Tanning

As tanning processing of raw animal hides into leather is called. In a tannery or leather factory, the skin structure is through the use of tanning stabilizes and thus made ​​leather. Tanning is one of the oldest cultural achievements of mankind.

For the tanning and finishing of fur skins of fur and fur finishing see.

Much of the chemicals used represents a threat to the environment if disposed of improperly dar.

• 4.1 Vegetable tanning
• 4.2 tanning with mineral salts
• 4.3 Fettgerbung
• 4.4 tanning with aldehyde tanning agents
• 4.5 tanning with synthetic tannins
• 4.6 Special tanning process

• 6.1 origins
• 6.2 antiquity
• 6.3 Middle Ages and Early Modern Times
• 6.4 19th century
• 6.5 20th century

• 9.1 Museums
• 9.2 leather production

Properties of hides

Significant effects of tanning are:

• Irreversible binding of tannins to the skin - that is, the leather can not be converted back without destruction. This is an essential difference from preservatives that are largely reversible.
• Resistance to natural self- decomposition by microorganisms.
• The leather-like drying ( raw skin dries horn-like translucent on ).
• Severely decreased swelling when placed in water.
• High resistance when heated in the wet state ( high shrinkage temperature). A raw skin of mammals begins when to glue at about 62 ° C heating in water, which can be recognized by a significant shrinkage. Depending on the leather tanning process may shrinkage temperatures in excess of 100 ° C to achieve; this is called kochgarem leather.

The animal skin

The animal skin consists of three layers of the upper, leather and subcutaneous tissue. Although the hair is often rooted with their roots deep in the dermis, they are still made ​​from the epidermis. The epidermis and its educational products consists mainly of the sulfur-containing protein keratin. It is made ​​up of cells, and is usually chemically dissolved ( liming ).

The subcutis, consisting of loose, fibrous, loosened up by muscle tissue, blood vessels and fat cells collagen is mechanically removed ( fleshing ).

The remaining middle part consists of thin papillary and reticular thick. These layers consist of one-third the fibrous protein collagen, and about 60 % water. The papillary layer has a denser and less entangled fiber structure and forms the finished leather, the smooth surface, which is referred to as grain. The reticular is formed of coarse collagen fibers, which are more intertwined. It is responsible for the mechanical strength of the skin and of the leather.

Other occurring in the dermis proteins such as elastin, blood components ( albumins and globulins ), cellular proteins, pigments and Keratinreste from the epidermis are removed as completely as possible before the actual tanning. Natural fats are released before or after tanning.

When skin finishing the chemical process of the epidermis and hair removal is not necessary.

The dermis has about the entire surface often significant structural differences. The area at the back and the rump ( core or butt ) is denser and stronger for most skin types, while the belly and armpits ( Flemings ) usually have a very casual, less valuable structure. The tanning process is to compensate for the possible, and prevent sticking during the drying of the collagen fibers ( fibrils ) and the horn-like drying. In addition to the insulation, the fibrils are cross-linked by the cross- over tannin compounds to increase the dimensional stability under mechanical load.

Steps in the tannery

Overview

In principle must be distinguished whether the skin to be made into leather without hair ( leather tanning ) or leather with hair ( or fur dressing Pelzgerbung ).

In the trade or industrial leather tanning following steps are common:

• Switch
• Skin shed during liming ( depilation and loosening of the skin fiber structure )
• Fleshing ( mechanical removal of the subcutaneous connective tissue)
• Columns ( with thick skins )
• Decalcification
• Enzymatic stain
• Degreasing (only for skin types with a lot of natural fat, such as pork, sheep)
• Pretanning ( pimples): Preparing for the tanning
• Tanning
• Dewatering ( withering )
• Thickness regulation ( folding)
• Wet dressing (bleaching, retannage, coloring, fats)
• Stretch out (lines and dewatering )
• Dehydration
• Moistening (conditioning )
• Softening by mechanical processing ( staking, milling )
• Dry
• Dry finish ( surface treatment)

The Pelzgerbung is similar:

