time in a single bath
if for example ox-hide be treated in a bath containing per litre of water:
4 grammes lactic acid (73%)
2.6 grammes Bichromate of Potash.
0-9 grammes Sulphuric acid.
In this bath the skin prepared for tanning as usual and previously slightly swelled
should be allowed to remain for about three days and then successively for a period
of eight days in a bath twice as strong and then four times as strong as that above
mentioned. The skin after only three days immersion appears to be thoroughly
penetrated with oxide of chromium or it looks green throughout. The skins are
afterwards washed, dried, and prepared as usual.
Thinner skins, such as calf and goat skins, may be transformed into leather
by treating them for about two days in a bath containing per litre of water :-
4 grammes lactic acid at 73%
.2.6
grammes Chromate of Potash.
0.9 grammes Sulphuric acid.
The skins should be swelled slightly for about half an hour in a bath containing
only the lactic acid, the other mordants being added subsequently. To accelerate
the process the bath may be heated to about 40° C.
Dated this 24th day of September 1896.

page_text: Improvements in Tanning Hides and Skins.
I, WILLIAM CLARK of the Firm of A. M. & Wm. Clark of 53, Chancery Lane in the
County of Middlesex, Fellow of the Chartered Institute of Patent Agents; do hereby
declare the nature of this Invention (a communication to me by the Firm of
C. H. Boehringer Sohn of Niederingelheim-on-the-Rhine, Germany, Manufacturers ;)
and in what manner the same is to be performed to be particularly described and
ascertained in and by the following statement:-
Tannin was formerly used exclusively in the manufacture of leather but of recent
years the practice of filling skins with mineral oxides (for example chromium oxides)
has become more and more common.
The fixing of the oxide of chromium in the skins may however be effected much 1
more easily than by the methods usually employed and even in a single bath if
the skins are treated with a mixture of trioxide of chromium or its salts and lactic
acid or salts thereof.
Very good leather may be made in a comparatively short time in a single bath
if for example ox-hide be treated in a bath containing per litre of water:
4 grammes lactic acid (73%)
2.6 grammes Bichromate of Potash.
0.9 grammes Sulphuric acid.
In this bath the skin prepared for tanning as usual and previously slightly swelled
should be allowed to remain for about three days and then successively for periods 2
of eight days each first in a bath twice as strong and then in a bath four times.
as strong as that above mentioned. The skin after only the first mentioned three
days immersion appears to be thoroughly penetrated with oxide of chromium or
it looks green throughout. The skins are afterwards washed, dried, and prepared as
usual.
Thinner skins, such as
calf and goat skins, may be transformed into leather
by treating them for about two days in a bath containing per litre of water :-
4 grammes lactic acid at 73%
2-6 grammes Chromate of Potash.
0.9 grammes Sulphuric acid.
The skins should be swelled slightly for about half an hour in a bath containing
only the lactic acid, the other mordants being added subsequently. To accelerate
the process the bath may be heated to about 40° C.
Although lactic acid has been chiefly mentioned it has been found that lactate
salts are more suitable because lactic acid causes the hide or skin to swell considerably
so that the resulting leather is brittle. Acetic acid also promotes swelling. The
acetic acid produced is caused to enter into combination when lactate salts are used
and thus an objection is obviated. A further objection is obviated when, instead
of using lactic acid and chromic acid compounds in one bath, they are employed in
succession in two baths. In that case the hides are first suspended in a solution of 4
chromic acid or in a solution of bichromate in hydrochloric or sulphuric acid at 4° Bé.
When the hide is saturated with the liquid it is removed therefrom, the liquid
is squeezed out, and it is suspended in a solution of about 2% strength of the lactate
salt at about 35° C.
By this means the excessive swelling action is reduced to the proper extent and
a considerable saving in chromium is effected. The skins are allowed to hang until
sample cuttings shew that they have become "green" right through. This takes

page_text: place in the case of thin skins in two to three days and in the case of thick skins in
five to six days. -
Having now particularly described and ascertained the nature of the said Invention
and in what manner the same is to be performed I declare that what I claim is :―
1. The herein described process of tanning hides and skins by treating them with
a mixture of chromium trioxide or a salt thereof, and lactic acid or a salt thereof,
substantially as specified.
2. The process of tanning hides and skins which consists in treating them first
in a bath of chromic acid and then after squeezing them out, in a lactate solution as
specified.
Dated this 25th day May
1907
A. M. & WM. CLARK,
Chartered Patent Agents, 53, Chancery Lane, London.
A. M. & WM. CLARK,
Chartered Patent Agents, 53, Chancery Lane, London.