HAND-CRAFTED LEATHER GOODS
How Leather is Produced
The nature of animal skins
The Epidermis is the surface layer of dead skin which flakes off during the life of the animal, being replaced from the living 'grain' layer that is immediately below. The Dermis is the true skin or corium which is made up of two sub-layers and is clearly seen in sheepskins, though not so much in others. The sub-layers are the grain layer, into which the hair shafts are embedded, and the fibre layer which is the main bulk of the dermis made of masses of tiny interlacing bundles of fibres giving the leather strength, flexibility, and allow it to breath.
Animal skin:- Animal skins, like other parts of animals, are colloidal and have been used since prehistoric times for a wide variety of things. It continues to be a very important raw material. According to Hewitt Bates, it was Eumenes II, King of Pergamom from 197 to 159 B.C., who discovered a method of cleaning sheep and goat skins on both sides. When dried in the sun, these skins became a very desirable writing material known as parchment.Even the Bible mentions leather. In the third chapter of the Book of Genesis, paragraph 21, a verse reads, "Unto Adam also and to his wife did the Lord God make coats of skins and clothed them." The Talmud describes the tanning art of the Jews which is, to some extent, still practised today. The ancient Greeks and Romans frequently refer to leather in their books and legends. Homer, in the Iliad, describes the process of chamoying hides by opening up their pores, forcing oil into them, and beating and rubbing the stacked-out skins in order to achieve softness and pliability in the leather. In Virgil's Aeneid, Dido, Queen of Carthage, was promised only as much ground for her kingdom as the skin of a bull could encompass. She cut the hide into a very thin continuous strip and was thus able to cover enough land to build Carthage. Romans used leather as a medium for money by cutting circles and stamping them. Our English word "pecuniary" comes from the Latin "pecus" meaning hide and "pecunia" meaning money.
In modern household collectibles, it will be found in a wide range of objects such as bookbindings, shoes, clothing, sports equipment, and furniture. Skin can be used without tanning in such forms as parchment and rawhide. Partially tanned skin includes many of the skin products prepared by Aboriginal peoples. These skins are very soft and flexible, but are sensitive to water and are prone to stiffening with age. Another process called alum tawing was used for gloves through the 19th century. Skin prepared in this way is very sensitive and should not be wetted at all. When skin is fully tanned by exposing it to chemical treatments, it becomes leather. This is a chemical process that is difficult to reverse. Some leathers are very durable while others disintegrate under certain conditions. Vegetable tanning, which uses barks rich in tannins, is the traditional method that has been used for millennia. Since the 19th century, mineral tanning using chromium and other metallic salts has become more popular. The leathers produced by this category of processes are extremely durable and water-resistant.
Cattle hide:- Cattle hide and cowhide are substantially the same, though strictly speaking, cowhide is leather from mature female bovines.
Fleshing and splitting of hides in lime
Hide splitting machine
Vegetable-tanned:- Vegetable-tanned cowhide is one of the most readily available leathers, but before tanning begins the hide has to be prepared. Before preparation, the hide of calves and sheep are about 1 cm thick. In other words, the flesh and hair has to be removed (there are tales from times past that some Welsh tanneries used to keep a few mastiffs to chew it off), and no better description than the following suffices (Gresham, I, 97, 1920).
"Practically every method of disintegrating the epidermis and loosening the hair leaves the cutis still covered with its outer coatings, which must be forcibly removed. For this purpose the hide is thrown, hair side upmost, over a slanted beam, thick and convexly rounded. Taking a blunt two-handled knife the blade of which is slightly concave, and, bending over the head of the beam, you push off the epidermis and hair. If the lime-pits or other depilatory agents have done their work, the job is very easy, calling for nothing more than consistent and regular pressure of the blunt edge downwards on the surface of the hide. The pressure of the body upon the part folded over the top of the beam holds the hide steady, and resists the pull of the knife. Inequalities on the inner side of the hide give some trouble, causing the knife to skip at times; but the main difficulty arises from ineffective action of the depilatory agent. Young hairs and hard places in the epidermis resist the blunt knife. Though contrary to regulations, and risky in itself, the common practice is to take a sharp knife to these parts, cutting and scraping the cutis clear. If carefully done, no harm need accrue, though a slip of the knife or too deep scraping may injure the hide seriously."