• Switch
• Fleshing ( mechanical removal of the subcutaneous connective tissue)
• Wash - Degreasing
• Pimple
• Tanning
• Greasing, possibly re-tanning and coloring
• Dehydration
• Moistening (conditioning ): treatment with moist sawdust ( Feuchtläutern )
• Softening by mechanical working ( tunnels )
• Grinding the leather side
• Dry
• Combing of the coat
• Trockenläutern ( treatment with dry wood flour)
• Ironing the coat

Similar steps were, in principle, very long. Compared with historical tanneries, the following changes:

• Until the 19th century, the wet processes were carried out in pits. The vessels were made of wood. The skins were rarely mixed because this was tedious manual work. This process times were sometimes very long.
• Today's standard work vessels for wet processes are the tanning drum in leather production and the reel in the skin finishing. In both areas, today a more or less thorough mixing and precise temperature control is possible. Thus, the processes were significantly accelerated. Vessels made of stainless steel or plastic are used. Thanks to modern regulation, control and metering of individual processes can run automatically.

The preparation

After the hides raw hides are first trimmed (trimmed, cut ). Wherein portions are removed, are not suitable for the manufacture of leather. The skin must be preserved as soon as possible to stop the organic decay and to avoid quality damage. This is done mostly by salts. In regions where the distances and the supply chain so permit, the skins are short -term preserved by cooling and thus dispensed with salt. In some regions where salt is scarce and the climate permits, hides and skins are also dried.

After transport, the skin goes through the Water Workshop, the non- leather giving constituents (hair, subcutaneous connective tissue, fat and unstructured proteins) are removed in the course.

At the first operation, the switch, the skin is soaked in water so that they will be cleaned and brought to the original, natural water content.

It is then up days inserted for skin digestion ( removal of the hairy epidermis and to loosen up the fiber structure ) a few hours in the liming. In the traditional method is usually used for this purpose only milk of lime. Today, the process is usually done in the highly alkaline range with lime and sulfides and / or enzymes, thus the dehairing is substantially accelerated. The more intense the skin shed, the softer the finished leather. The soaked and depilated skin is referred to as nakedness.

When fleshing the subcutaneous tissue is removed mechanically, so that finally only the dermis remains. Earlier it was the manually performed on Gerber tree - see picture. Today, this is done by machine.

In cleaving the dermis is horizontally cut one or more times over the whole surface. The grain splits obtained with the papillary and reticular and the flesh split, consisting only of reticular (see splits).

The resulting fleshing waste and also the inferior cleavage fragments are referred to as glue stock and processed to hide glue, gelatin or dog chews.

Due to the strong alkaline treatment of liming the skin swells greatly. When Deliming the liming chemicals are removed from the skin, the pH is largely neutralized and the skin gets its natural state of swelling. This allows the following chemicals and tannins penetrate the skin.

If you want to produce a soft leather, called the nakedness of a pickle with enzymes is subjected. Raw materials with a lot of natural fat still gets a special degreasing surfactants.

The tanning

The work described so far have indeed changed the skin chemically and mechanically, but it is still as a native protein with all its adverse properties before. Therefore now following the tanning. The tanning process basically consists of three phases: de-swelling of collagen, infiltration and penetration of the Gerbextrakts, loyalty and attachment to the skin fiber.

Takes place only through the effect of tannins on the fixation and cross-linking of proteins, the conversion to leather. This takes place via ionic bonds in the case of poorly soluble mineral salts (complex binding) or through covalent bonds with all other methods. There are also additional, hydrogen bonds and van der Waals forces for further stabilization. In addition to the binding tannins are stored in surplus between the collagen fibers.

The different Gerbstoffarten have different procedures, see below: tanning process and tannins.

So that the leather gives the desired characteristics, different tannings are often combined. Exotic tanning methods such as the Rauchgerbung mostly based on action of several tannins.

After tanning follows the mechanical dewatering ( withering ). With Folding the leather is then brought to a uniform thickness. In this thickness control chips are similar removed from the leather as with an electric plane.

Wet dressing

In wet finishing the final leather characteristics are determined, such as softness, color, abundance, structure, elasticity and water absorption.

