The examination of the chemistry of leather and its deterioration over time has been explained in detail (Haines 1991; Sykes 1991; Thomson 1991a; Thomson 1991b; Jordan–Lloyd 1943). More general process information about the production of leather and the implications of such processes for conservation have received far less attention. Although leather conservators are not common, staff at many museums need to understand the proper methods of caring for and conserving leather artefacts. Expanding beyond the basic chemical description by explaining the processing and deterioration of leather in an easily understood manner will provide these museum professionals more information to better care for the leather objects in their collection.
I reviewed the current literature available, including books, manuals, and monographs on the manufacture, processing, deterioration, aging, and conservation of leather and leather artefacts. What I discovered upon doing this was that there was little information about leather processing and conservation that was directed towards the average museum professional.
Leather is a particularly difficult material to conserve. Where conservation of leather is concerned one can not regard it as just a single material (Waterer 1971). Part of the reason for this difficulty is because one must consider the nature of the raw hide and the processes involved in its being made into leather. While this description sounds fairly uncomplicated, one must take into consideration that leather can be made from almost any animal hide, i.e. mammals, reptiles, amphibians, and processed in many different ways. To complicate this even further is the fact that the majority of leather objects are multicomponent. This means that they contain materials other than just leather. It is particularly important to understand the ways in which leather is processed and cared for to be able to predict aging problems.
Leather conservation is usually accomplished in one of two ways. In the interventive method, the conservator is trying to stop the deterioration process. One way in which this is accomplished is by treating the leather part of an object with an oil or lubricating dressing. In doing this, one is treating leather artefacts in a way similar to the care of brand new items. In the preventive method, the conservator is more passive in decreasing deterioration and most often focuses on proper storage methods and environmental conditions. Within the last five to ten years, conservators have moved away from the interventive approach and devoted more attention towards the preventive approach for working with leather objects.
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 (Waterer 1971). Of these, vegetable and mineral tanning are the most common. According to Thomson (1991a) 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 (1997) and Guldbeck (1969).
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 fibres, which in turn are composed of helically-twisted protein bipolymers, chemically linked to one another to allow for strength and flexibility" (Storch 1987). The important thing to remember here is that it is the unique chemical and structural properties of the collagen fibres 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 fibres 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 (Beruldson 1996; Haines 1991a; Storch 1987).
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 (Jordan-Lloyd 1943). 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 fibres 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 (Storch 1987). 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 (Guldbeck 1969). Upon completion of this process, oils and lubricants are applied to the leather to provide flexibility or for further manufacturing processes. The natural colour of vegetable tanned leather prior to finishing ranges from a pale brown to a reddish brown depending on the specific tanning agent used (Waterer 1971).
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 (Thomson 1991a).
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 (Canadian Conservation Institute 1992). 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 (Haines 1991c). 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 (Plenderleith 1970). According to Haines (1991c), 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 (Waterer 1971). 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 (Graham-Bell 1986; Guldbeck 1969). 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 (Color Plus 1997).
Mineral tans are also likely to show up in collections. Objects tanned by a mineral tanning method are usually lighter-coloured than objects tanned using a vegetable-tanning process (Canadian Conservation Institute 1992). 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 (Waterer 1971). The product produced by this method is not permanent leather, since the process can be reversed by immersion in warm water (Sykes 1992). 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 (Haines 1991b). Leather that has been tanned by the alum tanning process is pure white in colour and somewhat stiff. However, leather objects made after 1884 were probably not processed using alum tanning (Storch 1987).
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 (Thomson 1991a). 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 (Haines 1991b). 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 (Waterer 1971).
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 (Waterer 1971).
Finishing methods go beyond the altering of properties of leather resulting in changes to its overall appearance. According to Thomson (1991), 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 colouring 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.
The Aging of Leather and its Implications for Conservation
Like many other materials, leather undergoes a variety of changes as it ages. Red rot, described earlier, is one such deterioration (Plenderleith 1970). Red rot occurs as the leather reacts with sulfur dioxide or other air pollutants. Objects affected by red rot go through several stages. In the early stages of red rot, leather will exhibit a pinkish colour that becomes progressively darker as the decay progresses (Waterer 1971). The first sign of chemical decay appears as cracks in areas that are bent during use. As the deterioration advances, the cracks spread open and the underlying tissue begins to loose its resiliency and eventually disintegrates into a reddish-brown powder. With time, the entire object begins to loose its coherency as the more delicate parts disintegrate (Plenderleith 1970). When the leather is a dark red, it is in the final stages of disintegration and a decision must be made whether it is reasonable to attempt to save the object from complete decay (Waterer 1971). The degradation and disintegration caused by red rot cannot be reversed. However, the length of time between the beginning stages of red rot and the final disintegration of the object varies. Measures can be taken to slow the chemical disintegration (Waterer 1971). For instance, the object could be stored in a controlled air environment where contact with air pollutants would be minimized. Similarly, the object could be stored or displayed so that it is not handled, since handling the object causes physical distress to the object and increases chemical disintegration. For more information regarding red rot refer to Waterer (1971) and Plenderleith (1970).
