Manufacture of leather

Leather is made from animal skin, generally the skins of mammals. Evidence that prehistoric man used skins or leather comes from tools and images on rocks and cave walls. Sometimes prehistoric leather objects are found: a 6,000 year-old leather bowl, a 4,000 year-old shoe from the Buinerveen (the Netherlands) or leather items belonging to a man who lived in the Alps some 5,000 years ago.
In the Middle East alum and oak gall from the dwarf-oak (Quercus infectoria) were much used for tanning, but acacia pods (Acacia nilotica) were also used. In addition to these tanning agents, the Romans used the bark of oak trees. Starting in the eighth century, when the Moors arrived in Spain, Cordoba developed into a centre of leather making and indigenous and imported techniques merged. The term 'Spanish leather' referred not to one specific kind of leather but to leather (usually goatskin) that had been prepared and finished in a number of different ways. By the eleventh century the techniques known at the time were firmly anchored throughout Western Europe. In the fifteenth century sumac was introduced into Cordoba for tanning. Until the nineteenth century the tanning process altered very little, except for a few changes in actual procedure.
Leather making is a fairly complicated process. This chapter can do no more than sketch a general picture which does not pretend to record all operations. Furthermore, not only are there many different types of leather but many leather manufacturers also have their own methods of preparation so that even the same kinds of leather are not always prepared in the same way and various leathers enter the market under their own trademark. Many kinds of leather with old-established names, such as Russia leather, are nowadays made in a different way, while leathers bearing the same name were not always made in the same way even in the past. Bookbinding leather falls into a category known in the leather industry as 'light leather'. Other end-products for this kind of leather are gloves, bags, purses, watch-straps, and shoe linings. They are usually made from calf-, goat- or sheepskins.
'Light leather' must meet specific demands with regard to thickness, suppleness, durability, and suitability for the various finishing processes. Consequently, special conditions apply to the bating process, tanning method and tanning ingredients used in its manufacture. Preparation of skins used to make this kind of leather is much the same as for skins intended for other purposes.

Animal skin

A flayed skin that is not prepared will rapidly decompose due to the action of micro-organisms that secrete proteolytic enzymes. To prevent or slow down this action, the skin must be preserved. This can be done by salting and/or drying. This dehydrates the skin to some extent, thereby significantly decreasing the micro-organisms' chances of survival. Salt (NaCl), moreover, has a mild disinfectant effect. Drying causes the skin to lose its soft and supple nature, turning it as hard as horn and transparent. Preservation alone is not sufficient to guarantee maximum retention of the favourable properties of the material: the skin needs to be tanned as well. Under normal circum-stances tanning renders the skin impervious to bacterial activity. It also makes it possible to retain the suppleness, softness, and mechanical resistance of the skin. All leathers consist of a network of fibres made up of proteins. Collagen is the most important protein occurring in leather. The skin of mammals consists of the epidermis, the dermis, and subcutaneous tissue. For the manufacture of leather only the dermis is important (drawing 1).

Epidermis
The upper layers of the epidermis consist of (dead) hard keratin. The surface of the skin flakes, while from the bottom of the epidermis (stratum basale), which is made up of living cells, new tissue is constantly added to the epidermis. The stratum basale is made up of soft keratin and is highly susceptible to chemical attack. The epidermis is firmly rooted in the dermis by means of the hairs.

Drawing 1 Vertical cross-section of a calfskin

Dermis
The dermis consists of the grain (layer) and the fibre network layer.
The grain extends from the bottom of the hair follicles to the top of the dermis and consists of fine fibres, which are practically horizontal on the top where the fibres are very fine and compactly interwoven (grain membrane). At the level of the hair follicles - in other words, on the underside of the grain - are the sweat and sebaceous glands and also blood and lymph vessels. During leather manufacture the remains of the contents of these glands and vessels is removed, with the result that the junction (hyaline layer) between the grain and the fibre network layer is weak.
The tissue in the fibre network layer is made up of fibres that are thicker than those in the grain. They are all jumbled up together and are composed of fine fibrils. The fibrils are separated by an interfibrillar substance. The relative thickness of the grain and fibre network layers depends in the first instance on the kind of animal, its sex and its age. In general one can say that the thinner the coat of hair, the thicker the fibre network layer.
The thickness of the fibre network layer and the thickness and orientation of the fibres usually determine the strength and durability of leather. Horizontal fibres usually give good tensile strength, while the more upright fibres improve wear resistance.