In the re-tanning can be used in principle, all Gerbstoffgruppen. But are especially important synthetic tanning agents, vegetable tanning and resin and polymeric tanning, as these take a very favorable influence on the abundance and structure of the leather.

In the de-acidification, the acid residues of strong acids (for example sulfuric acid) is neutralized to give a better stability of the leather is obtained.

The coloring is done with dyes which form a chemical bond with the leather fiber. Modern dyes are distinguished not only the good Färbeigenschaften (for example, peel the leather, combinability, large color palette) and fastness by a low environmental impact from.

The natural leather color depends on the tanning agent used. Chromgerbungen give a bright, gray - green natural color, vegetable- can yellow brown - be reddish brown. Aluminum salts and synthetic tanning agents give white to light beige leather. Glutaraldehyde and Fettgerbstoffe stain the leather yellowish.

Not all types of leather must be dyed absolutely over the entire cross -section. Some leather types (for example chamois ) are colored only on the surface during dry dressing.

The greasing causes a better softness and smoothness. The Fatliquor wrap the leather fibers with a thin film of grease. The fibers do not stick by as much during drying and can be more easily move against each other. Fatliquor bind although there are more or less strongly to the leather fiber, but do not cause cross-linking as Fettgerbstoffe.

Through the use of water repellents leather may be dirt-repellent and water- repellent finish to waterproof.

In the vegetable tanning the wet dressing has less influence on the properties of leather. The properties of vegetable tanned leather are mainly dependent on the selection of the tannins and the leadership of tanning. The main tasks are fixing the tanning agent, bleaching or dyeing and greasing.

Drying, machining

Before the actual drying, the leather will stretch out when stretched, smoothed and partially dehydrated.

The actual drying is carried out by hanging, clamping on the frame or in a vacuum dryer. In vacuum drying, the leather on a heated steel plate are placed. Then, an air-tight cover is placed over it and a vacuum inside.

Some leathers are here completely dried ( 8-12 percent water content), then rehydrated to a water content of about 20 percent ( Stoll humidity) and made ​​soft and pliable by mechanical working ( tunnels or Millen ). In some modern Gerbprozessen the leathers are dried only to Stoll humidity and the same processed.

After staking or Millen, the leathers are dried to a final moisture content of about 12 to 15 percent.

Dry dressing

In the dry dressing the leather surface is treated. In addition to the final color appearance of the surface grip, structure, gloss and mechanical and chemical resistance properties of the surface are significantly affected. Even superficial leather errors can be compensated by the dry finish.

When Raul springs as suede, nubuck or Hunting the surface is sanded, dusted and equipped dirt and water -repellent with non-layer- forming chemicals.

For smooth leathers such as Nappa, box or Blank leather special paints are applied. The paints contain variants with different binders, the conventional paint additives that determine the gloss and surface grip, in addition may also include pigments and / or dyes. Do the finishes pigments, they look opaque and are distinguished by high durability. Are dyes or no coloring substances contained arise transparent Zurichtfilme that reveal the natural textures of the leather well. Most finishing products are water- in order one-or two - component systems based on polyurethane, polyacrylate or polybutadiene. In addition, polyamide, casein, cellulose esters and cellulose ethers are used as binder. After drying and curing produces water insoluble and more or less stable coatings. Only a few types of leather like patent leather require the use of dilutable only with solvents coating agents.

Application is made by casting, high pressure spray, low pressure spray, airless, air mix or by roller coating machine ( roller coater) in several steps. Also can be ironed onto the leather finishing to the surface more or less thin films.

The individual layers are compressed by means of ironing, stamping and polishing. A method now only rarely used is the gloss put. Here, a glass or agate cylinders at high pressure is quickly drawn across the surface. This gives very shiny surfaces, but can recognize the natural structure of the leather well. A similar, but not as strong smoothing effect results in the polishing stone or cloth rolls. When ironing the surface structure is leveled and in embossing the leather a new surface structure is given. In this case, temperatures can be used up to 160 ° C and pressures up to 200 bar.