While red rot is an example of the worst case scenario, there are other equally damaging circumstances that create problems for the conservator. Leather that has been exposed to excessive dryness will have cracking, breakage, and overall embrittlement of the object. Extreme exposure to light will cause similar problems and fading. High humidity levels will encourage the growth of mold that will lead to staining, odor, surface distortion, and a softening of the leather (Canadian Conservation Institute 1992; Storch 1987; Waterer 1971; Guldbeck 1969). Contact with insects may be demonstrated by the presence of small holes and loosened parts. The accumulation of dust can be a serious problem for leather conservation since dust particles can be difficult to remove from the irregular surface of leather. Dust creates problems because the particles act as tiny abrasives on the surface (Storch 1987). When objects are made so that leather touches metal, corrosion can cause problems with the leather. The degree and amount of corrosion depends on the type of metal used. With shoes or harnesses, where the metal cannot be removed without damaging the piece, it is best to make sure that the leather is fully supported and the metal clean. For information on cleaning metal objects refer to Canadian Conservation Institute (1988). Damage of a mechanical nature can occur when leather objects are folded or rested on their sides or edges. This can lead to structural damage of the object resulting in splitting and cracking of the leather (Canadian Conservation Institute 1992; Storch 1987; Waterer 1971; Plenderleith 1970; Guldbeck 1969).
In addition, consumer application of dressings, leather oils, or lubricants can create serious problems for the leather. The application of saddle soaps, which are highly alkaline, could produce a situation in which the breakdown of the tannage and leather would be eminent. These dressings were originally intended for use during the tanning process and were not intended for use as leather cleaners (Canadian Conservation Institute 1992; Storch 1987). The indiscriminate application of these dressings can result in stickiness, oxidation, migration to other objects, and the promotion of biological deterioration. They may also create problems related to leather swelling, dissolution of original adhesives, and distortion of surface finishes (Storch 1987).
Issues Regarding the Conservation of Leather Artefacts
Leather conservation is an area that is constantly changing as a result of research into the development and evaluation of historic and contemporary products. Such research illustrates the difficulties surrounding the conservation of leather. Currently there are two primary methods employed for leather conservation. The older method uses the application of dressings or treatments as a means of prolonging the life of leather objects. Recently, however, there has been a movement away from this interventive treatment of leather and towards a more preventive approach. This new method of leather conservation focuses on improving the way in which leather artefacts are stored. The advantages and disadvantages associated with each method will be addressed.
Because of the wide variety of dressings and treatments possible, space limitation prohibits a detailed discussion of the way in which this method of conservation works with specific interventive treatments. However, essentially a dressing is chosen based on the leather process used to create or care for the artefact and any dressing or finishing. The interventive dressing of interest is then applied to the artefact in the appropriate manner.
The principal reasons behind the application of a dressing or treatment for leather artefacts are outlined in great detail by Tuck (1983). In his monograph, Tuck describes six reasons for applying oils and fats to leathers. First, the fibrous structure of leather may require lubrication. According to Tuck, dressings will help control the internal friction between the collagen protein fibres and the cross links created during tanning resulting in a more durable leather product. Second, by incorporating fatty materials, the softness and extensibility of the leather will decrease the likelihood of its drying out. Third, dressings retard the penetration of other chemicals into the leather. Fourth, natural voids in the leather or those resulting from chemical modifications will be filled with oils and fats increasing the support and solidity of the leather. Fifth, the oxidation of oils will be less likely since dressings containing unsaturated fats increase the chemical stability of leather. Finally, Tuck states that dressing leather with oils and fats will improve the artefact's overall appearance.
Unfortunately, the application of dressings can produce unexpected and serious problems for conserving the leather. While dressings may improve overall appearance, current research indicates that these oils and lubricants are not effective in preserving leather. Landman (1991) questions the need for applying lubricants to museum objects. "For museum objects, which are handled only infrequently and where flexibility of the leather is no longer of prime importance is there a need for further lubrication?" He goes on to say that "in conservation the lubricant can only be applied to the leather surface, often only to one surface and with the minimum of mechanical action." This is unlike leather manufacturing where the leather can be fully immersed in the lubricant. Furthermore, the conservator needs to recognize that the lubricant might change over time, seriously affecting the object being conserved (Landman 1991).
