Chapter VI: The Language as Recorded (15)

Cutch, catechu or "dark catechu," is obtained from the wood of Indian
acacias, and is not to be confounded with mangrove cutch. It contains
60 % of tanning matter and a large proportion of catechin similar to
that contained in gambier, but much redder. It is used for dyeing
browns and blacks with chrome and iron mordants.

The willow and the white birch barks contain, respectively, 12 % to 14
% and 2 % to 5 % of tannin. In combination they are used to produce
the famous Russia leather, whose insect-resisting odour is due to the
birch bark. In America this leather is imitated with the American
black birch bark (_Betula lenta_), and also with the oil obtained from
its dry distillation.

In the list of materials two have been placed in a subsidiary class
because they are a mixture of catechol and pyrogallol tannin. Oak bark
produces the best leather known, proving that a blend of the two
classes of tannins gives the best results. It is the bark of the
coppice oak, and contains 12 % to 14 % of a reddish-yellow tannage.
Valonia is the acorn cup of the Turkish and Greek oak. The Smyrna or
Turkish valonia is best, and contains 32 % to 36 % of an almost white
tannin. Greek valonia is greyer in colour, and contains 26 % to 30 %
of tannin. It yields a tough, firm leather of great weight, due to the
rapid deposition of a large amount of bloom.

_Grinding and Leaching[1] Tanning Materials._--At first sight it would
not seem possible that science could direct such a clumsy process as
the grinding of tanning materials, and yet even here, the "scientific
smashing" of tanning materials may mean the difference between profit
and loss to the tanner. In most materials the tannin exists imprisoned
in cells, and is also to some extent free, but with this latter
condition the science of grinding has nothing to do. If tanning
materials are simply broken by a series of clean cuts, only those
cells directly on the surfaces of the cuts will be ready to yield
their tannin; therefore, if materials are ground by cutting, a
proportion of the total tannin is thrown away. Hence it is necessary
to bruise, break and otherwise sever the walls of all the cells
containing the tannin; so that the machine wanted is one which
crushes, twists and cuts the material at the same time, turning it out
of uniform size and with little dust.

The apparatus in most common use is built on the same principle as the
coffee mill, which consists of a series of segmental cutters; as the
bark works down into the smaller cutters of the mill it is twisted and
cut in every direction. This is a very good form of mill, but it
requires a considerable amount of power and works slowly. The teeth
require constant renewal, and should, therefore, be replaceable in
rows, not, as in some forms, cast on the bell. The disintegrator is
another form of mill, which produces its effect by violent concussion,
obtained by the revolution in opposite directions of from four to six
large metal arms fitted with projecting spikes inside a drum, the
faces of which are also fitted with protruding pieces of metal. The
arms make from 2000 to 4000 revolutions per minute. The chief
objection to this apparatus is that it forms much dust, which is
caught in silken bags fitted to gratings in the drum. The myrobalans
crusher, a very useful machine for such materials as myrobalans and
valonia, consists of a pair of toothed rollers above and a pair of
fluted rollers beneath. The material is dropped upon the toothed
rollers first, where it is broken and crushed; then the crushing is
finished and any sharp corners rounded off in the fluted rollers.

It must not be thought that now the material is ground it is
necessarily ready for leaching. This may or may not be so, depending
upon whether the tanner is making light or heavy leathers. If light
leathers are being considered, it is ready for immediate leaching,
i.e. to be infused with water in preparation of a liquor. If heavy
leathers are in process of manufacture, he would be a very wasteful
tanner who would extract his material raw. It must be borne in mind
that when an infusion is made with fresh tanning material, the liquor
begins to deposit decomposition products after standing a day or two,
and the object of the heavy-leather tanner is to get this material
deposited in the leather, to fill the pores, produce weight and make a
firm, tough product. With this end in view he dusts his hides with
this fresh material in the layers, i.e. he spreads a layer between
each hide as it is laid down, so that the strong liquors penetrate and
deposit in the hides. When most of this power to deposit has been
usefully utilized in the layers, then the material (which is now,
perhaps, half spent) is leached. The light-leather maker does not want
a hard, firm leather, but a soft and pliable product; hence he leaches
his material fresh, and does not trouble as to whether the tannin
deposits in the pits or not.

Whether fresh or partially spent material is leached, the process is
carried out in the same way. There are several methods in vogue; the
best method only will be described, viz. the "press leach" system.

The leaching is carried out in a series of six square pits, each
holding about 3 to 4 tons of material. The method depends upon the
fact that when a weak liquor is forced over a stronger one they do not
mix, by reason of the higher specific gravity of the stronger one; the
weaker liquor, therefore, by its weight forces the stronger liquor
downwards, and as the pit in which it is contained is fitted with a
false bottom and side duct running over into the next pit, the
stronger liquor is forced upwards through this duct on to the next
stronger pit. There the process is repeated, until finally the weak
liquor or water, as the case may be, is run off the last vat as a very
strong infusion. As a concrete example let us take the six pits shown
in the figure.