Subcutaneous layer
The subcutaneous layer forms the transition between the dermis and the flesh, and has a loose structure. This tissue is separated from the dermis with knives and is not used for the manufacture of leather.

Preparation of the skin

In the leather trade (but not in this book) a distinction is made between skins and hides. A hide has a fresh weight of up to about 15 kilogrammes. Anything over this weight is called a skin.

Preservation

To prevent the onset of decomposition, fresh (green) skins must be preserved within about three hours after stripping.
There are several methods for preserving skins: they can be dried, salted, dry-salted, pickled (a combination of treatments with salt and acids), or pre-tanned. In salting, the skins are first stirred in a salt bath, after which they are spread out and sprinkled with a layer of salt. Dry-salted skins are made by moving the skins in a salt bath and then drying them. Sheepskins are first shorn of wool in their country of origin, after which they are usually preserved by pickling and then dispatched in barrels. The skin of hybrid sheep - a race produced by crossbreeding wool and hair sheep - are often pre-tanned using vegetable tannins and then traded.
The dried and salted skins are stored at low temperatures (about 5º C) in the tannery stockroom. They then go to the 'wethouse' to be prepared for tanning, a process that in the past could take from several days to a few weeks and involved pits and paddles (drawing 2a). Nowadays drums (drawing 2b) are used in the wethouse and the skins remain in the same drum throughout treatment, thus reducing their time in the wethouse to 14 - 16 hours.

Drawing 2a Paddle

Drawing 2b Drum

Soaking, unhairing and liming
Because the skins lose a lot of moisture during preservation, they are soaked to restore their original water content. Initially the running water of a brook or river was used, lateron pits and paddles, and nowadays drums. Once the skins have been well soaked and have expanded sufficiently, they go to 'trimmers', who remove the claws, ears, and tails.
Next the epidermis (with hair) is removed. Before the middle of the last century this layer of hair was loosened by means of 'sweltering' or 'sweating'. This involved lightly salting the skins and folding them. Several skins were then piled on top of one other and covered over. The resulting rise in temperature caused the skins to start to rot and eventually the epidermis with the hairs came away from the base. In the middle of last century sodium sulphide was first used for unhairing.

The next step is liming, in which the bonds between the fibrils are partly broken, so that the fibres are slightly split up. The lime also removes part of the interfibrillar skin matter, so that the fibres become less fixed. At the same time liming loosens the subcutaneous layer so that it is more easily removed lateron. Liming takes 12 - 14 hours. At the end of the process the skin is very alkaline (pH 13-14).

Deliming, fleshing and scudding
After liming the skin must be delimed in order to neutralise most of the alkaline substances now present in the skin. If this were not done, the acid environment during tanning would result in a rapid and unnecessary hardening of the fibres, especially those of the grain. Lime, moreover, forms insoluble compounds with most tanning substances. For example, vegetable tannins and lime combine to form insoluble calcium tannates, which can cause lime spots at a later stage of the process.

For lighter kinds of leather a separate deliming process is omitted and a certain amount of deliming is achieved during bating.
The next phase in the manufacture of leather is fleshing: the removal of the subcuta-neous layer. Traditionally, this was done by means of a wooden beam and a fleshing knife (drawings 3 and 4), but nowadays it is done by machines.

The next stage is scudding which involves cleaning the grain once more of epidermis remnants, such as hair-roots, pigments and the content of glands and vessels. This was done using a blunt knife. For very thin or delicate skins a slate knife was used. After scudding the grain is more open, so that the tannin is able to penetrate more easily during tanning. Skins can be split lengthwise; very thick skins can even be split into more than two layers, leaving the grain on the 'outer split'. The grainless 'flesh split' was less valuable for certain applications, including bookbindings. Before the development of splitting machines - in the course of the nineteenth century - skins were shaved to make them thinner. Shaving was done using a flat shaving beam and a shaving knife (drawing 5) or by treating a stretched skin with a round shaving knife.

Drawing 3 Wooden beam

Drawing 4 Fleshing knives

Drawing 5 Shaving beam and shaving knife

Bating
Bookbinding leather and other light leathers which require a fine, silky, supple and elastic grain, are bated after liming and fleshing.
The object of bating is:

The bating preparation contains proteolytic enzymes which accomplish a controlled disintegration of the skin substance. Any remaining bits of epidermis are also removed during bating. Meticulous adjustment of dose, temperature, pH, and duration of bating make it possible to regulate the process very precisely. Nowadays bating substances based on pancreatic enzymes have replaced the dog and bird manure favoured in earlier times. As soon as the bating process is finished the skins are rinsed with water for a period of time varying from a few hours to several days. Finally the skin is purged of any remaining bits of connective tissue, etc.

