CERTAIN IMPROVEMENTS IN TANNING HIDES AND SKINS
Year: 1848

page_text: DUNCAN'S SPECIFICATION:
TO ALL TO WHOM THESE PRESENTS SHALL COME, I, JOHN
DUNCAN, of Brentwood, in, the County of Essex, send greeting.com
WHEREAS Her present most Excellent, Majesty Queen Victoria, by. Her
Letters Patent under the Great Seal of Great Britain, bearing' date at West-
minster, on or about the Twentieth day of January last, did, for Herself, Her,
heirs and successors, give and grant unto me, the said John Duncan, Her
special licence, full power, sole privilege and authority, that. I,.the said John,
Duncan, my exors, admors, and assigns, or such others as I, the said John
Duncan,.my exors, admors, or. assigns, should at any timejagree with, and no
others, from time to time and at all times during the term, of years. therein
expressed, should. and lawfully might make, use; exercise, arid vend, within
England, Wales, and the Town of Berwick-upon-Tweed, my: Invention of
"CERTAIN IMPROVEMENTS IN TANNING HIDES AND SKINS;" in which! said Letters
Patent is contained a 'proviso that.. I, the said John Duncan,: shall cause &
particular:. description. of the 'nature of my said Invention, and the manner.
in which the same is to be performed, to be inrolled. in Her said Majesty's
High Court of Chancery, within six calendar months; next and immediately.
after the date of the said in part recited Letters Patent, as in and by the same,
reference being thereunto had, will more fully, and at large appear...
NOW KNOW YE,. that in compliance with the said proviso, I,. the said
John Duncan, do hereby declare that the nature of my. Invention, and
the manner in which the same' is to be performed, are fully described and
ascertained in and by the following statement thereof::―

page_text: The conversion of the hides or skins of animals into leather by tanning
consists of a number of operations well understood, amongst which it will
be sufficient for the purpose of particularly describing and ascertaining the
nature of the present Invention to enumerate the solution of the tannin,
or the extraction of the tannic acid from the vegetable substance; and
the effecting the chemical combination of the vegetable principle called
tannin or tannic acid with the animal substance of the hide or skin. My
improvements in tanning relate to the mode in which the tanning principle
is obtained, and also to the mode in which it is applied. The infusion
of the tanning material may be obtained in the same pit in which it is
applied, that is, the preparation and the combination of the vegetable extract
with the animal substance may take place in the same pit; for instance, ground
bark and hides (properly prepared, may be laid in alternate layers in a tan
pit, and the pit then filled with water. When the bark and infusion are spent
or exhausted, the pit is emptied and refilled with layers of fresh bark and
water, and this operation may be repeated until the tanning is complete.
Recently, however, the more usual mode has been to prepare tanning infusions
in a more concentrated state than they would exist in the process just described,
and various modes of obtaining and applying the infusion have been adopted,
and the result has been a saving both of materials and of time in the process
of tanning.
Now, as has already been stated, my Invention relates both to the extraction
of the tannin, and to the subsequent application of it, and has for its
object,―
First, the extraction of tannic acid more effectively and economically from
the materials containing it.
Second, the shortening of the periods in which hides and skins become
leather, from the commencement of that part of the tanning process which
consists in subjecting the hides and skins to the tannic acid in tan pits.
First, as to the extracting the tannic acid. It is well known to tanners that
after the hides and skins have passed through the important operations of the
beam house, it is a matter of the greatest moment that there should be economy,
all practicable speed, and efficiency in the process of spending oak bark and
other like materials, for the purpose of extracting the tannin in which the
hides and skins after passing out of the beam house are to be placed. If the
process of spending be not effectively done, the tanning materials in use,
whether oak bark, mimosa, velonia, or any other bark containing tannic acid,
are wasted, and if not quickly done, time and labour are expended, and much
space in the tan yard occupied, and the liquor is subjected to exposure to