+-------+-------+-------+
| | | |
| 4 | 5 | 6 |
| | | |
+-------+-------+-------+
| | | |
| 3 | 2 | 1 |
| | | |
+-------+-------+-------+

No. 6 is the last vat, and the liquor, which is very strong, is about
to be run off. No. 1 is spent material, over which all six liquors
have passed, the present liquor having been pumped on as fresh water.
The liquor from No. 6 is run off into the pump well, and liquor No. 1
is pumped over No. 2, thus forcing all liquors one forward and leaving
pit No. 1 empty; this pit is now cast and filled with clean fishings
and perhaps a little new material, clean water is then pumped on No.
2, which is now the weakest pit, and all liquors are thus forced
forward one pit more, making No. 1 the strongest pit. After infusing
for some time this is run off to the pump well, and the process
repeated. It may be noted that the hotter the water is pumped on the
weakest pit, the better will the material be spent, and the nearer the
water is to boiling-point the better; in fact, a well-managed tanyard
should have the spent tan down to between 1% and 2% of tannin,
although this material is frequently thrown away containing up to 10%
and sometimes even more. There is a great saving of time and labour in
this method, since the liquors are self-adjusting.

_Testing Tan Liquors._--The methods by which the tanning value of any
substance may be determined are many, but few are at once capable of
simple application and minute accuracy. An old method of ascertaining
the strength of a tan liquor is by means of a hydrometer standardized
against water, and called a barkometer. It consists of a long
graduated stem fixed to a hollow bulb, the opposite end of which is
weighted. It is placed in the liquor, the weighted end sinks to a
certain depth, and the reading is taken on the stem at that point
which touches "water mark." The graduations are such that if the
specific gravity is multiplied by 1000 and then 1000 is subtracted
from the result, the barkometer strength of the liquor is obtained.
Thus 1029 specific gravity equals 29° barkometer. This method affords
no indication of the amount of tannin present, but is useful to the
man who knows his liquors by frequent analysis.

A factor which governs the quality of the leather quite as much as the
tannin itself is the acidity of the liquors. It is known that gallic
and tannic acids form insoluble calcium salts, and all the other acids
present as acetic, propionic, butyric, lactic, formic, &c., form
comparatively soluble salts, so that an easy method of determining
this important factor is as follows:--

Take a quantity, say 100 c.c., of tan liquor, filter till clear
through paper, then pipette 10 c.c. into a small beaker (about 1½ in.
diameter), place it on some printed paper and note how clear the print
appears through the liquor; now gradually add from a burette a clear
solution of saturated lime water until the liquor becomes just cloudy,
that is until it just loses its brilliancy. Now read off the number of
cubic centimetres required in the graduated stem of the burette, and
either read as degrees (counting each c.c. as one degree), to which
practice at once gives a useful signification, or calculate out in
terms of acetic acid per 100 c.c. of liquor, reckoning saturated lime
water as 1/20 normal.

The methods which deal with the actual testing for tannin itself
depend mostly upon one or other of two processes; either the
precipitation of the tannin by means of gelatin, or its absorption by
means of prepared hide. Sir Humphry Davy was the first to propose a
method for analysing tanning materials, and he precipitated the tannin
by means of gelatin in the presence of alum, then dried and weighed
the precipitate, after washing free from excess of reagents. This
method was improved by Stoddart, but cannot lay claim to much
accuracy. Warington and Müller again modified the method, but their
procedure being tedious and difficult to work could not be regarded as
a great advance. Wagner then proposed precipitation by means of the
alkaloids, with special regard to cinchonine sulphate in the presence
of rosaniline acetate as indicator, but this method also proved
useless. After this many metallic precipitants were tried, used
gravimetrically and volumetrically, but without success. The weighing
of precipitated tannates will never succeed, because the tannins are
such a diverse class of substances that each tannin precipitates
different quantities of the precipitants, and some materials contain
two or three different tannins. Then there are also the difficulties
of incomplete precipitation and the precipitation of colouring matter,
&c. Among this class of methods may be mentioned Garland's, in which
tartar emetic and sal ammoniac were employed. It was improved by
Richards and Palmer.

Another class of methods depends upon the destruction of the tannin by
some oxidizing agent, and the estimation of the amount required.
Terreil rendered the tannin alkaline, and after agitating it with a
known quantity of air, estimated the volume of oxygen absorbed. The
method was slow and subject to many sources of error. Commaille
oxidized with a known quantity of iodic acid and estimated the excess
of iodate. This process also was troublesome, besides oxidizing the
gallic acid (as do all the oxidation processes), and entailing a
separate estimation of them after the removal of the tannin. Ferdinand
Jean (1877) titrated alkaline tannin solution with standard iodine,
but the mixture was so dark that the end reaction with starch could
not be seen; in addition the gallic acid had again to be estimated.
Monier proposed permanganate as an oxidizing agent, and Lowenthal made
a very valuable improvement by adding indigo solution to the tannin
solution, which controlled the oxidation and acted as indicator. This
method also required double titration because of the gallic acid
present, the tanning matters being removed from solution by means of
gelatin and acidified salt.