Tanning

To put it very briefly, during tanning certain active groups in a tannin form chemical bonds with certain active groups in the collagen in the skin. This process is greatly affected by various circumstances, such as the condition of the skin, the pH, and the temperature. Tanning can be done in various ways, depending on the kind of tannin used.
Tannins can be divided into four different groups:

Tannins can be used alone or in combination.

Vegetable tannins
Vegetable tannins are obtained from the bark, wood, roots, leaves, fruits and growths of certain plants. In the past, the bark of oak and spruce trees was widely used in Europe and occasionally willow bark. Vegetable tannins can be classified by origin: barks, type of wood, fruit, leaves and gallnuts.
There is also a chemical classification (Proctor-Stenhouse) which differentiates between pyrogallol and pyrocatechol tannins and is based on the behaviour of vegetable tannins after being heated to 180-200 oC: the ensuing main products being pyrogallol and pyrocatechol respectively.
An even better classification (Freudenberg) is that between hydrolysable and conden-sed tannins. Hydrolysable tannins can be dissociated into smaller molecules by hydrolytic enzymes or by acids. As a result of the action of strong acids or oxidising substances, condensed tannins form high molecular, insoluble phlobaphenes (reds).
Examples of hydrolysable tannins are tannins derived from chestnuts, valonia, myrobalans, algarobas, sumac, divi-divi, and various types of galls. Mimosa, quebracho, mangrove, myrtle, and spruce are examples of condensed tannins. Oak bark has characteristics of both hydrolysable and condensed tannins. Leather tanned with hydrolysable tannins has a higher permanence than leather tanned with condensed tannins.
In the Netherlands the most popular tannin was coarsely ground oak bark (tan-bark). The bark of 10-15 year-old trees was the most suitable. The skins were put into a pit or vat, inter-spersed with layers of tan-bark. Water was poured over them and planks weighted with stones were placed on top. After about two months the tannin was completely absorbed. This treatment might be repeated several times, depending on the type and thickness of the skin. The lightest skins took 4-6 months to fully absorb the tannin.
It was possible to accelerate the tanning process by using vegetable tannin extracts. This method involved suspending the skins in vats containing a tannin solution. The skins passed through a series of vats containing increasingly stronger solutions. The leather was then retanned in a rotating tanning drum. Nowadays rotating drums are generally used from the very beginning of the process.

Mineral tannins
The most important mineral tannins are the alkaline chrome salts, followed at a great distance by aluminium and zirconium salts. It was not until the end of the nineteenth century that tanning with chrome salts was sufficiently developed to be used commercially. These days at least 80% of the leather produced is tanned partly or completely with chrome salts.
Chrome tannage requires an acid treatment, because the pH value after bating is about 8. For good chrome tannage the pH - depending on the process chosen - needs to be brought down into the acid range.
Acid conditions were obtained by using a mixture of acids and salt (pickling). The salt counteracts the swelling the acid would cause if it were working alone. After the tannin has been absorbed by the skin, the tanning bath is basified (alkalinised), which promotes the tanning effect, i.e. the formation of chemical bonds between the tannin and the skin fibres. Because alkalinization is a very exacting and time-consuming process, self-basifying chrome tannins have been developed.
One mineral tannage that was formerly used more often than now, is tanning with aluminium salts (metal salt tannage): alum tawing. The Assyrians and Egyptians used earth containing aluminium for tanning which makes this method one of the oldest, alongside tanning using smoke, unsaturated oils and fats, and vegetable tannins. Aluminium salts are a far less effective tanning medium than chrome salts. Aluminium is not a strong complex former, so that there is little bonding between the alkaline aluminium compounds and the skin fibres. Leather tanned with pure aluminium salts is consequently not waterproof. Tanning can be enhanced by the addition of extra complex formers. Aluminium salts turn the leather white. Alum-tawed bindings have a high sulphate content because of the presence of alum.