The thicker this coating is more uniform, durable and easy is the leather surface. With increasing Zurichtdicke the leather but often loses its natural character. A high quality finish gives the leather a smooth, durable surface with a natural appearance as possible and handle.

From a dry film thickness of 150 microns, the material must be referred to as coated leather material and may no longer be marketed as leather on the market.

Tanning methods and tanning

Vegetable tanning

Tannins: Tannins in the leaves, bark, wood and fruits

Still in the first half of the 20th century, the vegetable tanning ( vegetable tanning, tannery, Rotgerbung ) was the most common method of tanning, today it plays only a minor role. To obtain the Gerberlohe as quebracho, chestnut or oak wood, plant parts of mimosa and Sumachgewächsen and other wood or Rindengerbstoffe were used. From this use of the Lohwälder were once created. The vegetable tanning agents are milled in a Lohmühle and leached with water. The Sud used is known as broth or extract.

The nature of the vegetable tanning demands a gentle tanning to first stabilize the fiber structure of the skin. Previously, and in a very small extent even today, this stabilization was carried out by Angerben with little concentrated, already emaciated tanning solutions ( broths ) in several steps in the " color transition ". Only then was the Ausgerbung with ground Lohe and concentrated broths in Gerbgruben. This tanning process may take up to twelve months, and is called " Altgrubengerbung ".

In modern Vegetabilgerbungen the pelts are pre-tanned with synthetic tanning agents or aldehydes, and tanning in tanning drums rotating with concentrated tanning liquors. The tanning process can be shortened to a few days.

Tanning with mineral salts

Tannins: chromium, aluminum, zirconium or iron salts

Today, the tanning mineral salts, especially the tanning with chromium III salts, the most important tanning method. Aluminum salts - mainly alums - and zirconium salts are " tawing " for fur pelts used mainly in. As a pretreatment, all mineral tanning require a " pimple ". The pickling acid is made (usually sulfuric acid or formic acid ) and neutral salt (sodium chloride or sodium sulfate). By acidification of the skin, the mineral tanning can completely penetrate the skin. Subsequently, the tannins are fixed by the gradual addition of alkalis in leather ( Basifizieren ). The implementation is done in rotating tanning drums. The tanning process is completed in 10 to 15 hours. To complete the leather properties but neutralization, re-tanning, dyeing and greasing is required.

Fettgerbung

Tannins: fish and marine oils ( Trane ) or fats in the brain

When Fettgerbung ( Sämischgerbung ) special oils with gerbender effect ( Trane ) are rolled into the skins. Are the skins infused with tannin, an oxidation of the oils and they develop their tanning action. After drying, the excess tannins are washed out and the leather is dried again. A particular retanning is not required.

Tanning with aldehyde tanning agents

Tannins: earlier formaldehyde, glutaraldehyde today mainly

Aldehyde tanning agents are generally used in combination with synthetic tanning agents, or as pre-tanning agents for vegetable tanning or Fettgerbung. The workflow is similar to that of mineral tanning with pimples - penetration - basification for fixation.

Tanning with synthetic tannins

Tannins: resin tanning agents, polymer, polyphosphates, Paraffinsulfochlorid

Also, synthetic tanning agents ( Syntans ) are rarely used as Alleingerbstoffe. Your Gerbverhalten is similar to the vegetable tanning. They are used mainly for re- chrome for pre-tanning leather and as in the vegetable tanning.

Special tanning process

• Glacégerbung: Combination of aluminum salts with salt, egg and flour.
• Schrumpfgerbung: tanning with special tanning agents, the contraction ( shrinkage ) causes the skins.
• Hungarian tawing: tanning with aluminum salts and special greasing.
• Zweibadchromgerbung: Was used in the early days of chrome tanning. Chromium VI salts were incorporated into the skin and then be converted by reduction to chromium - III salts gerbfähige.