This lies at the bottom of the follicle.
Erector Pili Muscles
These muscles raises the hair on living animals and cause goose pimples.
The sebaceous glands secrete oils which lubricate the skin and hair. The sudoriferous glands exude sweat which is important in regulating the temperature of the skin and hence the animal.
The surface layer is called the epidermis. It consists of mainly dead cells which are relatively hard.
The hair root and the epidermis are made from a protein called keratin. It is the most stable part of the skin, but has to be removed during manufacture, to expose the grain layer which lies beneath it.
This deep layer is called the flesh layer. It contains high levels of meat and fat and is therefore of no use to the tanner. Unless the leather is to be left with the hair on, as in the case of woolled sheepskin, it is the section between the epidermis and the flesh layer which is important. This important section is called the dermis and consists mainly of densely inter-woven fibrous tissue. This tissue is made up of a protein called collagen. If you look under a microscope, you will clearly see all of the fibre bundles. It is these fibre bundles which make leather such a strong, flexible and unique material.
The dermis is broken down into 3 parts.
Here the fibre bundles are large and strong. They lie at varying angles to the grain layer above. This angle varies in different animals, but to some extent, the angle can be altered during the leather manufacturing process. This angle is known as the "angle of weave" and it effects the physical properties of the leather. A lower angle of weave produces a softer, weaker and less elastic leather.
It consists of more densely woven fibre bundles, which have a much finer construction. Towards the top of this layer, next to the epidermis, the fibres are extremely fine and form the layer we call the grain.
This is the area between the corium and the grain layer. In certain animals it can cause problems, e.g. in sheep (basil) where splitting can occur along the junction area. There are a few other things worth noting: Running throughout the skin are many veins and sweat glands. Between the fibre bundles are inter-fibrillary proteins. These proteins can cause a problem. They are not like the collagen proteins which make up the fibre bundles. When they dry out, they form hard glues that clog up the leather. This would make the leather hard and inflexible. There are many fat cells within the corium. These can vary depending on:
the type of animal
the foodstuffs used
the time of year it was slaughtered
Fats are easy to remove, but in sheep the fat can be up to 30% of the weight of the skin and once removed can leave large empty spaces, which can make the leather feel empty.
Vegetable-tanning appeared some c.8-10,000 years ago, and it has been suggested, may have resulted from skins being accidentally soaked in forest pools containing oak bark.
Briefly, vegetable-tanning is the commonest method used during recent centuries and involves soaking the hides in a solution of oak bark in water for up to 15 months.
Vegetable-tanning (or bark tanning) is done using typical materials such as any of the oaks, fir, some willows, chestnut, birch or heather. In fact an enormous amount of plants have at one time or another been important sources of tannin.
Presently, some 80% of all commercial vegetable-tanning is done with highly concentrated extracts from chestnut or mimosa.
Oak leather has a yellow-brown colour, as has fir. Before synthetic dyes started to come into general use, the Alder gave us some of the very finest. The wonderful Mrs. Grieves, author of A modern Herbal informs us in great detail: "Both bark and young shoots dye yellow and with a little copper a yellowish -grey. The shoots cut in March will dye cinnamon, and if dried and powdered, a tawny shade. The fresh wood yields a pinkish-fawn dye and the catkins green. The bark is used as a foundation for blacks, with the addition of copperas. Alone, it dyes woollens a reddish colour (Aldine Red — which was a favourite colour of our Celtic ancestors). An ounce (of bark), dried and powdered, boiled in ¾ pint of water with an equal amount of logwood, with solution of copper, tin and bismuth, 6 grains of each, 2 drops of iron vitriol, will dye a deep boue de Paris."