Aldehyde tannins
For the manufacture of chamois leather, skins are kneaded with fish oil, a procedure nowadays carried out in a vat. In the past this was done with pounders, usually driven by a mill or by foot. After being kneaded (12-36 hours) the skins are stacked in a warm place (35-40 ºC) and turned from time to time. The resulting oxidation generates heat which in turn releases acrolein (an aldehyde) that is bound by polymerisation to the skin fibres. Superfluous fish oil is then removed. Because it is washable, chamois leather was used from a very early time to make clothing. It was generally made from sheepskin but the skins of red deer, roe-deer, reindeer and goats are also suitable. For the manufacturing of chamois leather the grain has to be removed.
Skins can also be tanned using formaldehyde (another aldehyde) after which they are neutralised by being rinsed with ammonium sulphate. The dry leather can stand temperatures up to 110 ºC. The formaldehyde bonding is broken up by an acid environment.
Before the development of aldehyde tanning proper, skins were hung in the smoke given off by fresh plants and parts of plants, such as leaves, which release aldehydes as they burn. The smoke also contains tar particles and chinone.
Nowadays aldehydes are mostly used in combination with other tannins. Sometimes skins are subjected to retannage with fish oil.

Synthetic tannins (syntans)
The rise of the chemical industry has led to the development of synthetic tannins. The different types of synthetic tannins are classified according to their usage. Some are suitable for pre-tanning, others as combination tannins (used together with other tannins), replacement tannins, retannins, tannins to shrink leather, tannins for produ-cing alum-tawed leather, tannins with a bleaching, dispersive or neutralising effect, or auxilliary tannins.

Post-tannage processing

Once the skins have been turned into leather they must undergo several additional processes. Originally this was a task for the leather dresser, who received the leather from the tanner. Nowadays finishing takes place in the leather factory itself.

Fatliquoring or lubricating
As leather dries after tanning, it becomes hard and stiff, lacking in pliability, and with a fragile grain. The fibres now lie right up against each other and this can lead to considerable frictional resistance. Fatliquoring or lubricating reduces this resistance and renders the leather supple. There are various methods by which this can be accomplished. For some kinds of leather fat is added as an emulsion (dispersi-on in water), which is deposited chiefly in the outer layers. Vegetable-tanned leathers can be lubricated with oil. The oils also serve to inhibit oxidation of the tannins in the grain (darkening) and to prevent migration of non-bound tannins.

Staking
While the leather is still damp it is flexed from the middle to the sides with a blunt knife on a flat table. This makes the leather flatter and larger and improves its softness. Nowadays this is usually done by machine with blunt, rotating blades of a working cylinder.

Dyeing
There is evidence that people have been colouring leather (with natural dyes) since 5000 BC. The leather was placed in dye baths and coloured with red dyes such as carmine from the cochineal insect (especially alum-tawed sheepskin), turmeric and pomegranates, and with yellow dye from the berries of the rhamnus (Christ's thorn). Blue could be obtained with indigo, black with iron acetate, green with 'Spanish green' (alkaline copper acetate) or a mixture of yellow and blue. Until water soluble aniline dyes were developed in the nineteenth century, a lot of American woods containing dyes were also used: Brazilwood (redwood), fustic (yellowwood), and logwood (bluewood). These provided a large range of colours when mixed. Many simple general-purpose books are bound in alum-tawed, red-dyed sheepskin. The red colour has been applied to the leather in the form of a pigmented finish with a protein-containing binding medium. The paint layer is often found to have crac-kled, for example, as a result of usage.
Coloured finishing coatings began to be used at the beginning of the nineteenth century: first the water- and solvent-based finishes, and in the twentieth century the dispersi-on-based finishing coatings. These layers - which will be described in more detail under 'Finishing' - were used to add not only colour but also an artificial grain. The latter was usually applied in an attempt to imitate other (more expensive) kinds of leather.

Drying
After tanning and dyeing in a bath, the moisture content of the leather is about 65-70%. This must then be lowered to around 15%. When leather is being dried care must be taken to allow the water molecules to evaporate while leaving the fat molecules in place.
Leather can be dried in a variety of ways. Formerly it was hung in drying lofts fitted with shutters and sometimes there were several such floors (drying towers). Leather liable to heavy shrinkage (particularly chrome-tanned leather) is pegged out on stretching frames and tunnel-dried.