The indirect gravimetric hide-powder method first took form about
1886. It was published in _Der Gerber_ by Simand and Weiss, other
workers being Eitner and Meerkatz. Hammer, Muntz and Ramspacher did
some earlier work on similar lines, depending upon the specific
gravity of solutions. Professor H. R. Procter perfected this method by
packing a bell, similar in shape to a bottomless bottle of about 2 oz.
(liq.) capacity, with the hide-powder, and siphoning the tan liquor up
through the powder and over into a receiver. This deprives the tan
liquor of tannin, and a portion of this non-tannin solution is
evaporated to dryness and weighed till constant; similarly a portion
of the original solution containing non-tannins and tannins is
evaporated and weighed till constant; then the weight of the
non-tannins subtracted from the weight of the non-tannins and tannins
gives the weight of tannin, which is calculated to percentage on
original solutions. This method was adopted as official by the
International Association of Leather Trades Chemists until September
1906, when its faults were vividly brought before them by Gordon
Parker of London and Bennett of Leeds, working in collaboration,
although other but not so complete work had been previously done to
the same end. The main faults of the method were that the hide-powder
absorbed non-tannins, and therefore registered them as tannins, and
the hide-powder was partially soluble. This difficulty has now been
overcome to a large extent in the present official method of the
I.A.L.T.C.

Meanwhile, Parker and Munro Payne proposed a new method of analysis,
the essence of which is as follows:--A definite excess of lime
solution is added to a definite quantity of tannin solution and the
excess of lime estimated; the tan solution is now deprived of tannin
by means of a soluble modification of gelatin, called "collin," and
the process is repeated. Thus we get two sets of figures, viz. total
absorption and acid absorption (i.e. acids other than tan); the latter
subtracted from the former gives tannin absorption, and this is
calculated out in percentage of original liquor. The method failed
theoretically, because a definite molecular weight had to be assumed
for tannins which are all different. There are also several other
objections, but though, like the hide-powder method, it is quite
empirical, it gives exceedingly useful results if the rules for
working are strictly adhered to.

The present official method of the I.A.L.T.C. is a modification of the
American official method, which is in turn a modification of a method
proposed by W. Eitner, of the Vienna Leather Research Station. The
hide-powder is very slightly chrome-tanned with a basic solution of
chromium chloride, 2 grammes of the latter being used per 100 grammes
of hide-powder, and is then washed free from soluble salts and
squeezed to contain 70% of moisture, and is ready for use. This
preliminary chroming does away with the difficulty of the powder being
soluble, by rendering it quite insoluble; it also lessens the tendency
to absorb non-tannins. Such a quantity of this wet powder as contains
6.5 grammes of dry hide is now taken, and water is added until this
quantity contains exactly 20 grammes of moisture, i.e. 26.5 grammes in
all; it is then agitated for 15 minutes with 100 c.c. of the prepared
tannin solution, which is made up to contain tannin within certain
definite limits, in a mechanical rotator, and filtered. Of this
non-tannin solution 50 c.c. is then evaporated to dryness. The same
thing is done with 50 c.c. of original solution containing non-tannins
and tannins, and both residues are weighed. The tannin is thus
determined by difference. The method does all that science can do at
present. The rules for carrying out the analysis are necessarily very
strict. The object in view is that all chemists should get exactly
concordant results, and in this the I.A.L.T.C. has succeeded.

The work done by Wood, Trotman, Procter, Parker and others on the
alkaloidal precipitation of tannin deserves mention.

_Heavy Leathers._--The hides of oxen are received in the tanyard in four different conditions: (1) market or slaughter hides, which, coming direct from the local abattoirs, are soft, moist and covered with dirt and blood; (2) wet salted hides; (3) dry salted hides; (4) sun-dried or "flint" hides--the last three forms being the condition in which the imports of foreign hides are made. The first operation in the tannery is to clean the hides and bring them back as nearly as possible to the flaccid condition in which they left the animal's back. The blood and other matter on market hides must be removed as quickly as possible, the blood being of itself a cause of dark stains and bad grain, and with the other refuse a source of putrefaction. When the hides are sound they are given perhaps two changes of water.

Salted hides need a longer soaking than market hides, as it is not
only essential to remove the salt from the hide, but also necessary to
plump and soften the fibre which has been partially dehydrated and
contracted by the salt. It must also be borne in mind that a 10 %
solution of salt dissolves hide substance, thereby causing an
undesirable loss of weight, and a weak solution prevents plumping,
especially when taken into the limes, and may also cause "buckling,"
which cannot easily be removed in after processes. Dried and dry
salted hides require a much longer soaking than any other variety.
Dried hides are always uncertain, as they may have putrefied before
drying, and also may have been dried at too high a temperature; in the
former case they fall to pieces in the limes, and in the latter case
it is practically impossible to soak them back, unless putrefactive
processes are used, and such are always dangerous and difficult to
work because of the Rivers Pollution Acts. Prolonged soaking in cold
water dissolves a serious amount of hide substance. Soaking in brine
may be advantageous, as it prevents putrefaction to some extent.
Caustic soda, sodium sulphide and sulphurous acid may also be
advantageously employed on account of their softening and antiseptic
action. In treating salted goods, the first wash water should always
be rapidly changed, because, as mentioned, strong salt solutions
dissolve hide; four changes of water should always be given to these
goods.

There are other and mechanical means of softening obstinate material,
viz. by stocking. The American hide mill, or double-acting stocks,
shown diagrammatically in fig. 2, is a popular piece of apparatus, but
the goods should never be subjected to violent mechanical treatment
until soft enough to stand it, else severe grain cracking may result.
Perhaps the use of sodium sulphide or caustic soda in conjunction with
the American wash wheel is the safest method.

