Improvements in or relating to the Process of Tanning in the Manufacture of Leather.
Year: 1893

page_text: [Communicated from abroad by JOSEPH D. GALLAGHER, of Newark, New Jersey,
United States of America, Counsellor at Law.]
Improvements in or relating to the Process of Tanning in the
Manufacture of Leather.
I, JOSEPH SINCLAIR FAIRFAX, of 433 Strand, in the County of London,
Patent Agent and Consulting Engineer, 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:-
This invention, as communicated to me, relates to improvements in the process
of tanning, in the manufacture of leather. It consists of a novel method of fixing
the so-called gelatine of the hide or skin by means of a metallic oxide, and has for
its object the cheapening and simplifying of the process of making mineral tanned
leather, as well as the improvement of the manufactured product.
It has long been known that chromic oxide has the property of combining with
the hide gelatine to form an insoluble and non-putrescent body, and is therefore
capable of converting hides into leather. It has been but a few years, however,
that commercial leather has been made by the so-called chromic tannage; and
there is still much room for improvement in this kind of leather, as now placed
upon the market.
In this process, chromic acid is presented to the hide gelatine in the state of a
soluble combination in which it is so loosely held that it is readily given up to the
gelatine; rendering the fixing of the latter both rapid and complete. The soluble
combination which I use is designated by chemists as a basic or oxy salt.
When to solutions of a number of the normal salts of chromium is added a
quantity of a more powerful base, a part of the acid which was combined with the
chromium base is taken away, and the base is left in a combination which may be
regarded as basic; that is, containing two or more equivalents of the metallic base
or oxide, to one of the acid. In this case the excess of metallic base, over what
the acid would hold in the normal salt, is readily yielded to any body having an
affinity for it.
The effectiveness of my method, as far as the mere fixing of the gelatine is con-
cerned, lies in bringing the latter in contact in a neutral or nearly neutral solution,
with chromic acid, so held as to be readily taken up by the gelatine. This result is
accomplished by solutions of the combinations which I have called basic salts of
chromium, and they furnish the only practicable means of presenting to the hide
gelatine chromic oxide in soluble form, and yet not in stable combination with
acids.
When hides or skins, properly prepared, are treated with a solution of one of
these basic chromic salts, the gelatine quickly takes up the excess of the chromium
base, (in combination with which it is insoluble), and leaves only the remaining
normal salt in solution.
Attempts have been made to use chromic alum for converting hides into leather,
but without practical success. When chromic alum is used, deoxydation or reduc-
tion is unnecessary, since the chromium exists in the alum in the oxide form, but as
chromic alum is a very stable body, and gives up its chromic oxide with great
difficulty, the process of fixing the hide gelatine is exceedingly slow and
unsatisfactory.
The central idea upon which the practical methods of chromic tannage, other
than mine, are based, is to impregnate the hide or skin with chromic acid, and then
to reduce this acid to chromic oxide in the substance of the hide, by means of some
reducing agent; for example, sulphurous or oxalic acids, sulphuretted hydrogen or

page_text: proto-sulphate of iron. The chromic acid is usually introduced into the hides or
skins by immersing them in a bath, to which bichromate of potassium and a strong
acid like hydrochloric, have been added.
Chromic acid, however, is a powerful oxidizing and corrosive agent, and
invariably does more or less injury to the hide or skin; and, unless it is handled
with very great skill and caution, will produce leather which will become hard and
crack, and exhibit other defects. In fact the utmost care and experience often fail
to prevent the finished leather from showing the destructive effect of this powerful
chemical agent.
One of the probable causes of the deterioration of leather made by use of chromic
acid is, that chromic acid itself appears to enter to a great or less extent, into some
sort of combination with the hide gelatine, in which combination it is not wholly
reduced to chromic oxide by the sulphurous acid or other reducing agent employed,
but after the leather is finished and placed upon the counter or manufactured into
shoes, the oxidizing and destructive effects of the chromic acid become manifest by
its losing its strength and suppleness, and becoming hard and brittle and " papery
to the touch.