page_text: the air from fifteen to thirty days, passing from one spender pit to another,
whence gallic acid is formed to the deterioration and diminution of the tannic
and which is so being prepared for application to the hides and skins.
I shall now proceed to describe that portion of my improvements which
5 relate to the extraction of the tannin to be applied to the hides and skins.
The spender pit intended to be used may be of the ordinary size and shape
of spender pits usual in tan yards, but it must have a false bottom perforated
.with holes, and a well below it like the pits at present in use, and there must
be the means also by an eye in the corner of the pit of fixing a pump to draw
off the contents of the well. In these respects the present spender pits in
tan yards require no alteration. For the purposes, however, of carrying out
the process properly which I am about to describe, it will be necessary to fix
a close bottom of wood underneath, and parallel to the false bottom, and
within half an inch of it, and there must be one opening only from this close
bottom to the well, which opening must be made to enter the eye before
mentioned, and so that a plug or sluice acted upon by a rod passing up
the eye may be opened or shut at pleasure. When this plug or sluice
is open, the liquor in the pit will have a passage into the well, and when
shut the liquor will be confined and kept within the pit. If bark be the
material used, whatever may be the kind of bark it must be well ground
in the usual manner. The ground or powdered bark being placed in the
pit constructed as above mentioned, and the said plug or sluice being shut,
I then, by means of the percolating trough hereafter mentioned, sprinkle
water over the whole surface of the bark, until the entire quantity of powdered
bark in the pit has become thoroughly saturated. As a guide to the workman to
prove. when saturation has been effected, I recommend him to stop pouring on
water or liquid whenever the bark or other material in the pit begins in the
least degree to float. I leave the saturated bark when thus saturated for the
first time to stand for about six hours, and I then apply the method for
extraction of the tannin known to operative chemists by the name of displace-
ment. The process I pursue is as follows:-I construct a large tray or trough
of a size sufficient for its bottom to cover exactly the top of the spending pit.
The sides of this tray or trough should be of wood and water-tight, and not
-exceeding twelve inches in depth. The bottom should be formed of zinc or, other
materials not acted upon by tannic acid, perforated with extremely small holes
all over it. This tray or trough, which I call a percolator, being placed over
the mouth of the pit in which the saturated or wetted bark is lying in steep,
water must be made to pass into and: upon or across the bottom of the
percolator by any suitable apparatus in a sheet or streams, in such manner that

page_text: being thrown equally over the whole bottom, the water may fall by means
of the perforated holes equally and at the same time all over the surface of
the bark. The descent of the water equally over the entire surface of the
bark will be assisted by placing a cloth, which I recommend to be a hair
cloth, over the bottom of the percolator. Previous to the water descending
'on the surface of the bark, the said. plug or sluice must be opened to allow
the displaced tanning liquor in the pit to descend through the holes of the
false bottom on to the close bottom, and thence through the opening aforesaid
into the well. During the time of this process, which will in every case last
a short period only, the workman may, with the usual instrument called a jet,
be trying or testing the strength of the liquor passing out of the opening
where the said plug or sluice is fixed, and whenever he finds that the tanning
liquor is losing its strength, or, in other words, contains a small portion of
tannic acid, this will be a proof to him that the liquor held in saturation has
'been displaced, and he must then shut the said opening and stop, the further
passage of water into the percolator; the water or liquor which has displaced
the saturated liquid will then remain in the pit to form a fresh infusion. This
process of displacement may be repeated several times, so long as the material
'gives out tannic acid; but between each process a period varying from twelve
to eighteen hours must elapse, during which time the said plug or sluice being
'shut, the bark or other material in the pit will be laying in steep or in a state
of saturation. Care, however, must be taken on each occasion of applying the
water or liquor to effect, in the manner before described, the displacement
of that liquor which has been laying in contact with the bark.or other
material in the pit, that the workman is cautious to observe the rule before
mentioned, that when the plug or sluice. has been shut, no more water or
:liquor be poured into the pit than will saturate the bark or other material
without causing the same to float. One material will be found by the tanner in
practice to require a longer period for steeping than another, and the third and
subsequent periods of steeping should be twice, and even three times as long
as the first. period of steeping. The tanner can, however, always test the
strength of the liquor which passes out of the opening when the displacement
commences, and if it be weak liquor he can shut the opening, discontinue the
displacement, and let the saturation continue for a more prolonged period.
When the bark or material' becomes exhausted of tannic acid, it. must be
removed and replaced with fresh bark or other material. The tanner, will
.be enabled on pumping up the contents of the well, to use the liquor for, such
purpose as he thinks fit. If found to be of the strength required, it may be
removed into a tanning pit, where it will be: ready for use, to be applied to

