When Did Tanning Begin?
Leather tanning is among the oldest activities carried out by men and women. Initially, men and women used skin from domestic and wild animals for clothing and making a shelter. The skins would stiffen during cold weather and rotted during hot weather. Attempts were then made to increase the flexibility and durability of skins through various ways such as rubbing animal fats on them which is the first crude tanning process as recorded in Assyrian texts and also in Homers Iliad. The history of leather tanning is a long one.
An animal skin can exist in either wet or dry state. Wet hides are prone to rotting while dry hides are hard thus difficult to use for clothing. Soon, it was discovered that exposing hides to the sun or applying salt would cause them to lose moisture through dehydration.
Through tanning, the hide’s fiber is combined with another substance to allow the skin to dry, thus avoid rotting and become flexible for use.
Tanning can be done using three methods: fat, plant and mineral-based tanning.
The history of leather tanning can be traced to the early Stone Age; around 8000 BCE.
Men and women created waterproof leather by rubbing fatty substances on rawhides. Five thousand years later, people from Egypt and Mesopotamia discovered plant-based tanning. The new tanning method used the bark of the Gum Arabic or Acacia tree.
Oak bark tanning started by removing hair from the hide using hand-held large blades pushed in a forward direction away from the body of the person shaving. The clean-shaven hides were then hung to dry on frames, then dipped daily for four weeks into different pits containing varying tanning solution concentrations.
After the initial surface tanning, the hides were put into racks and dipped into pits containing tanning fluids. The hides spend up to twelve weeks submerged in the tanning fluids.
The final step involved dipping the hide in oak-lined pits of a depth of nine feet full of tanning agents and solution, for instance, spruce, oak, valonea fruit and bark of mimosa.
There was no heating or moving the hides in the solution where they sat for nine months.
Tanning using plants is also called formaldehyde tanning since formaldehyde is present in the vapors that result from the combustion of green leaves and branches.
Vegetable tanning uses naturally occurring tannins in plant leaves and bark and is thus a slow hide treatment process. The process results in stiff leather. Vegetable tanning relies on tannic acid obtained from organic plants such as trees and nuts. Tannic acid is the oldest tanning agent used in the history of leather tanning and was the primary method of leather-making until 1900. 150 years ago, vegetable leather took one year to make.
The Egyptians also used mineral alum. The mineral can be found in plenty especially in volcanic areas. In 800 BCE, the Romans tanned a different kind of leather such as corium which was used in making sandals and soft leather named Aluta using alum tanning.
An unusual method of tanning also existed among people living in Polar Regions such as Alaska and Greenland. Hair would be removed from seal skins using specially designed stones and knives. The skin was them tumbled and softened using urine. Workers in the leather industry then, who were predominantly women would masticate the hides using their teeth until they soften after which they would use fat and fish oil to prepare them.
During the fifth century, conversion of hides to leather was done by first removing the layer of fat from the hide using clay. The hide was then covered using mixed animal fat, brain, and salt. The hides were then joined to form a round tent using bone or horn needles and then smoke-dried in an open fire. The smoke contained phenol which is a key tanning ingredient. Skins were worn as long dresses by Sumerians in Mesopotamia between the third and fifth century. Leather was also used for footwear, storing liquids and for making inflated floaters for use in rafts. The leather known as “Morocco” in the present day was the first to be processed in the oldest civilization of India. Leather tanning techniques were greatly enhanced during Roman times owing to the demand for leather products in the Roman Empire.
Traditional natural vegetable tanning method is still present in Marrakesh and Fez cities in Morocco where tanneries ensure to use plant-based natural coloring agents. Bright red color is obtained from bell pepper or red poppy, pink color from rose, orange color from henna, black color from a mixture of henna and sugar, green color from mint, and yellow color from a mixture of pomegranate and saffron.
Cordovan leather was developed in Spain in the 8th Century during Moors dominion. The leather has gained prominence in Europe over the years. The fine art of leather tanning then spread from Europe to other continents such as America and Asia.
Tanning underwent a revolution during the 12th century. Tanning systems did not substantially change from then to the twentieth century. Protective garments were produced using oil tanning. Leather tawing using oil was also prominent though it had not been perfected to produce satisfactory results. Tawed -leather required dye finishing to make it malleable and enhance its appearance. Leather products were basic though they were decorated. Quicklime was discovered in leather tanning during the Middle Age. The technique is valid to date and is in use in many tanneries. Lime was used to remove hair from the hide and also remove impurities from skin fiber.
In the late 18th Century, chrome tanning was developed in France. The technique was later commercialized in America before becoming available in other parts of the world. About 95% of leather produced globally is made using the technique since it is fast and inexpensive. Leather tanning takes only one day in the present day. Tanning pits were also replaced with the rotating drum while new variants of tannins were discovered during the Century.