Finishing
After it has gone through all previous processes the leather is ready to be given a finishing touch: a dry finish or a finishing coating (the finish). A finishing coating may embellish the leather without affecting its character, but it can also be so thick as to make the leather look like imitation leather. On the whole the dry finish is meant to smooth the colour of the grain, to make the leather pleasant to the touch, to adjust the reflection characteristics of the leather, to smooth away any blemishes, and to protect the sensitive grain from damage.
The kind of finish that is applied depends on the quality of the leather. If its surface is undamaged and has an even (aniline) basic colour, a transparent glazed layer containing a little aniline dye can be applied. If the basic colour is not even, an opaque layer with added aniline dyes can be applied. If leather is not too badly damaged various finishing coatings can be used.
Several of the most important finishing coatings used on bookbinding leather, especially on nineteenth-century leather bindings, will be discussed below. For the most part they have a characteristic (vivid) crimson colour although one also finds green, blue and other colours. They have often been embossed in order to achieve a particular structure, for example an imitation of goatskin (formerly often described in the literature as imitation saffian or morocco), a pattern of fine checks, or small 'dots' in imitation of shark's leather.

1. Water-based finishes
The binding agent of water-based finishes consists of a protein, such as casein, albumen, or gelatin, applied separately or in combination. Natural and synthetic pigments are used as colorants, and sulphonated oils (castor oil, neatsfoot oil) as plasticisers to give the paint layer the necessary elasticity. Emulsified waxes are added for glazing and, if necessary, phenol compounds, which have a strong bactericidal and fungicidal effect.
To promote adhesion between leather and dye, fixatives such as formalin or chrome solutions are used. The dye is applied in very thin layers with a plush cloth or a soft brush, with brief pauses for drying between each layer. This procedure serves to retain the profile of the grain. In order to make the dye less soluble in water and to give a lustrous finish to the leather, the dyed leather is 'glazed' (polished) using casein and albumen. In glazing an agate or frosted glass cylinder is moved mechanically and horizontally under some pressure over the skin in the direction of the fibres (hair growth directi-on). The wax in the finish is plasticised by the generated heat and forms a smooth, lustrous surface when cool. A grained effect can be achieved by boarding: the skin is folded double, the grain surface inside, and pressure is applied by means of special graining boards (drawing 6).
The water-based finishing coatings are not very resistant to contact with alkaline solutions such as ammonia or water containing neutral soap (non-ionic soap). Also they have a small wet-rub resistance.

Drawing 6 Hand boarding / Arm boarding

2. Solvent-based finishes
Often the binding medium of solvent-based finishes is cellulose nitrate (collodion). Natural as well as synthetic pigments are used for colouring. Cellulose nitrate is dissolved in organic solvents consisting of ketones and/or esters. The preparation is diluted as required with alcohols and/or esters to which plastici-sers such as sulphonated castor and linseed oil have been added. Nowadays synthetic plasticisers, such as dibutylphtalate, are used in addition to these oils. No fixatives are added to the dye.
Solvent-based finishes were - and are - nearly always applied with a spraying machine, and more recently also by means of curtain coating. The paint layer applied in this way cannot be glazed; instead the lustre is achieved with a kind of iron or a flat plate in a process known as 'ironing'.
Solvent-based finshing coatings blur the grain structure and often make the leather resistant to cracks, because the softeners seep through the dye into the leather. This is particularly likely when there are great fluctuations in the moisture of the leather, which cause the plasticiser in the finish to be absorbed into the leather.
Dirt does not stick easily to solvent-based finishing coatings, so that dust and other impurities can be easily removed. They have a high wet-rub resistance.

3. Dispersion-based finishes
The binding medium of dispersion-based finishing coatings consists of synthetic polyme-rs and co-polymers, which are available as dispersions in water and are also mixed with casein. Natural as well as synthetic pigments are used as dyes.
As with water-based finishes, sulphonated castor or neatsfoot oil is used, with phenol compounds as bactericides. Dispersion-based finishes are also applied in very thin layers with brief pauses for drying between each layer. After sufficient even layers have been applied and allowed to dry, the leather is 'ironed' or embossed with the required grain pattern, at a temperature of 45 - 50ºC. Dispersion-based finishes are also frequently used as primers for other finishes. Dirt does not stick easily to dispersion-based finishes, so that dust and other impurities can be easily removed. They have a high wet-rub resistance; the paint layer does not absorb liquids.

