Tanning Process

Penrith City Library Photographic Collection

(Extract from Birmingham, et al, and Australian Pioneer Technology)

“Tanning is the preparation of hides (skins of cattle or horses) or skins (skins of sheep, goats, pigs, seals) for uses ranging from boot soles and heavy harnesses (the hardest leathers) to soft harness and boot uppers (soft leathers), together with many more varied uses. It is a chemical process in which the gelatinous part of the skin (corium) is permanently changed by impregnation with tannic acid, a substance found in the bark of a number of trees of which the black wattle, abundantly found near early settlements, turned out to be particularly rich…

There are numerous nineteenth century descriptions of the stages in the process of vegetable tanning, with few or minor variations. Hides were usually cured by air-drying or by salting before arrival, so they were first soaked in the water pit for softening or removal of salt. Then followed the preliminary stages: soaking in three successively stronger solutions of lime to allow the removal of the hair with the two-handled tanner’s knife over the beam or tanneries work-horse; then the removal of the flesh from the other side of the hide with a very sharp fleshing knife. De-liming followed, done by squeezing with a scudding-knife…. thinner skins were not scudded, but thrown into three mastering pits at the back of the beam house where the lime was dissolved by unattractive mixtures called ‘bates’ (hen or pigeon dung and ‘cold water) or ‘drenches’ (dog excreta and warm water). After washing and cutting (or rounding’) hides and skins so that the thickest and thinnest parts of each could be treated in thick or thin batches, they were ready for actual tanning.

The tanning liquor was made by grinding wattle bark in a stone mill… and then soaking it in cold water in what were called leaching pits. Hides now passed through a series of pits with successively strong liquor. In the first pits (the ‘suspenders’) they were suspended in a weak solution; in the next group of pits (the ‘floaters’ or ‘handlers’) they were floated; and then in the last group (the ‘layers’) they were laid away or stacked with alternating layers of bark.”(1 )

Description of the Tanning Process Used at  Andrew Thompson’s Tannery,  St Marys 1908

(Derived from an article in the Australasian Leather Trades Review, 2 March 1908)

Soaking the hides
“When green salted hides are received at the tannery the preparatory process in converting them into leather is to soak them in water before removing the hair or commencing the tanning process. This soaking is carried out with the object of thoroughly softening the hides and extracting salt, dirt and blood. In doing so every care is taken to prevent the substance of the hide from losing its weight from remaining too long in the water, which, of course, would result in the hide becoming flaccid…. A loss of gelatine is often caused by soaking green hides too long, and as insufficient soaking is apt to leave marks on the finished leather, it can be seen that constant care is necessary in this preliminary process. After the hides have remained for a number of hours in clean water they are then either taken from the pits, into which fresh water is run onto the hides, or hung over sticks and suspended in the water. The latter method is followed by most tanners, because it is then an easy matter to drain off the water and replace it by fresh. Changing the water depends on the condition of the hides, those that do not carry much foreign substance only requiring one change of water. An advantage of frequently changing the water is the prevention of an accumulation of salt and dirt in the soaks. The quality of water used is a matter of equal importance, as different results are obtained from different qualities of water. Many tanners contend that hard water affects the substance of the hides owing to the presence of salts of lime and magnesia and to remedy this and soften it about five pounds of borax, dissolved in hot water, are poured into the soak pit before putting in the hides. Sulphide of sodium and boric acid are also used for the prevention of decomposition and loss of weight. The general rule is to soak hides of ordinary substance and condition from twenty-four to forty-eight hours, and the necessity of getting rid of all blood, salt and dirt is recognised by up-to-date tanners, who realise that if putrefaction (sic) injures the hide in the slightest degree an unpleasant odour clings to the finished leather.” (2)

“When dry hides are used by tanners they require very careful treatment. Through being dried in the raw state they are necessarily almost waterproof, and being very thick and, heavy it is difficult to thoroughly soften them. ‘Decay sometimes sets in before the hides are thoroughly dry, and although this fault may not be noticed when they are purchased, the putrification readily becomes apparent when during the soaking process, every spot that was not perfectly cured revealing itself by the grain peeling. To hasten the softening of dry hides so as to obviate the necessity of long soaking, which results in the loss of much of the hide gelatine, it is necessary to use borax and sulphide of sodium, two to five pounds of the former being taken for each one thousand gallons of water. Sulphide of sodium being rapid in its action and comparatively cheap, is recommended as the best softener that can be used, the quantities in this case being the same as with borax. After dry hides are soaked and softened they are further prepared for tanning, and handled in much the same manner as salted hides are treated.” (3)