Environment

The tannery is connected due to the use of toxic substances always been available with risks for environment and health, environmentally harmful tannins were discharged without further treatment into the river, some of them seeped into the ground. Today mainly arise in the chromium process in third world countries and emerging economies problems, as they often exist only inadequate environmental protection measures. Especially in densely populated India repeatedly occur, such cases. The two Indian cities Ranippettai and Hazaribagh landetem for this reason on the top 10 list of the most contaminated places in the world, which is published by the American Blacksmith Institute since 2006.

History

Origins

The use of hides and skins goes far back in the history of mankind and is supported by numerous illustrations. Whether the materials used were actually already tanned or made more or less preserved only by appropriate methods, can not be clearly established. While leather can be increasingly replaced by modern materials today, it was until the 20th century, a material for its properties there were no substitutes.

Because of the rapid decomposition of organic materials in the soil, the Fund situation is extremely low with leather objects. Since leather as a by- product of the food procurement is one of the oldest materials used, you can probably date the beginnings of the tannery in the Stone Age. Finds of tools suggest that initially was mainly mechanical processing of raw materials.

With the mastery of fire, the coating effect of the smoke was known. The Rauchgerbung counts with the tanning of animal fats to the oldest tanning methods. The effect of different substances such as smoke, grease, parts of plants or mineral salts has been empirically and probably often discovered by accident. The development of the tannery was strongly dependent on the general level of development of the population and of the materials immediately available.

Antiquity

The Egyptians knew the processing of hides and skins before 5000 BC illustrations in tombs dating from around 1600 BC, they represent the processing, but also the production of leather tanning agents represents the effect of alum plants and were known.

The oldest written records about the manufacture of leather go to the Greeks (1200 BC) and the Assyrians (700 BC). Preservation methods were the vegetable tanning and Fettgerbung.

Even at the time of the Greeks, but of course also with the Romans, the economic importance of leather production was remarkable. Probably because of the unsanitary working conditions, the environment, and not least because of the strong Aasgeruchs, who also clung to the tanners, they were not very respected; the work was mainly done by slaves. The Romans used as Gerbmaterialien pine, alder and pomegranate tree bark, galls, sumac and oak. Excavations at Pompeii, which was buried in 79 AD, parts could be exposed to a tannery.

Middle Ages and Early Modern Times

In medieval times, leather was a preferred material. In Europe emerged leather workshops in monasteries and towns. The tanneries often reached impressive sizes. However, their operators in the cities in their own district had to retreat: The production of leather was a dirty and literally disreputable trade, so the tannery was a socially recognized and not very dangerous work. Dealing with the rotting skin and the ( poisonous ) chemicals put extreme stench free, also you could easily become infected with anthrax and other diseases. A worker who had survived anthrax was very valuable and enjoyed better treatment by his employer. The Gerber (also tanners, Löher, Loher ) were among the unclean crafts. In some cities a Löhergasse or Lohgerberstraße reminiscent of their craft.

The guilds regulated the market and also the craft. A technical progress thus was very difficult and was, if anything, by the influence of foreign cultures. From the 14th century the production of leather manufacture parted ( tannery ) and leather processing. Subsequently, the tannery in areas Rotgerber, Corduaner ( vegetable tanning ), tanner ( tanning with aluminum salts ), Sämischgerber was divided ( tanning with fat) and Pelzzurichter.

From about 1700, we addressed scientifically with the tannery. Has been the production of usable leather the essential problem, they were sought after now stronger method for accelerating and streamlining of tanning. There new tanning methods have been developed and published. 1803 appeared the first magazine about tanning.

The Berlin Tannery won since 1734 by French immigrant significant expansion and improvement. The first in France operated successfully patent leather fabrication was soon operated in Germany, as well as the white tannage of goat, lamb and sheep skins, which was initially a special industry of the city Annonay and its environs. Later, the British and Americans competed in the training of the quick tannery.

19th century

With the beginning of the 19th century the first steam engines were used. In addition to the drive machine of the steam has also been used for the production of concentrated Gerbextrakte. This allowed a reduction of the tanning times from several months to a few weeks. This created a rational working, fast growing tanning industry. Middle of the 19th century was the leather craft the third largest branch of industry in the German Reich.