Leather terminology

The kinds of leather used by bookbinders were also frequently used for other purposes. Since very few manufacturers tanned especially for the bookbinder, he had to scour the leather market for the most suitable products. Certain leathers with a long history of use in the binding trade are still used for bookbinding today. One of the oldest Dutch publications on the manufacture of leather is a treatise by P.J. Kasteleijn, entitled De leerlooijer, leertouwer, wit- en zeemlooijer (The tanner, leather dresser, and tanner of alum-tawed and chamois leather). The book appeared in 1789 as the fourth volume of the series 'Volledige beschrijving van alle konsten, ambach-ten, handwerken, fabrieken, trafieken, derzelver werkhuisen, gereedschappen, enz.' (Complete description of all arts, crafts, trades, factories, processing industries, places of work, tools, etc.), published by A. Blussë & Son in Dordrecht. Besides discussing ordinary, widely used kinds of leathers, Kasteleijn also deals with less well-known kinds, such as Russia leather, Hungarian alum-tawed leather, English sole and calfskin leather, 'Bauzener' leather, Danish glove leather, French or 'Erlanger' glazed leather, Brussels leather, granular/embossed or English saddle leather, black leather for mourning, morocco, cordovan, and shagreen. Kasteleijn drew on books by foreign writers for his treatise: La Lande, von Justi, Jung, Schreber, Sprenger and Beckman.
A study of older literature on tanning, binding, etc., reveals a lack of any consistent or coherent leather terminology. It is almost impossible to discover whether certain regularly encountered names are indeed the original ones. Leather called after a particular region or place (of origin) may already be in use somewhere else where it is known by a different name referring to a different place of origin. This often makes it difficult to find out from historical sources what kind of leather is meant by a particular name. One must remember that a term denoting origin used in one country, does not necessarily refer to the same leather when translated into a foreign language.
Further complications arise with names which, though they originally denoted a specific kind of leather, have become a mere phantasy name or trademark and where nothing is known of the kind of processing the leather has undergone. Nor is this situation improved by the occasional addition of 'genuine' or 'imitation' (as in genuine or imitation morocco).
It is almost impossible to make an exhaustive descriptive inventory of all 'historical' names. Moreover, such a task lies outside the framework of the present publication. To prevent confusion it is necessary to use unambiguous terms and notions, which is something that can be achieved by national and international agreements. In 1992 the Koninklijke Bibliotheek published Kneep en Binding - Een terminologie voor de beschrijving van constructies van oude boekbanden. In this publication the authors WK Gnirrep, JP Gumbert and JA Szirmai also defined terms for a number of materials, including leather and parchment.
As they are also relevant to the conservation of bookbindings, a number of terms from Kneep en Binding referring to leather and parchment are listed below.

Goatskin
Goatskin leather has a characteristic follicle pattern consisting of rows of hair pores, sometimes predominantly parallel and lying in the grooves of the grain. The grain exhibits numerous variations. Terms best avoided: Turkish leather, morocco, shagreen, and saffian.

Sheepskin
As a rule sheepskin leather - notably that of wool sheep, rather than hair sheep - has a smoother surface and a less pronounced grain than goatskin; the hair pores are arranged in groups rather than rows. Sheepskin makes an inferior covering material: the removal of the high amount of fat (up to 30 %) during manufacture gives it a loose structure. Moreover, the upper layer is weak and easily damaged (chafed). Sheepskin is often impressed with an artificial grain (especially in the nineteenth century after being finished with water- and solvent-based finishes - editorial note) in imitation of better/more expensive kinds of leather.
Alum-tawed, red-dyed sheepskin is often used for binding purposes. Basan is a vegetable-tanned, natural sheepskin (its beige colour comes from the tanning).

Calfskin
Calfskin has a smooth surface and a very dense and random follicle pattern. Alt-hough usually vegetable-tanned it is occasionally alum-tawed. It is often dyed, sprinkled and so forth. Cowhide is too thick to be used for covering material, but has been used for straps and (usually alum-tawed) for bands.

Russia leather
The leather of young cattle, tanned with willow bark and impregnated with birch tar oil is called Russia leather. In most cases it is finished with an artificially applied check pattern.

Pigskin
Pigskin has a characteristic follicle pattern made up of groups of three and visible to the naked eye, and a not particularly pronounced grain. It is usually alum-tawed (and has often turned sallow and stiff).

Sealskin
Sealskin comes in fine and coarse grain variants. The follicle pattern is irregular and independent of the grain (hence it is found on the rises as well as in the hollows). There is some resemblance to goatskin and it has an oily feel.