page_text: the hides or skins; but if it be desired to increase its strength, the process of
displacement must be carried on with it instead of using plain water, and it
must for this purpose be pumped back into the percolator, and made thereby
to descend progressively through the same bark, or through fresh or other
bark in another spender pit or other spender pits. This pumping up and
progression of the same liquor through the same bark, or through fresh or other
bark in another spender pit or other spender pits, may be repeated until the
extract has attained the desired strength. Hot water may be used instead of
cold.. It will be obvious that various mechanical contrivances and arrange-
ments could be adopted by which the apertures in the false bottom.might be
opened and shut at pleasure without having recourse to the fixing of a close
bottom as above described, and by means of such contrivances, and by great
attention on the part of the workman in throwing on water to the bark or
other tauning materials, as above-mentioned, the quanty of water 'thrown on
each time might be regulated .so as to be sufficient only to saturate the bark
or other materials, and to displace exactly the quanty of the liquid infusion
in contact. with the bark or other materials under treatment. If catechu
or other like extractive matter containing tannic acid is to be employed in the
process of tanning according to my improvements, the same must be
ground to a fine powder, and in that state be well mixed with about equal
weights of sawdust or bark which has been previously exhausted. I recom-
mend exhausted oak bark to be used for this purpose in preference to any
other, because by adopting the displacement process, and using cold water
only, the product of tannic acid from the catechu gives a nearly similar
color to leather to that which is produced by the tannin from oak bark alone.
The reason why I mix the ground catechu with sawdust or bark is on
account of the tendency which catechu has when alone to aglutinate, and
thereby to obstruct the due action of the displacement process. Hot water,
in this case, I recommend to be altogether avoided. "The powdered catechu
and sawdust or bark mixed together having been placed. in the spender pit,
the self-same process, as above described with respect to bark alone, must be
carried on with cold water or cold tanning liquor, as before mentioned, and
may be repeated until the tannic acid has been exhausted from the catechu.
My second improvement in tanning consists in the application of a
recurring vertical downward pressure, or repetition of vertical downward
pressure, at deferred intervals, by means of weights or force upon the hides
or skins, when laying or floating in the ordinary manner in tanning liquor in
pits or tanks, whereby the tanning process is much expedited, with economy
of labour, and without the necessity of tanners purchasing and putting up

page_text: expensive apparatus or machines; or altering materially the construction
of their tan yards, or sowing their hides or skins into bags, and without
handling them to the extent now practised; or exposing them to the air, or
to the chances of being torn, abbraised, or injured. It is well known to
tanners that hides or skins under the process of tanning are frequently taken
out or handled and placed in heaps at the sides of the tanning pits, and
that the object of this in a great measure is to cause the liquor absorbed by
the skins to run out, and that while thus heaped upon each other, the weight
or pressure of the upper hides upon the lower ones forces from the latter
the spent liquor, and renders them more absorbent of the liquor charged
with tannic acid into which they are to be replaced. Mechanical apparatus
has also been employed for giving pressure outside the tanning pits to hides or
skins in the process of tanning; which after being removed from the pits and
squeezed by means of such mechanical apparatus in the open air, are made
again to enter the pits to absorb fresh tanning liquor. Mechanical apparatus
into which skins or hides are placed, or to which they are suspended or hung,
and put in motion by steam power, for constantly and rapidly embracing and
squeezing the skins or hides, has also been attempted; also arrangements
have been adopted for raising and lowering the hides in the pit during the
process of tanning. Such apparatus and arrangements have been either
altogether abandoned or but slightly used by tanners in consequence of
practical difficulties. Now the object of this part of my Invention is to
obviate the objections to the use of such processes for expelling the spent
liquor as hitherto practised, and to introduce simplicity and economy in the
operation of causing the hides and skins to discharge such spent liquor and
-imbibe fresh tannin. The process I pursue for carrying out this part of my
Invention is as follows:-The hides or skins being well grained and colored,
terms well known to tanners,. and being thus prepared for immersion in
tanning liquor to undergo the process of tanning, the same are placed in an
ordinary tanning pit filled with such liquor. The bottom of the tanning pit
'must be constructed so as to bear the pressure hereafter described. Between
each hide or skin a piece of net the size of a hide or skin may be placed
as an interposing medium to prevent the hides or skins sticking together; but
this is not absolutely necessary. Upon the top of the hides or skins a strong
thick flat board, perforated with holes, of about an inch diameter each, must
be placed: This board must be of a size about one inch less in width
and length than the pit into which it is to be placed or used. When
the weight after mentioned is not resting upon this board it will of course float,
and need not be taken out of the pit during the whole time that the hides or