During the Fourteenth and Fifteenth Centuries, plant tanning was widely practiced. Leather cutting was the choice mode of ornamentation and could only be done on pieces of leather made from plant tannins. The grain side of a leather was decorated by figuratively drawing an image from its fleshy side. Plant-tanned leather was also decorated using punching where a steel rod was used to emboss moist leather.
Today, leather tanning is a sophisticated process that involves the use of industrial chemicals which ensure a faster process. To easily understand how leather tanning is done presently, it is important to elaborate on layer separation. The method splits leather into two across its thickness to obtain the top layer also called hair side and an underlayer. The top layer is identified as full grain due to its exposure to the elements thus made durable and malleable and is considered fine leather. The underside is stiff and less durable compared to the top layer. The stiffer underside layer requires a further coating before it can be used.
Tanning process has five stages as explained below. Each of the complex stages has further steps.
Soaking. Leather usually arrives in the tannery soaked in salt for preservation. The leather is soaked in water contained in revolving drums to remove the salt. During traditional days, pre-tanning was not necessary since law hides did not have to be bulked before being taken to the tannery.
Liming. Calcium hydroxide and sodium sulfide are used to clean the hide and remove hair and epidermis. Liming also softens the hide making it more flexible for handling during later stages of development.
Splitting. The hide is then split into two layers. The top layer is called grain layer and produces high quality, fine and smooth leather. The bottom layer is also called split leather and produces low-quality leather that is used to make suede.
During this stage, the pre-tanned hide is converted into leather. The most common forms of tanning today include chrome tanning, vegetable tanning, and latigo tanning.
Alkaline chrome salts are commonly used in mineral tanning. Chromium cross-links leather’s collagen fibers. Chrome tanning was invented in 1858. Hides are tanned in chromium sulfate combined with other salts of chromium instead of using natural tannins. The process is fast, only taking one day to be completed. Chrome tanning results in a light blue hide. Tone variation in the hide is neutralized by chromium resulting in an even colored piece of hide. After the hides have been tanned, they are then dipped in a drum for dying. Dye absorption rate of chrome-tanned leather is higher than vegetable-tanned leather.
The final leather moderately stretches, is supple and very soft. However, the leather requires being reinforced by other materials to maintain shape and prevent over-stretching. About 4 to 5 % of chromium tightly combined with skin protein is present in leather tanned using this method. The bonding takes 24 to 48 hours and results in a pale blue color. The leather acquires a soft, fine and modern finish upon processing.
Latigo tanning is done by combining chrome and vegetable tanning. The cowhide is first soaked in the acidic salt solution, then dried before it is further processed. The hide is then soaked in different vats containing increasing tanning solution concentrations. Once the tanning process is complete, the hides absorb oil and waxes from large drums where they are placed. The hides become soft and supple and at the same time retain their strength. Chrome tanning is first done succeeded by vegetable tanning thus producing flexible leather that does not stretch. Latigo tanning is used to produce leather for making belts and straps which require heavy leather. Latigo tanning is the most expensive method of tanning leather due to its complexity. Latigo tanning is at most times used to process cowhides.
Once tanning is complete, excess water has to be removed.
The hides are then ranked based on their quality, natural feature locations, and imperfections.
Hides of high quality produce aniline and nubuck leather which have a natural look. Aniline leather is dye colored and does not contain polymers. Semi-aniline leather comes second in quality classification of leather. The leather is more compared to aniline since it has a light pigmented surface coating though it has a reduced natural appearance. The leather has a consistent color and is resistant to stain.
The third classification is pigmented leather. Pigmented leather lasts the longest and is used to make furniture and car upholstery.
Pigmented leather contains a polymer coating which enhances its strength. This surface coating gives manufacturers greater control over leather properties such as fading and scuffing resistance.
After the hides have been dried and classified depending on quality, they have to be dressed. Dressing ensures flaws which might have occurred during treatment are solved before the final stage.
The hide is shaven to ensure it has a uniform thickness. The hides are also dyed to make sure they have an even tone color. The leather is once again tanned to customize its physical characteristics to match its final use. Re-tanning is customer focused and is commonly done to fit a client’s specifications. Hides are mechanically stretched to remove creases during the setting process. Creases might hold water during drying and affect color tone if the final leather product. The hides are dried when stretched using vacuum dryers or flames of fire to retain a water content of between 10-20%. Finally, rough and jagged edges are trimmed out to give the hides a smooth feel all over.
Here, cosmetic final touches are made on the hides before the final leather product can be produced. Finishing reduces grain blemish appearance without the leather product losing its natural beauty. The leather product is also given the needed degree of gloss. Softness, flexibility, and malleability of leather are ensured during finishing. The leather product is given a more protective surface that is easy to clean. Finishing provides an opportunity to effect special effects on the final product, for example, antique look.
Various surface coating techniques for example spraying, padding and roller coating are used during the finishing process. Additionally, mechanical processes including embossing and buffing are employed to give the final product the required appeal.