Section 1020: tanneries
This publication is intended for Valuation Officers. It may contain links to internal resources that are not available through this version.
This category is subject to co-ordination in accordance with the relevant practice note.
Tanneries specialise in different grades of leather, ie heavy (sole leather and other) and light (upper leather, lining leather and other).
A full description of the process is set out below and may be of assistance to Local Offices.
2.2 Description of the tanning process
2.2.1 In the trade the term “hides” applies only to the pelts of larger beasts such as cattle, horse and buffalo and “skins” to those of smaller animals such as calf, goat, sheep and pig. There is an intermediate size of cattle pelt between that of a calf and a full grown animal known as a “kip”. Sheep skins follow an ancient custom in that they reach the tanner via the fellmonger, whose job it is to separate the wool from the pelt and to distribute both products in their respective markets.
2.2.2 Leather manufacture may be divided into three main stages:- a. Preparation for tanning
The majority of hides and skins are imported from abroad and reach the tanner having travelled distances which involve journeys of anything from a week to several months after slaughter. To enable this to be done without putrefying, hides and skins must be preserved or “cured”. The principal methods are drying, dry-salting, wet-salting or salting followed by drying. In preparation for tanning the first process is soaking to remove dung, earth and albuminous matter. In the case of fresh hides and skins and salted skins, this is a simple operation consisting of soaking for a few hours in one or two changes of cold water. In the case of dried skins however the skins are treated with alkaline chemicals such as caustic soda or sodium sulphide.
The skins as received by the tanner have three zones:-
i. the outer portion of epidermis;
ii. the middle portion of corium;
iii. the under layer of adipose tissue or flesh.
The epidermal system, including the hair and wool, nails, scales, horns and hoofs, together with the epidermis which is very thin, is removed in the early stages of leather manufacture; the linings, the hair follicles, the fat glands and fragments of hair or wool which tend to remain behind, are removed by scudding. This is done by first “liming” the hides and skins, that is they are immersed in liquors prepared with slaked lime “sharpened” with sodium sulphide or similar chemicals to hasten the action. The lime liquors eat away the hair roots and lower layers of the epidermis so that both can be removed by scraping. In former days unhairing would be done by throwing the skins over a sloping beam and scraping off the hair manually with a special blunt tool, but now machines with special blades on rollers have almost entirely replaced the hand method.
In the case of sheep skins, where it is desired to preserve the wool, a different method is necessary. The skin is painted on the flesh side with a thick paste of lime and sodium sulphide. The chemicals penetrate to the roots in a few hours and the wool can then be pulled off by hand.
After unhairing or de-wooling, the skins are “fleshed”, that is to say the adipose tissue is cut away. Hand fleshing, still employed to some extent for high class work, especially sole leather, was carried out over a beam with a special two-handled knife. Hand fleshing has now been largely replaced by machinery. After removal of hair or wool and fleshing, the hides and skins go back into the limes for a further period during which the fibrils become separated through the removal of albuminous matter and also “plumped up” by water absorption; this is desirable to ensure proper penetration by the tanning materials. Liming has in the past been carried out in pits, the pelts being regularly hauled out, piled and returned. With hides, liming is sometimes speeded up by suspending them in the liquors and rocking them; still speedier action is occasionally obtained by liming in revolving drums.
Skins are commonly limed in paddles, that is semi-cylindrical vats fitted with paddle wheels to keep the liquors and skins in motion. Up to the present point, which is known as the “white pelt” or simply “pelt” stage, all types of hides and skins receive much the same treatment, but henceforward treatment varies according to the type of pelt and the kind of leather required.
Where hides are required for bag, case or upholstery work, it is usually at this stage that they are split (in the lime) although in some cases this is delayed until tanning is completed. The splitting is accomplished by the band knife splitting machine. In this machine the hide is pushed edge on against a rapidly rotating endless band knife. Of such precisions is this machine that the hide or skin can be split into two or more layers from the thickness of tissue paper upwards. The top layer, known technically as the “grain split” forms what is generally known as a “split leather” whilst the under layers produce “leather split” or simply “split”.
In the case of light leathers (skins) the lime is usually completely removed before tannage. This is achieved by water washing and acid treatment. But where softness or stretchiness and a smooth grain are desired the skins are subjected to a “bating” or “puering” process. “Bating” consists of treating the skins with artificial or synthetic bates which contain sal ammoniac. After “bating” occurs scudding which removes the epidermal system by lining and scraping. “Drenching” may follow and consist in treating the skins with a mildly acid liquor produced by fermenting bran; the object is to remove the last traces of lime and scud. A further preparatory process employed in certain cases is “pickling”, that is drumming in a solution of sulphuric acid and common salt, the object being to attain a uniform degree of acidity to facilitate tanning and also to effect a thorough cleansing. After washing in clear water the hides and skins are now ready for tanning.
The purpose of tanning is to bring about chemical changes which will render the hides and skins imputrescible and water resistant whilst preserving the fibrous structure from which ultimate strength and flexibility are derived. The usual methods of tanning may be considered under five headings:-
i. Vegetable tanning- is achieved through the agency of water infusions of tannin or “tannic acid” obtained from certain barks, leaves, woods, or nuts. This tanning is used for the bulk of sole, belting and harness leather, for bag, case and upholstery leathers, and for the majority of light leathers for fancy leather work. Oak bark tannin applied to sole leather was a long process, occupying up to fifteen months or more. Formerly tanners prepared their tanning infusions by steeping the ground material in warm water; this process is known as “leaching”. The resultant liquors were weak and tanned slowly. Today the tendency is for tanners to drop leaching in favour of manufactured extracts. These concentrated extracts come in the form of viscous liquors (30 per cent tannin) or toffee-like extracts (60 per cent tannin); their use, enabling the tanner to prepare stronger liquors, has greatly speeded up tanning. Since the beginning of this century, artificial tannins have been discovered and used alone or in conjunction with natural materials, they facilitate speedy tannage.