Chestnut oak (a white oak) is high in tannin as well as acid-forming sugars, and is among the most desirable of barks for tanning. In general it takes about twice the weight of hide in bark to effect a good tan, and the more finely shredded it is the more tannin you get for a given quantity.
Hemlock: This was the bark of choice by the tanners of the 1800s in the United States. Because tanners especially favoured hemlock and stripped the bark from this plant, it became almost extinct in the north-eastern U.S. Formerly hides were sent from South America to New York and New England and then hemlock was used to tan them. The leather was then sent to Europe. This continued until the hemlock was almost all gone. This leather has a similar colour to quebracho. There were several drawbacks to this type of bark. One is its inability to hold black when dyed with iron mordents. Since hemlock had problems dyeing black, the tanners stained the leather with logwood in order to get a tannin that would bond with the iron mordents. A faster tanning time was gained with hemlock over oak. This allowed the tanner to tan more leather in a give period of time. So along with the higher weight gain of hemlock over oak this made it very popular with tanners in New York and Pennsylvania. The importance of this stems from the fact that leather was sold by the pound until roughly 1850 when measuring machines were invented, and leather switched to being sold by the square foot, as it is today.
Quebracho is obtained from the heart-wood of the quebracho (either of two anacardiaceous South American trees, Schinopsis lorentzii or S.balansae) tree which grows chiefly in Argentina and Paraguay. Ordinary or water soluble quebracho is the natural extract, rich in condensed tannins. This type of bark gives a leather colour closer to what the hemlock bark tanned leather would have looked like during the 1860s. This is the most common bark used in vegetable tanning done today in the US. This wood was first used in combination with hemlock bark in the 1870s. For the past eighty-five years quebracho has been used on its own.
Rowan has a strong, flexible, yellow-grey wood, and was used for great variety of purposes: magical spears, wands, or a talisman inscribed with runes and other meaningful patterns. All parts of the tree were used for tanning hides and for dyeing cloth black. Pine Bark is primarily used in Central Europe producing a red-brown colour to the leather.
The main ingredients for the tanning liquors will vary from country to country, and will have a profound affect on the qualities of the finished leather. The amount of calcium in the water will make a profound difference to the finished product. Areas like Walsall in England, or near the European Alps, where there is a high limestone (calcium) content in the water, produce some of the strongest leathers in the world.
Tanneries were traditionally situated near rivers and streams because they used so much water. Tannin, being water soluble, the warmer the water, the faster the tannin is extracted. Consequently, warmer water gives a darker colour and, cooler, a lighter colour, to the end product.
This traditional method is still in use today by a few tanners who produce leather for high quality leather-goods makers. Generally, though, the introduction of chemicals has superseded this way of tanning.
After chemical tanning, the resulting product is a distinctive blue shade – therefore known as "Wet Blue"! Each wet blue hide is then cut down the length of the backbone into two sides before further processing – hence the phrase "side-leather". Side leather is predominantly used for footwear and leather goods which have smaller pattern pieces. The hides would be left whole for upholstery leathers, for example, where larger areas need to be cut.
The sides (which at this stage are fairly thick), are then split horizontally through their structure to produce two thinner pieces. The uppermost piece, i.e. the outer skin surface, goes forward for processing as full grain leather, the underside is suede – sometimes also known as "split-leather".
It was an inventor, Samuel Parker, who devised a means of splitting leather to make it more usable with less waste. Prior to that, the thickness of finished leather had been determined largely by the thickness of the hide. If a thinner product was desired, the hide was shaved, and the shavings became strictly waste. Parkerís process permitted splitting the hide to specification with practically no waste at all. As with chrome tanning, hide splitting remains much the same today as when Parker invented the process.