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32
It is true that the above method as far as fixing the gelatine is concerned, although
it is done in an acid solution, is very effective. In fact it is, outside of this process,
the only practical way known of accomplishing that result. The effectiveness is
due to the fact that chromic oxide (which is the real fixing agent) is formed by the
reduction of the chromic acid while in actual contact with the hide gelatine.
Therefore the gelatine can take up the chromic oxide while in the so-called
nascent" state, and thus accomplish in solution, which is necessarily acid, what it
could not easily do if the oxide were not formed in direct contact with it; for
instance, as when the hide is treated with an acid or even neutral solution of a
normal salt of chromium. But the destructive action of chromic acid on the fibre
of the hide or skin, (particularly when the highest skill is not employed), renders
it extremely desirable to use a method in which the hide is not exposed to the
action of chromic acid or other destructive chemical agent, or to intensely acid
solutions of any kind, but in which the chromic oxide is brought in contact with
the hide gelatin in a nearly neutral and mild solution, and yet in a form in which
it can be easily and quickly taken up. All this is accomplished in the improved
method, which I will now describe.
The salt of chromium which is found well adapted for my purpose is the chloride,
although other salts might be used, and this is used to illustrate my process in the
following description;
I first prepare a solution of common chloride of chromium. This may be done by
dissolving the commercial oxide of chromium, in commercial hydrochloric acid which
has been diluted with an equal bulk of water, care being taken to use more chromic
oxide than the acid will take up, in order that the resulting solution may be as
nearly neutral as possible. About eight ounces of the commercial acid is sufficient
to dissolve a pound of commercial oxide of chromium. To this solution of chloride
of chromium add slowly and carefully a solution of a more powerful base and for
which purpose I prefer carbonate of sodium, or, as it is known in the trade,
"sal soda," until permanent precipitation of chromic hydrate begins; or if
carbonate of sodium is used, until rapid effervescence ceases. It usually takes
about one-half pound of sal soda crystals to each pound of chromic oxide dissolved
as above.
I have now a nearly neutral solution of oxy or basic chloride of chromium. It
may be considered as consisting of the normal chloride of chromium and chromic
oxide in soluble combination, (free from chromic oxide hydrated or otherwise being
insoluble in water) and a quantity of chloride of sodium formed by the union of a
part of the acid which was in combination with the chromium, with the sodium base
of the carbonate of sodium, and which also serves a useful purpose, which will be
mentioned later on.
The hides of skins, after having been suitably prepared by soaking, liming,

page_text: CC
unhairing, baiting, washing etc. are immersed in the solution of the oxy or basic
chloride of chromium prepared as above, and of strength of about three gallons of
the concentrated solution to one hundred gallons of water. To the solution is also
added, besides that formed by adding carbonate of sodium to the solution of
chloride of chromium, a few pounds more of chloride of sodium (common salt) for
the purpose of preventing the grain of the leather from drawing under the astringent
effect of the chromic salt, and to facilitate the tanning action. The treatment of
the hides or skins in the above solution is accomplished in a paddle" or other
equivalent apparatus, as it is quite necessary that they be frequently moved in
order that the action of the chromic salt may be uniformly distributed. According
to the thickness of the skins and the strength of the solution, this tanning process
requires from about ten to forty eight hours. Ordinary goat-skins, for the so-called
Dongola leather, do well by running them in the paddle during the day, then
allowing them to stand over night in the liquor, and running the paddle about an
hour in the morning when the skins will be found well tanned. The strength of the
chromic solution may be varied according to the thickness of the skins, and may be
strengthened from time to time during the tanning operation if necessary.
After the absorption of chromic acid has ceased, and all the gelatine of the hide
has been rendered insoluble by its combination with the chromic oxide, which
condition is readily recognized by one familiar with leather making, the skins are
washed in clean water, and are then agitated in a bath containing finely divided
carbonate of calcium or its equivalent.
Instead of carbonate of calcium, carbonate of barium or the carbonates of lead,
zinc, etc., may be used. When carbonate of calcium is used, the form known in
commerce as whiting is well adapted.
The object of agitating the skins with these carbonates which are insoluble in
water, is, that every kind of acid present may be neutralized without subjecting the
skins to the action of an alkaline bath.
The skins may now be again washed, after which they are ready for the so-called
fat-liquor, for drying previous to dyeing, or for any other of the processes belonging
to the currier's art.