Formerly all tannage, both of hides and skins, was carried out by steeping in pits full of liquor but this was a slow process. Pit tannage is still largely used for sole, belting and harness leathers although of recent years drumming at certain stages has become common. The revolving drums are fitted with pegs or shelves which pick up the hides and drop them again more or less violently into the liquor. Whilst satisfactory for thin hides and skins, for thick leathers the quality probably suffers owing to the violent mechanical action. For such leathers a more satisfactory method of speeding up without danger of damage is by a “rocker” system in which the hides are suspended in pits on frames which are gently rocked by mechanical action. Light leathers such as calf, sheep and goat skin are almost exclusively tanned in the drum.
ii. Mineral tannage - is done with the salts of certain minerals, of which the most important are those of chromium and aluminium.
The first mineral tannage was undoubtedly the alum process which is still in use today. Chrome tannage is generally much quicker than vegetable tannage. Skins such as calf, goat and sheep are tanned by drumming in the chrome salt solution the strength of which is gradually increased and tannage is completed in a few hours. Hides for sole or belting leathers may be tanned by suspension in the pits, the hides being gradually shifted into stronger and stronger liquors.
iii. Aldehyde tannage - is done with formaldehyde or formalin, well-known as disinfectant and preservative. The leather is snow white, is washable and is principally used for gloves.
iv. Oil tannage - is achieved by the use of certain fish oils, of which cod oil is the principal, which have the power of oxydizing. It is the products of oxydation which are responsible for the tannage. The process is known as “shamoying” from the fact that it was formerly used for chamois skins. These are rarely used today, “chamois” and wash leathers being made from sheep skin splits.
v. Combination tannage - e.g. mineral and vegetable or two mineral tannages.
a. Dressing and finishing
The leather now has to be dressed and finished according to its ultimate purpose.
In the case of sole leather the finishing processes or “shed work” are carried out in the same factory as tannage. A simplified form of shed work is as follows:-
After coming from the layers or steeping pits, the butts, or tanned hides, are piled up to drain; they are then scoured by machine to clean out the grain and rinsed through a weak tan liquor. After drainage again they are slightly oiled on the grain and slowly dried to a semi-dry condition. In this state they are “set out” to remove wrinkles and smooth the grain. After again oiling they are further dried. When nearly dry they are heavily rolled to press the fibres together, are dried a little more and again rolled. Finally they are completely dried.
The finishing of chrome sole leather is simpler and consists of treatment with borax to render the leather nearly neutral, washing, drying and then impregnation or “stuffing” with a mixture of paraffin wax or stearine and rosin. This makes the leather thoroughly waterproof.
Many vegetable tanned leathers are finished by “currying” - for example, harness, belting and mechanical leathers, upper leathers for special purposes where waterproofness is essential and welting. The characteristic feature of currying is the introduction of grease into the leather; the final product may contain over 30 per cent of grease.
As regards the light leathers, the finishing of glazed kid and box calf is always carried out by the tanner. One reason for this is that rough tanned chrome leathers, if once dried up, cannot be easily wetted back properly for finishing.
Radical changes in drying techniques have been perfected. Tunnel drying, in which hides and skins move continuously through an enclosed tunnel under controlled heat and humidity, are now used. Uniform rates of evaporation are thereby achieved with the final condition of leather pre-determined.
3. Survey requirements
3.1 Basis of Measurement
As with other industrial hereditaments tanneries should be measured to Gross Internal Area (GIA). Care should be taken in describing the various buildings, particularly those of a specialist nature, for example buildings containing the liming and tanning pits and buildings which have been used for a considerable time for treatment involving the use of salt or other chemicals which have tended to foul the buildings.
The survey should also highlight the advantages and disadvantages of the site, particularly with regard to the factors affecting the choice of the site (see para 4 below).
3.2 Plant & Machinery
It is not usual to find much rateable plant and machinery within a tannery. However, for the purpose of the supply of water, treatment and disposal of effluent and of heat and power some plant and machinery would be required which would be rateable under Valuation for Rating (Plant and Machinery) Regulations. Items of plant rateable under Class 4 would include any pits. For guidance in this subject, reference should be made to RM 4:3.
4. Factors affecting choice of site
Pure water is needed in very large quantities in the tanning industry, which accounts for the fact that so many are sited close to a river. In addition many tanneries have their own bore holes, though this water may only be suitable for washing the hides, clean pure water being needed for the soaking pits.
The disposal of effluent is a special problem to the tanning industry as without treatment the effluents are extremely obnoxious and may be excessively acid or alkaline according to the process carried on.
4.3 Heat and power
Power is required for revolving the drums and moving the rocking frames in the pits; also for the various machines used in the process. Steam is required for the drying rooms and the boilers may be used for the dual purpose of generating the electricity required and also for providing the process steam.
5. Basis of Valuation
Tanneries should be assessed by means of the rental method based on the local industrial tone.
Rateable plant and machinery which is not reflected in the rent passing or for which there is no separate rental evidence should be valued on the contractor’s basis. Guidance may be found in the VO Cost Guide and RM 4:3. Paragraph 3.2 deals also with this item.
No adjustments should be made to reflect any sui generis argument relating to this class nor should any special value be ascribed to these hereditaments because the use is classed as an offensive trade.
6. Valuation Considerations
In the post-war period the introduction into world markets of plastics and other synthetic materials resulted in a recession in certain parts of the tanning industry. The part of the industry most affected by competition was that dealing with heavy sole and other similar leathers
Today, the number of tanneries remaining reflect these changes and many of them have been the subject of refurbishment and modernisation.