Whatever means are used the ultimate object is first to swell and open
up the fibres as much as possible, and secondly to remove putrefactive
refuse and dirt, which if left in is fixed by the lime in the process
of depilation, and causes a dirty buff.

After being thus brought as nearly as possible into a uniform condition, all hides are treated alike. The first operation to which they are subjected is _depilation_, which removes not only the hair but also the scarf skin or epidermis. When the goods are sent to the limes for depilation they are, first of all, placed in an old lime, highly charged with organic matter and bacteria. It is the common belief that the lime causes the hair to loosen and fall out, but this is not so; in fact, pure lime has the opposite effect of tightening the hair. The real cause of the loosening of the hair is that the bacteria in the old lime creep down the hair, enter the _rete Malpighi_ and hair sheath, and attack and decompose the soft cellular structure of the sheath and bulb, also altering the composition of the _rete Malpighi_ by means of which the scarf skin adheres to the true skin. These products of the bacterial action are soluble in lime, and immediately dissolve, leaving the scarf skin and hair unbound and in a condition to leave the skin upon scraping. In this first "green" lime the action is mainly this destructive one, but the goods have yet to be made ready to receive the tan liquor, which they must enter in a plump, open and porous condition. Consequently, the "green" lime is followed with two more, the second being less charged with bacteria, and the third being, if not actually a new one, a very near approach to it; in these two limes the bundles of fibre are gradually softened, split up and distended, causing the hide to swell, the interfibrillar substance is rendered soluble and the whole generally made suitable for transference to the tan liquors. The hide itself is only very slightly soluble; if care is taken, the grease is transformed into an insoluble calcium soap, and the hair is hardly acted upon at all.

The time the goods are in the limes and the method of making new limes depends upon the quality of the leather to be turned out. The harder and tougher the leather required the shorter and fresher the liming. For instance, for sole leather where a hard result is required, the time in the limes would be from 8 to 10 days, and a perfectly fresh top lime would be used, with the addition of sodium sulphide to hasten the process. Every tanner uses a different quantity of lime and sulphide, but a good average quantity is 7 lb. lime per hide and 10-15 lb. sodium sulphide per pit of 100 hides. The lime is slaked with water and the sulphide mixed in during the slaking; if it is added to the pit when the slaking is finished the greater part of its effect is lost, as it does not then enter into the same chemical combinations with the lime, forming polysulphides, as when it is added during the process of slaking.

For softer and more pliable leathers, such as are required for harness and belting, a "lower" or mellower liming is given, and the time in the limes is increased from 9 to 12 days. Some of the old mellow liquor is added to the fresh lime in the making, so as just to take off the sharpness. It would be made up as for sole leather, but with less sulphide or none at all, and then a dozen buckets of an old lime would be added. For lighter leathers from 3 to 6 weeks' liming is given, and a fresh lime is never used.

"Sweating" as a method of depilation is obsolete in England so far as
heavy leathers are concerned. It consists of hanging the goods in a
moist warm room until incipient putrefaction sets in. This first
attacks the more mucous portions, as the _rete Malpighi_, hair bulb
and sheath, and so allows the hair to be removed as before. The method
pulls down the hide, and the putrefaction may go too far, with
disastrous results, but there is much to recommend it for sheepskins
where the wool is the main consideration, the main point being that
while lime entirely destroys wool, this process leaves it intact, only
loosening the roots. It is consequently still much used.

Another method of fellmongering (dewooling) sheepskins is to paint the
flesh side with a cream of lime made with a 10% solution of sodium
sulphide and lay the goods in pile flesh to flesh, taking care that
none of the solution comes in contact with the wool, which is ready
for pulling in from 4 to 8 hours. Although this process may be used
for any kind of skin, it is practically only used for sheep, as if any
other skin is depilated in this manner all plumping effect is lost.
Since this must be obtained in some way, it is an economy of time and
material to place the goods in lime in the first instance.

Sometimes, in the commoner classes of sole leather, the hair is
removed by painting the hair side with cream of lime and sulphide, or
the same effect is produced by drawing the hides through a strong
solution of sulphide; this completely destroys the hair, actually
taking it into solution. But the hair roots remain embedded in the
skin, and for this reason such leather always shows a dirty buff.

Arsenic sulphide (realgar) is slaked with the lime for the production
of the finer light leathers, such as glace kid and glove kid. This
method produces a very smooth grain (the tendency of sodium sulphide
being to make the grain harsh and bold), and is therefore very
suitable for the purpose, but it is very expensive.

Sufficient proof of the fact that it is not the lime which causes
skins to unhair is found in the process of chemical liming patented by
Payne and Pullman. In this process the goods are first treated with
caustic soda and then with calcium chloride; in this manner lime is
formed in the skin by the reaction of the two salts, but still the
hair remains as tight as ever. If this process is to be used for
unhairing and liming effect, the goods must be first subjected to a
putrid soak to loosen the hair, and afterwards limed. Experiments made
by the present writer also prove this theory. A piece of calf skin was
subjected to sterilized lime for several months, at the end of which
time the hair was as tight as ever; then bacterial influence was
introduced, and the skin unhaired in as many days.

