The Making of Leather
Leather is a manufactured product that can be made from the skin of any living creature using a variety of methods. Because the last fifty years have brought great changes in the processing of leather, it is impossible in the scope of this paper to describe in detail all of the developments that have occurred in leather manufacture. I will focus on the major processes of leather manufacture. The majority of historic and contemporary leather has been manufactured by one of three different processes. Historically, these processes were known as tanning, tawing, and chamoising but are now referred to as vegetable tanning, alum tanning and mineral (chrome) tanning. Of these, vegetable and mineral tanning are the most common. According to Thomson the majority of leather objects found in museums were vegetable tanned. A general description of the vegetable and mineral tanning processes and their implications for conservation will be discussed below. Although the tanning of leather is only one step in the overall production of leather objects, it offers the most issues affecting conservation. One should not forget that methods of dressing and/or finishing leather also give particular qualities and characteristic appearances to leather. Because the breadth of information and variety of leather processing makes it impossible to examine all forms of leather creation, dressing, or finishing in this paper, readers interested in the processing of leather should refer to Ekornes and Guldbeck.
The skin of any animal is made up of three layers. The outer layer or epidermis, the middle layer or corium and the bottom layer or flesh. When processing leather, one is concerned primarily with preserving the corium from putrefaction. "The corium is made up of collagen fibers, which in turn are composed of helically-twisted protein biopolymers, chemically linked to one another to allow for strength and flexibility". The important thing to remember here is that it is the unique chemical and structural properties of the collagen fibers that allow for the processing of raw skin into the durable material known as leather. It is equally important to understand that the chemical and physical structure of the collagen fibers differs from species to species. Such differences can influence the conservation of the objects. The most commonly used skins for leather processing, both past and present, are those of cattle, sheep, horse, goats, bison, and pigs.
Once the animal dies, the dead skin begins to lose water through evaporation, leaving the skin inflexible and stiff. It is the job of the tanner to give the dead skin stability similar to that which it experienced during its life. Tanning is a chemical process that stops the natural decay process of raw skin. Prior to being tanned, leather must go through a preparatory process. This is the same regardless of the final tanning method employed. During the preparatory processes the hair and epidermis of the hide are loosened so that they can be removed by scraping off. The bottom layer of skin is then cut away and the corium is then ready for its conversion into leather. Without these processes, the tanning solution would be unable to infiltrate in and around the collagen fibers and the skin and any attached muscle tissue or fat would decay.
In vegetable tanning, tannins are used. These are derived from various plant sources, most commonly oak and chestnut trees. The chemical process of tanning occurs when the tannin molecules bond to the collagen fibrils and separate them. Vegetable tanning is a slow process consisting of placing the leather in baths containing the tannin materials. For this reason, vegetable tanned leather can take up to two years to produce. It is worthwhile however, since it yields a tough, durable, and workable leather. Upon completion of this process, oils and lubricants are applied to the leather to provide flexibility or for further manufacturing processes. The natural color of vegetable tanned leather prior to finishing ranges from a pale brown to a reddish brown depending on the specific tanning agent used.
The characteristics of leathers made using vegetable tanning vary, since the tannin can be derived from a wide variety of vegetation. Where conservation is concerned, the vegetative material source for the tannin is particularly important. The overall life of the final product may be related to the tannin used in the processing of the leather. Likewise, the ageing characteristics of the leather vary considerably depending upon the type of vegetative materials used.