Alum-tawed leather
Leather tanned with alum (with or without the addition of other minerals) is often made of pigskin, but other skins are also used. It is white or light yellow/sallow and is used for bands, straps and covering material.

Chamois leather
Leather tanned with oils or fat is called chamois leather. It is often made from the skin of wild animals such as red deer and roe-deer, but sheep and goat skins are also used. Tannage consists of rubbing fat or oil into the skin and exposing it to smoke (which causes the fatty acids to oxidise). The hair side is usually ground away. Chamois leather is yellowish and very supple. It is used as covering material and also for straps and protective spines.
Chamois leather is sometimes confused with 'reversed' leather, that is to say, leather used as covering material with the flesh side turned outside.

Manufacture of parchment

Parchment was used chiefly for writing, first on a scroll - as is still the case in Israel - and from the second century BC onwards in book form. To make a book, the rectangular cut sheets might be folded one or more times. The skins of sheep and goats from the areas round the Mediterranean were rarely more than 50 cm long by 40 cm wide. In northern regions we find larger skins and also calfskins being used to make books.
The term pergamena is first used in the Edict of Diocletian (301 AD); until that time the term membrana had been used. It is generally accepted that the use of a new term indicates a new or modified product, but so little is known about the parchment of those days that it is impossible to say with any certainty whether this was the case here.
One of our few informants about pre-Christian times is the (unreliable) Roman historian Pliny. He writes that the king of Pergamon (in present-day Turkey), Eumenes II (197-159 BC), was forced to look for alternative writing materials when the import of papyrus from Egypt was suspended. This is supposed to have led to the invention of parchment. Although parchment had been known at least eight hundred years before this date, Pergamon did have a reputation for good quality parchment in classical antiquity. The great change occurred around the fourth century AD, when people started manufacturing parchment using lime water. Until the fourth century skins were mostly treated with salt, flour and other vegetable products that were used to remove the hairs and to prepare the skin. The lime water method may have been introduced by Jews and Arabs to Spain in the early Middle Ages, after which it spread throughout the rest of Europe. Jewish parchment was lightly tanned on the surface with vegetable tannins. Another technique, the splitting of skins, was also known to the Jews and Arabs, even before the Middle Ages. In the West the traditio-nal procedure to obtain the required thickness was to shave the full skin.
Formulas and depictions of parchment manufacturing have come down to us, especially from the late Middle Ages. There is considerable correspondence between these mediaeval formulas and those used by modern parchment makers, and even the processing and tools have not changed fundamentally. For the most part, parchment manufacture is still a matter of handwork.