After liming it is necessary to unhair the goods. This is done by stretching a hide over a tanner's beam (fig. 3), when with an unhairing knife (a, fig. 4) the beamsman partially scrapes and partially shaves off the hair and epidermis. Another workman, a "flesher," removes the flesh or "net skin" (_panniculus adiposus_), a fatty matter from the flesh side of the skin, with the fleshing knife (two-edged), seen in b, fig. 4. For these operations several machines have been adapted, working mostly with revolving spiral blades or vibrating cutters, under which the hides pass in a fully extended state. Among these may be mentioned the Leidgen unhairer, which works on a rubber bed, which "gives" with the irregularities of the hide, and the Wilson flesher, consisting of a series of knives attached to a revolving belt, and which also "give" in contact with irregularities.

At this stage the hide is divided into several parts, the process being known as "rounding." The object of the division is this: certain parts of the hide termed the "offal" are of less value than the "butt," which consists of the prime part. The grain of the butt is fine and close in texture, whereas the offal grain is loose, coarse and open, and if the offal is placed in the same superior liquors as the butt, being open and porous, it will absorb the best of the tannin first; consequently the offal goes to a set of inferior liquors, often consisting of those through which the butts have passed. The hides are "rounded" with a sharp curved butcher's knife; the divisions are seen in fig. 5. The bellies, cheeks and shoulders constitute the offal, and are tanned separately although the shoulder is not often detached from the butt until the end of the "suspenders," being of slightly better quality than the bellies. The butt is divided into two "bends." This separation is not made until the tanning of the butt is finished, when it is cut in two, and the components sold as "bends," although as often as not the butt is not divided. In America the hides are only split down the ridge of the back, from head to tail, and tanned as hides. Dressing hides are more frequently rounded after tanning, the mode depending on the purpose for which the leather is required.

The next step is to remove as much "scud" and lime as possible, the degree of removal of the latter depending upon the kind of leather to be turned out. "Scudding" consists of working the already unhaired hide over the beam with an unhairing knife with increased pressure, squeezing out the dirt, which is composed of pigment cells, semi-soluble compounds of lime, and hide, hair sacks and soluble hide substance, &c. This exudes as a dirty, milky, viscid liquid, and mechanically brings the lime out with it, but involves a great and undesirable loss of hide substance, heavy leather being sold by weight. This difficulty is now got over by giving the goods an acid bath first, to delime the surface; the acid fixes this soluble hide substance (which is only soluble in alkalies) and hardens it, thus preventing its loss, and the goods may then be scudded clean with safety. The surface of all heavy leathers must be delimed to obtain a good coloured leather, the demand of the present day boot manufacturer; it is also necessary to carry this further with milder leathers than sole, such as harness and belly, &c., as excess of lime causes the leather to crack when finished. Perhaps the best material for this purpose is boracic acid, using about 10 lb. per 100 butts, and suspending the goods. This acid yields a characteristic fine grain, and because of its limited solubility cannot be used in excess. Other acids are also used, such as acetic, lactic, formic, hydrochloric, with varying success. Where the water used is very soft, it is only necessary to wash in water for a few hours, when the butts are ready for tanning, but if the water is hard, the lime is fixed in the hide by the bicarbonates it contains, in the form of carbonate, and the result is somewhat disastrous.

After deliming, the butts are scudded, rinsed through water or weak acid, and go off to the tan pits for tanning proper. Any lime which remains is sufficiently removed by the acidity of the early tan liquors.

The actual tanning now begins, and the operations involved may be divided into a series of three: (1) colouring, (2) handling, (3) laying away.

The colouring pits or "suspenders," perhaps a series of eight pits, consist of liquors ranging from 16° to 40° barkometer, which were once the strongest liquors in the yard, but have gradually worked down, having had some hundreds of hides through them; they now contain very little tannin, and consist mainly of developed acids which neutralize the lime, plump the hide, colour it off, and generally prepare it to receive stronger liquors. The goods are suspended in these pits on poles, which are lifted up and down several times a day to ensure the goods taking an even colour; they are moved one pit forward each day into slightly stronger liquors, and take about from 7 to 18 days to get through the suspender stage.

The reason why the goods are suspended at this stage instead of being
laid flat is that if the latter course were adopted, the hides would
sink and touch one another, and the touch-marks, not being accessible
to the tan liquor, would not colour, and uneven colouring would thus
result; in addition the weight of the top hides would flatten the
lower ones and prevent their plumping, and this condition would be
exceedingly difficult to remedy in the after liquors. Another question
which might occur to the non-technical reader is, why should not the
process be hastened by placing the goods in strong liquors? The reason
is simple. Strong tanning solutions have the effect of "drawing the
grain" of pelt, i.e. contracting the fibres, and causing the leather
to assume a very wrinkled appearance which cannot afterwards be
remedied; at the same time "case tanning" results, i.e. the outside
only gets tanned, leaving the centre still raw hide, and once the
outside is case-hardened it is impossible for the liquor to penetrate
and finish the tanning. This condition being almost irremediable, the
leather would thus be rendered useless.