page_text: skins under it are undergoing the process of tanning, as the necessity of
handling them is in my process obviated. Having thus placed the hides or
skins in the pit, and the perforated board above them, they may be allowed to
lay or float in the pit for about an hour; I then lower down upon the board
near its centre a box of wood, or of metal not acted upon by tannic acid,
with a flat bottom, and heavily loaded with weights, shot, or stones. This
box should be allowed to rest upon the top of the board for about a quarter
of an hour and then be removed. The lowering of this box with weights in a
similar manner, and resting for a similar period upon the perforated board,
should afterwards take place about every three hours, or oftener if the tanner
thinks fit, until the skins or hides have been completely tanned. After the
box has been removed each time, the workmen with a blunted hook attached
to a long pole or handle, should lift or stir up the skins at their edges and
corners from top to bottom all round the pit, letting the parts lifted fall back
into their places, so as to accelerate the fresh liquor being imbibed by the
hides or skins. No injury will take place, if during the night the process of
lowering the box be deferred, and if the same be continued only during the
ordinary hours of labour; but if the process be pursued during the night, the
tanning of the hides and skins will be more rapid. When the box has been
lowered down upon the skins, the weight may of course be increased by
putting additional weighty materials into the box, or by placing several boxes
with weights, upon the board. When the liquor becomes weak and ineffective,
it must be renewed or replaced in the usual manner.
I recommend that the ordinary tanning pits should be made less deep than
at present, and that the foundations for the bottom boards be so constructed
that the proposed pressure shall not cause any leakage. I also recommend
that the box should have handles attached to it, approximating to those of a
hand barrow, so as to enable the workmen to carry the box from pit to pit;
and to lower it into each pit in which skins or hides shall have been placed to
be tanned. By lessening the depth of the pits, the heaps of hides or skins
will reach nearly to the top level of the pits. This is desirable as saving
labour with the weights. Care however has to be taken to allow for the
swelling of the hides or skins as the tanning progresses, and also that the
liquor shall not overflow and be wasted when the weight is lowered into
the pit.
Another mode of giving to hides or skins the recurring vertical downward
pressure above mentioned is to use one or more heavy weights or monkeys,
which may be lowered down into the pits by means of tackle placed
over the