Manufacture

The definition of parchment used in this publication and taken from Kneep en Binding, states that it is a skin treated with lime water and dried while stretched. This implies that all parchment-like skins that are treated with other substances, such as alum and enzymes, or have been given a surface tanning, or been dried unstret-ched, cannot properly be called parchment. However, these variants are seldom encountered in bookbinding conservation.
One of the oldest and most detailed descriptions of this lime water method is found in an early twelfth-century formula. (Theophilus Presbyter, Schedula diversarium artium. British Museum MS. Harley 3915, fol. 128r.) More modern formulas (i.e. up to the end of last century), indicate how parchment is made 'nowadays'. If we compare the different formulas we find that the oldest does not explain the process whereas modern formula preparations often give very elaborate explanations.
Modern formulas, like those of the twelfth century, begin by soaking the skins in water so as to restore the moisture lost between fleecing and preservation. Soaking swells the skin, thus allowing the lime to penetrate more deeply, and rinses away the salt used as preservative.
In the twelfth century the skins were then put into a lime-water bath that had usually already been used for unhairing. The skins remained in this bath for about eight days (but twice as long in winter). In modern formulas the rinsed skins are not put into a lime-water bath, but placed over a wooden beam and trimmed on the flesh side with a blunt knife. This stretches them a little and removes dirt and remnants of flesh. It seems reasonable to assume that this was also done in the Middle Ages.
In the Middle Ages unhairing was done on the wooden beam straight after the lime-water bath. Modern formulas sometimes refer to a 'lime dressing' applied to the grain side of the skins, after which unhairing takes place. However, lime water was also used.
In the course of the nineteenth century, sodium sulphide was added to the lime water to speed up the unhairing process. During unhairing - with a curved or straight knife - the hair roots and the content of the sebaceous and sweat glands were also removed as much as possible. The skins were then put into a fresh lime-water bath (about eight days in the Middle Ages, one to three weeks in around 1900, depending on the size of the skins). This opens the skins up for the new, fresh lime and care must be taken not to 'burn' the skins with an excessively strong lime solution. After removal from the lime water the skins are returned to the wooden beam to be scraped clean once again on the flesh side.
Threads attached to tapered wooden pegs are then fastened to the edge of the wet skin. The pegs in turn are inserted in holes around the edge of a square, round or rectangular stretching frame. By turning the pegs the skin can be stretched into a smooth, creaseless surface. Stretch drying is essential for making parchment for it causes the fibres to lie in a flat plane parallel to the surface. Stretching also makes the parchment opaque. Rewetting manufactured parchment allows the stretched fibres to relax with the result that when the material dries again it becomes rough and horn-like.
Consequently, parchment should only be dampened if it is allowed to dry in a stretched condition. In the medieval formula stretched skins are scraped on the flesh side with a sharp, semicircular knife (drawing 7), and left to dry in the shade for two days. After drying they are dampened again and the flesh side scraped with powde-red pumice.
In modern formula preparations the skin is also scraped smooth on the flesh side with a similar semicircular knife. Scraping is accompanied by the continuous addition of lime water. For this operation slaked lime is used, prepared by temporary exposure to air in order to diminish the etching effect. During this treatment the stretcher is in a horizontal position, with the flesh side up. The grain side is treated only with powdered chalk, which has a polishing effect. This is followed by further polishing with pumice or powdered pumice. Split skins are treated in the same way on both sides. Scraping and polishing is done several times until all loose fragments of skin on the flesh side and the papillary layer of the grain side have been removed. Removing the papillary layer is important because it also removes the pigmentation from the skin (black-patched calves!).
After scraping and polishing the parchment is dried in the shade for some days. When it is thoroughly dry, it is taken from the stretcher and, if necessary, cut to measure.
The old and the later formula preparations use much the same method of treatment. Similarly, tools and equipment, such as stretchers and knives, are virtually the same. In eight hundred years little has changed in the manufacturing of parchment; the main difference between then and now is in the use of chemicals. Modern machines make it possible to split the skins to the required thickness before they are turned into parchment. This produces a flesh split and a grain split. In antiquity sheepskins were also split into two layers, without the help of machines. This was possible because sheepskins consist naturally of two clear layers separated by a loose, fatty layer. The practice of splitting skins ended after the third century, however. In order to give both sides of the skin the same surface, the grain was scraped off with a razor-sharp, semicircular or round knife. A knife-sharpener was used to make a burr on the edge of the blade, thus turning it into a kind of scraper. With the grain removed, the surface became velvety to the touch. This parchment was particularly suitable for books, because there was little difference between the verso and recto sides of the pages.
Nowadays, too, the grain is often scraped (using machines and sandpaper) instead of being split. Where the whole skin (grain and fibre network layer) is left unscraped, as for instance for bookbinding parchment, pigmentation is removed with alum, enzymes or a bleaching agent (hydrogen peroxide).

Parchment terminology

Like leather, parchment is also given various trivial names. Many of the terms we use hail from abroad, and this may cause confusion. It all started in France, where the term velin was used alongside parchemin. This gave rise to vellum which is used especially in Great Britain and, sometimes incorrectly, in the Netherlands. The British parchment manufacturer makes a distinction between parchment and vellum. Parchment is traditionally used for the split skin, vellum for the complete skin. In British technical jargon the terms sheepskin vellum and sheeps-kin parchment are used, although not always consistently. Dutch terms such as francijn and forril give no indication as to either the original animal or the method of manufacture.
It is important, especially for documentation relating to parchment conservation, that the terminology used be as unambiguous as possible. For parchment, as for leather, this publication has adopted the terminology as used in Kneep en Binding where parchment is defined as an 'animal skin, preserved by treatment with lime, stripped of hair and remnants of flesh, and dried while stretched, which causes the arrange-ment of the skin fibres to change, and its characteristic qualities to appear (slight thickness, a certain transparency and a light colour). It is sometimes possible to distinguish between the hair side (traces of hair pores; often smoother) and the flesh side (rougher structure); but some forms of treatment make it almost impossible to make this distinction.
Most parchment is made from sheep, goat or calves: sheep, goat or calf parchment; these are distinguished mainly by the hair patterns.'

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