After the "suspenders" the goods are transferred to a series of "handlers" or "floaters," consisting of, perhaps, a dozen pits containing liquors ranging from 30° to 55° barkometer. These liquors contain an appreciable quantity of both tannin and acid, once formed the "lay-aways," and are destined to constitute the "suspenders." In these pits the goods, having been evenly coloured off, are laid flat, handled every day in the "hinder" (weaker) liquors and shifted forward, perhaps every two days, at the tanner's convenience. The "handling" consists of lifting the butts out of the pit by means of a tanner's hook (fig. 6), piling them on the side of the pit to drain, and returning them to the pit, the top butt in the one handler being returned as the bottom in the next. This operation is continued throughout the process, only, as the hides advance, the necessity for frequent handling decreases. The top two handler pits are sometimes converted into "dusters," i.e. when the hides have advanced to these pits, as each butt is lowered, a small quantity of tanning material is sprinkled on it.

Some tanners, now that the hides are set flat, put them in suspension again before laying away; the method has its advantages, but is not general. The goods are generally laid away immediately. The layer liquors consist of leached liquors from the fishings, strengthened with either chestnut or oakwood extract, or a mixture of the two. The first layer is made up to, say, 60° barkometer in this way, and as the hides are laid down they are sprinkled with fresh tanning material, and remain undisturbed for about one week. The second layer is a 70° barkometer liquor, the hides are again sprinkled and allowed to lie for perhaps two weeks. The third may be 80° barkometer and the fourth 90°, the goods being "dusted" as before, and lying undisturbed for perhaps three or four weeks respectively. Some tanners give more layers, and some give less, some more or less time, or greater or lesser strengths of liquor, but this tannage is a typical modern one.

As regards "dusting" material, for mellow leather, mellow materials are required, such as myrobalans being the mellowest and mimosa bark the most astringent of those used in this connexion. For harder leather, as sole leather, a much smaller quantity of myrobalans is used, if any at all, a fair quantity of mimosa bark as a medium, and much valonia, which deposits a large amount of bloom, and is of great astringency. About 3 to 4 cwt. of a judicious mixture is used for each pit, the mellower material predominating in the earlier liquors and the most astringent in the later liquors.

The tanning is now finished, and the goods are handled out of the pits, brushed free from dusting material, washed up in weak liquor, piled and allowed to drip for 2 or 3 days so that the tan may become set.

_Finishing._--From this stage the treatment of sole leather differs from that of harness, belting and mellower leathers. As regards the first, it will be found on looking at the dripping pile of leather that each butt is covered with a fawn-coloured deposit, known technically as "bloom"; this disguises the under colour of the leather, just like a coat of paint. The theory of the formation of this bloom is this. Strong solutions of tannin, such as are formed between the hides from dusting materials, are not able to exist for long without decomposition, and consequently the tannin begins to condense, and forms other acids and insoluble anhydrides; this insoluble matter separates in and on the leather, giving weight, firmness, and rendering the leather waterproof. It is known technically as bloom and chemically as ellagic acid.

After dripping, the goods are scoured free from surface bloom in a
Wilson scouring machine, and are then ready for bleaching. There are
several methods by which this is effected, or, more correctly several
materials or mixtures are used, the method of application being the
same, viz. the goods are "vatted" (steeped) for some hours in the
bleaching mixture at a temperature of 110° F. The mixture may consist
of either sumach and a light-coloured chestnut extract made to 110°
barkometer, and 110° F., or some bleaching extract made for the
purpose, consisting of bisulphited liquid quebracho, which bleaches by
reason of the free sulphurous acid it contains. The former method is
best (though more expensive), as it removes less weight, and the light
shade of colour is more permanent than that obtained by using
bisulphited extracts.

After the first vatting the goods are laid up in pile to drip;
meanwhile the liquor is again heated, and they are then returned for
another twenty-four hours, again removed and allowed to drip for 2 to
3 days, after which they are oiled with cod oil on the grain and hung
up in the sheds to dry in the dark. When they have dried to an
india-rubber-like condition, they are piled and allowed to heat
slightly until a greyish "bloom" rises to the surface, they are then
set out and stretched in a Wilson scouring machine; using brass
slickers instead of the stone ones used for scouring, "pinned" over by
hand (with the three-edged instrument seen in c, fig. 4, and known as
a "pin") to remove any bloom not removed by the machine, oiled and
dried. When of a damp even colour they are "rolled on" between two
heavy rollers like a wringing machine, the pressure being applied from
above, hung up in the dark sheds again until the uneven colour so
produced has dried in, and then "rolled off" through the same machine,
the pressure being applied from below. They are now dried right out,
brushed on the grain to produce a slight gloss, and are finished.

As regards the finishing of harness leather, &c., the goods, after thorough dripping for a day or two, are brushed, lightly scoured, washed up in hot sumach and extract to improve the colour, and are again laid up in pile for two days; they are then given a good coat of cod oil, sent to the sheds, and dried right out. Only sufficient scouring is given to clean the goods, the object of the tanner being to leave as much weight in as possible, although all this superfluous tan has to be washed out by the currier before he can proceed.

_Currying._--When the goods are dried from the sheds they are purchased by the currier. If, as is often the case, the tanner is his own currier, he does not tan the goods so heavily, or trouble about adding superfluous weight, but otherwise the after processes, the art of the currier, are the same.