As the invention does not appertain to the beam house work, by which the hides
or skins are prepared for tanning, and which would be substantially the same for
my process as for any other method of tanning or tawing, I will not specify any
particular procedure for that part of the tanner's work. Neither is it necessary to
give directions as to the treatment of the skins after they are removed from the
bath of suspended carbonate of calcium, for it is as various as the different uses to
which leather put, and does not belong to the realm of tanning proper, with which
alone this invention has to do.
Among the advantages possessed by my method may be mentioned the
following;-
1. The tanning or fixing of the gelatine is effected entirely in one bath, instead, as
in the old method, of impregnating the skins with chromic acid in one bath, and then
reducing the chromic acid to chromic oxide in another bath. Thus the labour of
one handling is saved.
2. The skins are not exposed to the destructive action of chromic acid, whereby
the leather is stronger and has much more suppleness which it long preserves,
shows a better grain and does not become "papery," as compared with the product
of the ordinary method of chromic tannage.
3. There are no offensive or suffocating smells evolved as when in the ordinary
methods, the chromic acid is reduced to chromic oxide by the use of sulphurous
acid or sulphuretted hydrogen.
4. Since my tanning bath is entirely non-corrosive there is no danger of allowing
the skins to remain in it too long, and not nearly so much skill and care are necessary
as in the old methods.
5. Not only is there a saving in labour in my process, but my tanning bath is
much cheaper than those of the old methods, since the chromic acid in my tanning

page_text: bath costs much less as prepared by me than as prepared from bichromate of
potassium and a reducing agent in the old methods.
6. The tanning is effected in my process in a bath, which is not strongly acid,
as is always the case in the old methods in which chromic acid is employed. This
nearly neutral bath is of great advantage to the skins and naturally assists the
currier in the subsequent work of dyeing and finishing the leather.
In the foregoing Specification, and in the claims to follow, the term basic is
meant to include metallic salts which contain more than one equivalent of the
metallic base to one equivalent of the acid, as above described; and in the claims,
the terms carbonate of sodium and carbonate of calcium are meant to include their
chemical equivalents for the purpose specified.
The word "tanning" is meant to include all methods of fixing or rendering
insoluble the so-called gelatine of the hides or skins for the purpose of converting
them into leather.
The gist of the invention consists broadly in the use, as a tanning agent, of a
basic or oxy salt of chromium.
Specifically it includes the use as a tanning agent of a basic or oxy chloride of
chromium. It also includes a tanning bath prepared by adding to a solution of
chloride of chromium, carbonate of sodium, or its equivalent, until the chromium
salt is rendered basic. Further it includes the use of a bath, for tanning, containing
both basic of chloride chromium and chlorium of sodium. And lastly, it includes
the process of subjecting hides or skins in which the gelatine has been fixed by
treatment with oxy or basic chloride of chromium to the action of a neutralizing
bath of suspended carbonate of calcium.
I am aware that compounds of chromium have been used in calico printing, and.
in dyeing, but my invention does not extend beyond the process of tanning, and
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. In the manufacture of leather, a tanning bath containing a basic salt of
chromium in solution, substantially as and for the purpose herein described.
2. In the manufacture of leather, a tanning bath containing a basic chloride of
chromium, substantially as and for the purpose herein described.
3. In the manufacture of leather, a tanning bath containing basic chloride of
chromium and chloride of sodium substantially as and for the purpose herein
described.
4. In the manufacture of leather, a tanning bath containing chloride of chromium
which has been rendered basic by the addition of carbonate of sodium, substantially
as and for the purpose herein set forth.
5. In the manufacture of leather, a tanning bath prepared by dissolving chromic
oxide in hydrochloric acid and then rendering the resulting chloride of chromium
basic by the addition of carbonate of sodium, substantially as and for the purpose
herein described.
6. In the manufacture of leather, subjecting skins which have had their gelatine
fixed by means of a bath of basic chloride of chromium, to the action of a
bath of suspended carbonate of calcium, substantially as and for the purpose
herein described.
Dated this 14th day of April 1893.
FAIRFAX & WETTER,
433, Strand, London, Agents for the Applicant.