In 1840, the first tannery machines have been developed. Until then, the editing was done still manually changed little since the Stone Age. There were only uses simple pumps or transport facilities. Developing Gerbgefäßen which allowed a movement of the skin material, especially, the rotating the tanning drum, was of particular importance. The chemical processes could be substantially accelerated. Many modern tannery machines are still working on the same principle.

From the new colonies, tannin-rich vegetable tanning were introduced (1822 mimosa, quebracho 1867 ). In 1861, the first chrome - tanning process has been patented. Greater practical importance won the chrome tanning but only in the 20th century.

Some of the mid-19th century to accelerate the tanning process the tanning agents added chemicals set had a negative effect on the aging resistance of the leather. The resulting induced accelerated decay of leathers gives a reddish dust, called red decay.

Environmental protection, for example, the wastewater treatment, was already an important issue.

End of the 19th century, the first relevant research and educational institutions have been established, for example in 1874 in Vienna, 1889 in Freiberg (Saxony), in 1891 in Yorkshire, later in Leeds. The development of the tannery was thus considerably accelerated, it took place primarily in Europe and the United States.

20th century

The further development is closely linked with the development of the chemical industry. 1850, the first synthetic dyes were developed. 1907 Otto Röhm began with the production of enzymatic stains; the unhygienic use of dog feces or bird droppings could be avoided.

In 1911, the first synthetic tanning agents have been developed. Originally, it should be a replacement for the vegetable tannins temporarily difficult available. In fact, they have, however, been established as a valuable tool for speeding up the vegetable tanning and chrome-tanned leather as retanning for. With the help of fat chemistry as important aids such as surfactants and water- oiling agents ( Licker ) have been developed. The development of the first man-made varnishes and binders for the dressing falls into this time.

The faster and more economical chrome tanning was further developed, notably through the efforts of Heinzerling, who represented first chrome tanned leather. It created new types of leather with new properties. Latest from the mid-20th century, the chrome tanning was the most important kind of tanning, it has remained so to this day.

1921, the " Kaiser- Wilhelm Institute for Leather Research" was founded in Dresden under the direction of Max Bergmann. It was rededicated in 1933 for political reasons and has since been meaningless. ( Start-up in 2002 as " Max Bergmann Center of Biomaterials ", jointly operated by the Technical University of Dresden and the " Institute for Polymer Research " in the Leibniz Association. )

The progress of chemistry brought benefits not only for the production of leather. New materials were discovered, developed competing products. Also, the general technical development, for example the car, let the demand for certain leather products shrink greatly. Quite a few tanneries in Europe, mainly those that were set up in the production of vegetable-, did not make the move and had to close.

The technical development has been increasingly operated in the chemical companies of the industry. Especially the dressing benefited from the development of new binders ( acrylates, butadienes, polyurethanes) and procedures. These international companies acquired in the course of time essential application knowledge they marketed internationally. That is, modern technology was and is available internationally.

In the second half of the 20th century, the European and North American leather industry came under pressure. In Asia and South America, established a strong, competitive leather industry. In addition to the difficult economic situation, the environmental requirements in Europe have been tightened in the 1970s. The investment required and sometimes high production costs caused further shrinkage of the leather industry in Europe.

Since 1937 there is an apprenticeship Gerber. 1981 updated to the rules and created a state-recognized training occupation by the Vocational Training Act.

The technical developments of recent years have mainly focused on ecological processes and products. In addition, however far-reaching as possible rationalization and automation of production processes plays an important role.

Museums

• Annweiler, museum below Trifels with tannery workshop
• Calw, Tannery Museum
• Dippoldiswalde, Lohgerbermuseum
• Doberlug- Kirchhain, Weißgerber Museum
• Enger, Enger Tannery Museum
• Grefrath, tannery Bremen in Lower Rhine Open-Air Museum
• Hirschberg, Museum of tannery and town history
• Mülheim an der Ruhr, leather and tanning Museum
• Offenbach am Main, German Leather Museum
• Ohrdruf, Old Tannery
• Weida, Tannery Museum

• Malmö, Sweden, Tannery Museum