Currying consists of working oil and grease into the leather to render it pliable and increase its strength. It was once thought that this was a mere physical effect produced by the oil, but such is not the case. Currying with animal oils is a second tannage in itself; the oils oxidize in the fibres and produce aldehydes, which are well-known tanning agents; and this double tannage renders the leather very strong. Then there is the lubricating effect, a very important physical action so far as the strength of the leather is concerned. Mineral oils are much used, but they do not oxidize to aldehydes, or, for the matter of that, to anything else, as they are not subject to decomposition. They, therefore, produce no second tannage, and their action is merely the physical one of lubrication, and this is only more or less temporary, as, except in the case of the heavier greases, they slowly evaporate. Where animal fats and oils are used, the longer the goods are left in contact with the grease the better and stronger will be the leather.

In the "Einbrennen" process (German for "burning in"), the hides are thoroughly scoured, and when dry are dipped into hot grease, which is then allowed to cool; when it is nearly set the goods are removed and set out. This process is not much used in Great Britain.

In hand-stuffing belting butts the goods are first thoroughly soaked in water to which has been added some soda, and then scoured and stretched by machine. They are then lightly shaved, to take off the loose flesh and thin the neck. The whole of the mechanically deposited tannin is removed by scouring, to make room for the grease, and they are then put into a sumach vat of 40° barkometer to brighten the colour, horsed up to drip, and set out. If any loading, to produce fictitious weight, is to be done, it is done now, by brushing the solution of either epsom salts, barium chloride or glucose, or a mixture, into the flesh, and laying away in pile for some days to allow of absorption, when, perhaps, another coat is given. Whether this is done or not, the goods are hung up until "tempered" (denoting a certain degree of dryness), and then treated with dubbin. This is manufactured by melting down tallow in a steam-jacketed pan, and adding cod oil, the mixture being stirred continually; when quite clear, it is cooled as rapidly as possible by running cold water through the steam pan, the stirring being continued until it has set. The tempered leather having been set out on a glass table, to which the flesh side adheres, is given a thin coat of the dubbin on the grain, turned, set out on the flesh, and given a thick coat of dubbin. Then it is hung up in a wind shed, and as the moisture dries out the grease goes in. After two or three days the goods are "set out in grease" with a brass slicker, given a coat of dubbin on the grain slightly thicker than the first coat, then flesh dubbined, a slightly thinner coat being applied than at first, and stoved at 70° F. The grease which is slicked off when "setting out in grease" is collected and sold. After hanging in the warm stove for 2 or 3 days the butts are laid away in grease for a month; they are then slicked out tight, flesh and grain, and buck tallowed. Hard tallow is first rubbed on the grain, when a slight polish is induced by rubbing with the smoothed rounded edge of a thick slab of glass; they are then hung up in the stove or stretched in frames to dry. A great deal of stuffing is now carried out by drumming the goods in hot hard fats in previously heated drums; and in modern times the tedious process of laying away in grease for a month is either left undone altogether or very considerably shortened.

In the tanning and dressing of the commoner varieties of kips and dried hides, the materials used are of a poorer quality, and the time taken for all processes is cut down, so that whereas the time taken to dress the better class of leather is from 7 to 10 months, and in a few cases more, these cheaper goods are turned out in from 3½ to 5 months.

A considerable quantity of the leather which reaches England, such as East India tanned kips, Australian sides, &c., is bought up and retanned, being sold then as a much better-class leather. The first operation with such goods is to "strip" them of any grease they may contain, and part of their original tannage. This is effectually carried out by first soaking them thoroughly, laying them up to drip, and drumming for half an hour in a weak solution of soda; they are then washed by drumming in plenty of water, the water is run off and replaced by very weak sulphuric acid to neutralize any remaining soda; this is in turn run off and replaced by weak tan liquor, and the goods are so tanned by drumming for some days in a liquor of gradually increasing strength. The liquor is made up as cheaply as possible with plenty of solid quebracho and other cheap extract, which is dried in with, perhaps, glucose, epsom salts, &c. to produce weight. Sometimes a better tannage is given to goods of fair quality, in which they are, perhaps, started in the drum and finished in layers, slightly better materials being used all through, and a longer time taken to complete the tannage.

The tannage of dressing hides for bag and portmanteau work is rather different from the other varieties described, in that the goods, after having had a rather longer liming, are "bated" or "puered."

Bating consists of placing the goods in a wheel or paddle with hen or
pigeon excrement, and paddling for from a few hours to 2 or 3 days. In
puering, dog manure is used, and this being rather more active, the
process does not take so long. This bating or puering is carried out
in warm liquors, and the actions involved are several. From a
practical point of view the action is the removal of the lime and the
solution of the hair sacs and a certain amount of interfibrillar
substance. In this way the goods are pulled down to a soft flaccid
condition, which allows of the removal of short hair, hair sacs and
other filth by scudding with an unhairing knife upon the beam. The
lime is partially taken into solution and partially removed
mechanically during the scudding. A large quantity of hide substance,
semi-soluble and soluble, is lost by being pressed out, but this
matters little, as for dressing work, area, and not weight, is the
main consideration. Theoretically the action is due to bacteria and
bacterial products (organized ferments and enzymes), unorganized
ferments or vegetable ferments like the yeast ferment, such as
pancreadine, pepsin, &c. and chemicals, such as ammonium and calcium
salts and phosphates, all of which are present in the manure. The
evolved gases also play their part in the action.