Certain tannins require processing that produces leathers that are particularly susceptible to attack by a destructive chemical decay known as red rot. Red rot is a deterioration of leather that produces a red, powdery surface. Red rot occurs when the tannin reacts with sulfuric acid. Leather objects affected by red rot will go through a variety of stages. Vegetable tanned leather made between 1850 to 1900 is particularly susceptible to this reaction. Museums with large shoe collections and libraries with books dating from this period will attest to this. This is due in part to the removal of what are called non-tans while manufacturing leather during this time period. Non-tans are protective enzymes usually found in animal skins. When the animal is alive, non-tans help to protect the animal's skin from environmental influences as well as to increase its durability. According to Haines, before 1850 organic acids were used during the hair removal process. Because these acids are not as active as mineral acids, they did not remove all of the calcium salts (non-tans) in the leather. After 1850, however, liquid sulfuric acid, a more active mineral acid, was used and it removed all of the calcium salts. Use of sulfuric acid produced the more uniform finish desired by leather manufacturers. Although the calcium salts contribute nothing to the processing of leather, they did offer protection against the ill effects of contact with sulfuric acid in a gaseous form. With the complete removal of the non-tans, these leathers are much more susceptible to red rot. Unfortunately, there is no cure for leather objects affected by red rot. All that can be done is to try to preserve the object in as good a condition as possible for as long as it will last. Leather objects dating before 1886, when chrome tanning, a type of mineral tanning, was perfected, were probably processed using a vegetable tanning method.
Mineral tans are also likely to show up in collections. Objects tanned by a mineral tanning method are usually lighter-colored than objects tanned using a vegetable-tanning process. Mineral tans include the alum and chrome tanning processes. Alum tanning was originally produced using a solution of alum and salt into which skins from goats and sheep were steeped for 10 to 15 minutes, removed and then dried. This process, also known as tawing, produced a pure white leather that was soft and resistant to microorganisms. The product produced by this method is not permanent leather, since the process can be reversed by immersion in warm water. Therefore, objects made from this type of leather should not come in contact with water. Contact with water will reverse the tanning method and the leather will begin to decay. Outside from contact with water, this form of leather processing is quite stable. It is long lasting, not subject to red rot, and requires little or no treatment. In fact, alum tannages have a reputation for long term durability even in polluted atmospheres. Leather that has been tanned by the alum tanning process is pure white in color and somewhat stiff. However, leather objects made after 1884 were probably not processed using alum tanning.
The most common form of mineral tanning in more recent times uses chromium salts. Beginning in 1884, the chrome tanning process began to be used on a large scale. The adoption of chrome tanning methods revolutionized the leather manufacturing industry since it decreased the amount of time it took to process leather. What took days to vegetable-tan could be done in a matter of hours with the chrome tanning process. Today, over 80% of leather is produced using variation of chrome tanning. Chrome-tanned leathers are hard-wearing, supple, stable, not subject to the ravages of red rot and can withstand hot, even boiling water. In fact, the chrome tanning process produces leather that can be used under conditions that would be damaging for leather produced using any other tanning process. Unfortunately, the resilient, open texture of chrome tanned leather carries with it less desirable qualities. For this reason, chrome tanned leather holds water, feels clammy and does not hold its shape as well as vegetable tanned leather. Generally, chrome tanned objects are pale blue in colour and can not be made pure white.
Different leathers are selected to fit different end purposes. In some cases, the changes that leather undergoes do not occur until the leather enters the dressing and/or finishing stage of processing where it could experience any one of a variety of different treatments. These treatments change the appearance of the leather and add desirable qualities to its overall performance. Dressing of leather follows the actual conversion of raw hides and skin into leather and is used to modify the character of leather for different end uses. For example, sole leather undergoes the further process of being rolled or hammered to flatten the leather. Then it is lightly oiled. These final processes give it solidity and a degree of pliability with which to perform its intended function.
Finishing methods go beyond the altering of properties of leather resulting in changes to its overall appearance. According to Thomson, leather finishes can be considered as analogous to any other paint or surface coating since they are applied to make the surface more attractive and to protect it. Traditional finishing processes include staining or coloring the surface, dyeing, graining or embossing a pattern on the surface, plating to produce a smooth and glossy surface, enameling as with patent leather, and abrading to produce a suede or velvet finish. Each dressing and finishing possibility offers its own issues regarding the conservation of leather objects.