page_text: pit's mouth, or by travelling cranes, and be raised again, and be, shifted, or
run from pit. to pit, so that the weight or weights may be lowered thereby
successively upon the floating boards before described.
. Another convenient method of carrying out my last-mentioned improve-
ment, and of applying the recurring vertical downward pressure upon the
hides or skins when immersed in liquor, can be carried out in tan yards where
the tanning has been or shall be conducted in large shallow tanks or vessels
of; many feet in length and breadth, and with the liquor of a depth varying
from twelve to twenty-four inches. In this case the following measures may
be adopted. Place the hides or skins to be tanned in heaps of eight to. twelve
bides or skins upon the floor of the tanks completely immersed in the liquor.
The heaps should be in a row, or, straight line along the length of the floor.
Each heap, should be nearly close to its neighbour heap, and the hides or
skins in each heap, and each row of heaps, should be nearly of the same size.
Boards of three inches in thickness, flat and smooth on, both sides, with the
edges rounded off, and equal in their superfices to the size of the heaps over
which, they are placed,.,and perforated all over with holes, should be con-
structed and so attached by ropes or chains to the walls or sides of the tank,
as to make each board when laying down flat on a heap of hides or skins to
have its side, next the wall parallel to and close to the wall, and as also to
enable the board when pressed down by the roller after mentioned to yield
freely to that pressure. If, for, instance, there be six heaps in a row, there
would be six corresponding boards. The heaps having been made, and the
hides and skins being left for one hour in this state, with the boards either
floating and laying upon them or raised up and supported, it will then be
proper to apply to them the first squeeze or pressure. This is to be effected
by passing very slowly over the boards a heavy roller, like a garden roller
of great weight, and sufficient width to extend nearly across each board; the
roller should rest from five to ten minutes on each board, and be worked
backwards and forwards. There must be room for the roller at each end
of the tank to be put away in a resting place clear of the boards. At the
expiration of three hours, or less if the tanner thinks fit, the roller should
be again put in motion in like manner, and drawn slowly to the opposite end
of the tank, where also it must have a resting place clear of the boards; and
so at intervals the roller should be made to traverse 'slowly from one end
of the tank to the other until the hides or skins have been tanned. During
the night this operation may be suspended, the only consequence being that
that the hides and skins will occupy a longer period to tan than they would if

page_text: squeezed at intervals by night as well as by day: After each operation of
squeezing, the boards may be left floating, or be lifted by the workmen and
raised, and if lifted may in any convenient manner be made to rest against the
wall of the building, or be propped' up and supported. The workmen should
then take hold of the end of each 'skin or hide furthest from the wall, and lift
it gently a few inches high, so as to disorder its position; and thus the fresh
liquor will be more quickly and effectively brought in contact with, the entire
surface of each skin or hide. Attention must be paid to keeping up the
strength of the liquor in the tank, as the hides and skins by this process absorb
the tonnic acid greedily. Provision must be made, which can be done by
any ordinary mechanic, to arrange the coupling of the boards, so that when
the roller is passing over them no tilting shall take place.
Having thus described the nature of my said Invention, and in what
manner the same is to be performed, it will be obvious that various other
arrangements, or variations in the details of the arrangements above described,
may be made for the purpose of carrying out my improvements in tanning,
and the application of the displacement principle, or, the recurring vertical
downward pressure in the tanning of hides. I wish it, therefore, to be under-
stood that I do not claim the exclusive use of such apparatus except when
employed in connection with and for the purpose of the application of the
improvement in tanning by means of the displacement principle and recurring
vertical pressure; but what I claim as the Invention intended to be secured by
the said Letters Patent is,--
First, the improvement in tanning by the adaptation and application of the
process of displacement as herein-before described, for facilitating the extraction
of tannic acid from materials containing it, when such materials are placed in
the ordinary size spender pits of tan yards, and to be used for the tanning of
hides or skins.
Second, the improvement in tanning by a recurring vertical downward
pressure at deferred intervals, or by a repetition of a vertical downward
pressure at deferred intervals, applied by means of perforated boards to hides
or skins, when subject in the ordinary way to tannin liquor in pits or tanks,
as above described.
•
In witness whereof, I, the said John Duncan, have hereunto set my hand
and seal, this Twentieth day of July, in the year of our Lord One
thousand eight hundred and forty-eight.
JNO (L.S.) DUNCAN.

page_text: AND BE IT REMEMBERED, that on the Twentieth day of July, in the
year of our Lord 1848, the aforesaid John-Duncan came before our said
Lady the Queen in Her Chancery, and acknowledged the Specification
aforesaid, and all and every thing therein contained and specified, in form
above written. And also the Specification aforesaid was stamped according to
the tenor of the Statute made for that purpose.
AND BE IT REMEMBERED, that on the Twentieth day of July, in the
year of our Lord 1848, the aforesaid John-Duncan came before our said
Lady the Queen in Her Chancery, and acknowledged the Specification
aforesaid, and all and every thing therein contained and specified, in form
above written. And also the Specification aforesaid was stamped according to
the tenor of the Statute made for that purpose.
Enrolled the Twentieth day of July, in the year of our Lord Ono
thousand eight hundred and forty-eight.