There are several bates upon the market as substitutes for dung bate.
A most popular one was the American "Tiffany" bate, made by keeping a
weak glue solution warm for some hours and then introducing a piece of
blue cheese to start fermentation; when fermenting, glucose was added,
and the bate was then ready for work. This and all other bates have
been more or less supplanted by "erodin," discovered after years of
research by Mr Wood (Nottingham) and Drs Popp and Becker (Vienna).
This is an artificial bate, containing the main constituents of the
dung bate. It is supplied in the form of a bag of nutrient material
for bacteria to thrive on and a bottle of bacterial culture. The
nutrient material is dissolved in water and the bacterial culture
added, and after allowing the mixture to get working it is ready for
use. Many tons of this bate are now being used per annum. Its
advantages are: (1) that it is clean, (2) that it is under perfect
control, and (3) that stains and bate burns, which so often accompany
the dung bate, are absolutely absent. Bate burns are caused by not
filtering the dung bate through coarse sacking before use. The
accumulation of useless solid matter settles on the skins if they are
not kept well in motion, causing excessive action in these places.

After pulling down the goods to a soft, silky condition by bating or puering, it is necessary, after scudding, to plump them up again and bring them into a clean and fit condition for receiving the tan. This is done by "drenching" in a bran drench. A quantity of bran is scalded and allowed to ferment. When the fermentation has reached the proper stage the goods are placed, together with the bran liquor, in a suitable pit or vat, and are allowed to remain until they have risen three times; this rising to the surface is caused by the gaseous products of the fermentation being caught by the skin. The plumping action of the bran is due to the acids produced during fermentation and also in part to the gases, and the cleansing action is due to the mechanical action of the particles of bran rubbing against the grain of the skins. After drenching, the goods are washed free from bran, and are ready for the tanning process.

Drenching, now that all kinds of acids are available, is not so much
used for heavy hides as for light skins, it being found much more
convenient and cheaper to use acids. In fact, bating and puering are
being gradually replaced by acid baths in the case of heavy leathers,
the process being carried out as deliming for sole leather, only much
more thoroughly in the case of dressing leather.

The tanning of dressing hides, which are not rounded into butts and offal, is briefly as follows. They first enter a series of colouring pits or suspenders, and then a series of handlers, by which time they should be plump and coloured through; in this condition they are split either by means of a union or band-knife splitting machine (fig. 7).

This latter is the most popular machine, and consists essentially of
an endless band knife _a_, which revolves at considerable speed with
its cutting edges close to the sides of a pair of rollers through
which the leather is fed and pressed against the knife. The lower of
these rollers is made of short segments or rings, each separately
capable of yielding so as to accommodate itself to the unequal
thicknesses of various parts of a hide. The thickness of the leather
to be cut is gauged to the utmost minuteness by means of the hand
screws _b b_ which raise or lower the upper roller. The knife edge of
the cutter is kept keen by rubbing against revolving emery wheels _c_
as it passes round. So delicately can this machine effect its work
that slices of leather uniform throughout and as thin as paper can be
easily prepared by it, and by its aid it is quite common to split
hides into as many as three useful splits.

The dressing hides are usually split in two. Here we will leave the split (flesh) for a time and continue with the treatment of the grain. After splitting, they enter another series of handlers, are then piled up for a day or two, and thrown into a large drum with sumach mixed to a paste with hot water and a light-coloured extract. They are drummed in this for one hour to brighten and mellow the grain, washed up in tepid liquor, piled for two days, and drummed with cod oil or some other suitable oil or mixture; they are now piled for a day or two to absorb, dried out, flattened on the grain, and flesh folded.

The splits are rinsed up in old sumach liquor and drummed with cheap extracts and adulterants, such as size, glucose, barium chloride, epsom salts, &c. after which they are piled up to drain, dried to a "sammied" condition, rolled to make firm, and dried right out.

In the dressing hide tannage very mellow materials are used. Gambier
and myrobalans form the main body of the tannage, together with a
little quebracho extract, mimosa bark, sumach and extracts.

_Upper Leather._--Under the head of upper leather are included the thin, soft and pliable leathers, which find their principal, but by no means exclusive, application in making the uppers of boots and shoes, which may be taken as a type of a class of leathers. They are made from such skins as East Indian kips, light cow and horse hides, thin split hides, such as those described under dressing leather, but split rather thinner, and calf. The preparatory dressing of such skins and the tanning operations do not differ essentially from those already described. In proportion to the thinness of the skin treated, the processes are more rapidly finished and less complex, the tannage is a little lighter, heavy materials such as valonia being used sparsely if at all. Generally speaking, the goods have a longer and mellower liming and bating, the lime being more thoroughly removed than for the leathers previously described, to produce greater pliability, and everything must tend in this direction. The heavier hides and kips are split as described under dressing leather, and then tanned right out.

_Currying of the Lighter Leathers._--The duty of the currier is not solely directed towards heavier leathers; he is also entrusted with the dressing and fitting of the lighter leathers for the shoemaker, coachbuilder, saddler, &c. He has to pare the leather down and reduce inequalities in thickness, to impregnate it with fatty matter in order to render it soft and pliable, and to give it such a surface dressing, colour and finish as will please the eye and suit the purposes of its consumers. The fact that machinery is used by some curriers for nearly every mechanical operation, while others adhere to the manual system, renders it almost impossible to give in brief an outline of operations which will be consistent with any considerable number of curriers.