The Liming Process
“After the soaking process is completed, the next stage is to remove the hides to the limes, after they have been drained, fleshed and split. In making good lime the scientific knowledge of the tanner comes into play, and varying quantities of lime and arsenic are used, according to the leather manufacturer’s requirements in making different classes of leather. Liming is conducted with the object of swelling and distending the hide fibres, so as to loosen the hair roots, and to dissolve the perishable animal matter of the hides, in order to remove it before tanning. Where hides are intended for soft pliable leather, the animal matter is dissolved quicker than the swelling of the fibres, the result being a loose, spongy leather. Insufficient liming gives a hard, firm leather.” (4)

The Bating Process
“After leaving the lime pits the next stage is to rid the hides of all the lime and alkaline sulphides assimilated by the raw material during the liming process, and this is secured by what is termed bating”, the object being to put the hide into the necessary stage for accepting the tannage. Unless all the lime is removed it is exceedingly difficult to get even-colored leather. Manures were formerly invariably used in this stage of tanning, but the unpleasantness and attendant dangers of using that material have resulted in the use of chemical bates, which possess th6 advantages of cleanliness, safety and cheapness. Lactic acid is held to be one of the best articles that can be used in bating. To apply it the method generally followed is to wash the hides in warm water after taking them from the limes, and place them in a paddle vat containing water heated to about 95 degrees Fah. One gallon of lactic acid is used to each one hundred gallons of water. After being paddled about in the liquor for one or two hours the sides are in a proper condition for tanning. Splitting the hides after liming necessitates the use of less bating than when they are not split until after being tanned. A decided advantage accruing from lactic acid is the fact that it dissolves the lime without affecting the tissues or fibres of the hides. A method followed by many tanners is to use solutions of caustic acid and calcium chloride, the combination of the two chemicals forming calcium hydrate, or lime and salt, in the interior of the hide. By this process it is claimed that quick tanning is obtained, four hours sufficing to do what required ten days under the old process.” (5)

Fleshing and Unhairing
“When the hair has been loosened the hides are dehaired on a beam and placed in clean water for five or six days, prepatory to being put on the beams to be scudded. This treatment is sometimes carried out with specially prepared scrapers, or shaving knives, which remove all the short hair or other matter in the grain. That is the method followed in many tanneries, but when a yard is fitted with the latest machinery…. modern mechanical aids are requisitioned …. for tanners operating on anything over three hundred hides per week (a fleshing and unhairing machine) means a considerable saving in labour.” (6)

The Tanning Process
“When the hides have reached this stage of the beam house treatment, they are placed in water pits for further cleansing, and afterwards suspended on poles in the first lot of tanning pits, called “handlers”, from the fact that the liquid is constantly being agitated. After remaining there for three or four days they are transferred to other pits, containing a mixture of Mimosa bark, myrabolams and valonia. Ten to twelve days are allowed for this stage of tanning, after which the hides are treated with a stronger mixture of the same composition for three or four weeks, and a third handling is obtained in a still stronger liquor for a similar period. After being sorted out, and washed in a hot bath of valonia and myrabolams, they are placed in the drying room to drain, preparatory to being placed under a striking machine…. This machine consists of a large wheel running over -a solid table, the sides of the leather being placed under the wheel, which makes the leather firm, removes the creases, and imparts a good finish. The sides are then thoroughly oiled and hung up. The succeeding stage is the treatment at the rolling machine… the process in this department being on the same principle as that followed in connection with the striking machine, except that a steel roller is used instead of the wheel. Like the striker it works on a solid metal table, of brass mountings, and sloping away on either side of the roller is a modern table, which enables the operator to adjust the side as required in shifting the different sections under the roller… A second application by the roller after the side is three-parts dry makes it firm and solid, in readiness to be folded on a table. The final stage in the preparation of sole leather is commenced in the hot air chamber where the sides are hung up until thoroughly dry, after which they are allowed to stand for two days, and are then pressed, forty sides to the bale, for delivery in Sydney, or consignment to London.” (7)

1. Birmingham, et al, and Australian Pioneer Technology, p 145-9
2. The Australasian Leather Trades Review, 2 March 1908, p 67, 70-71
3. The Australasian Leather Trades Review, 2 March 1908, P 71-2
4. The Australasian Leather Trades Review, 2 March 1908, P 71
5. The Australasian Leather Trades Review, 2 March 1908, P 71
6. The Australasian Leather Trades Review, 2 March 1908, P 72
7. The Australasian Leather Trades Review, 2 March 1908, P 72