In addition to the problems surrounding the use of a lubricant in leather conservation, their use does not meet the principle of reversibility set forth in the American Institute for Conservation of Historic and Artistic Works (AIC) Code of Ethics. The principle of reversibility states that the conservator "should avoid the use of materials which may become so intractable that their future removal could endanger the physical safety of the object. They also should avoid the use of techniques, the results of which cannot be undone if that should become desirable" (American Institute for Conservation of Historic and Artistic Works 1979).
In setting up guidelines for the proper storage of leather artefacts, it is important to look at the problems surrounding current storage practices for leather artefacts. Leather may experience problems with humidity and temperature levels, light, dust, fungus, mold, atmospheric pollution, and damage due to improper care. An assessment of these problems prompted the Canadian Conservation Institute (1992) to set up guidelines for proper storage methods. They recommend regulating the environment in which leather objects are stored by maintaining humidity and temperature levels in an acceptable range. Leather artefacts should be stored in an area with a humidity level in the range of 45% to 55% [Calnan (1991) suggests 55% – 65%] and a temperature of 18 degrees Celsius to 20 degrees Celsius. Regulating humidity levels is extremely important since leather responds to even the slightest change in humidity (Calnan 1991). By maintaining environmental control, conservators and other museum professionals will avoid problems with mold, which requires a relative humidity in excess of 65% to grow. Monitoring objects in storage will help to identify any potential problems with both mold and insect infestation.
The light to which an object is exposed is also of importance since painted and dyed leathers are extremely light sensitive. For this reason, the Canadian Conservation Institute (1992) recommends that leather objects should have minimal exposure to sunlight or direct light. Leather objects should always be stored in the dark. As dust is also an enemy of leather, objects should be protected from dust settling on them. Whenever possible, metal components that come in contact with leather should be removed.
Of primary importance in preventive conservation is the full support of leather artefacts while they are in storage to prevent stiffness or loss of shape over time (Canadian Conservation Institute 1992; Storch 1987; Guldbeck 1969). Full support minimizes the cracking and splitting of leather due to mechanical stresses resulting from use. Supportive devices should be developed for all areas of the artefact that may need them. For footwear, this means both internal support to help maintain their shape and external support to prevent the footwear from tipping over and to prevent abrasion from other materials.
When cleaning or handling leather one should have clean hands and should not use objects that may leave a permanent mark on the leather. The method of cleaning recommended by the Canadian Conservation Institute (1992) is brushing leather with a soft bristled brush to remove dust and dirt particles. While brushing, a vacuum cleaner with a fiberglass cover attached to the nozzle should be held so that the dust brushed off the leather will be removed.
The movement towards the proper storage of leather artefacts is partially in response to the questions surrounding the use of leather dressings. According to the Canadian Conservation Institute (1992), several of the problems outlined in the previous section could be avoided or minimized if leather were stored and displayed in a proper manner. For this reason, most leather conservators focus their efforts on identifying the proper methods for the storage and display of leather objects (Canadian Conservation Institute 1992; Storch 1987; Guldbeck 1969). One of the obvious advantages to the implementation of proper storage methods is that it adheres to the AIC Code of Ethics. It is completely reversible and does not endanger the physical safety of the object. Also this method is cost-effective in that the conservator does not have to buy or apply special dressings. By using material remaining from other conservation projects to create supports for leather artefacts, savings on expenses for supplies can be realized. Finally, the biggest advantage to this method is that a person with little knowledge of conservation methods could implement proper storage practices. This is of particular importance when one considers the large number of museums with a small budgets and staffs.
The main disadvantage associated with the adoption of proper storage methods for leather objects is that it does not treat leather that is exhibiting serious problems such as those discussed previously. Also the size and nature of some leather artefacts, i.e. saddles, may require rather large and elaborate storage. While these procedures would be the best for the preservation of the leather, they may lead to problems connected with the amount of storage space available. Finally, the multicomponent nature of many leather artefacts is not addressed by the implementation of proper storage methods. What may be a good environment for the storage of leather may be bad for the storage of wood, metal, and other materials.
The methods used to conserve leather objects are always under review. Many of the articles, monographs, manuals, and books that address the conservation of leather are outdated. For this reason it is the responsibility of the museum professional to stay abreast of the latest developments in order to determine the best method for the conservation of leather artefacts. Future research into the different finishing and dressing processes associated with the manufacture of leather goods would contribute significantly to understanding the degradation of leather and determining proper conservation and storage methods.
Dr. Sara Kadolph and Dr. Jane Farrell-Beck, Department of Textiles and Clothing, Iowa State University.
References and Bibliography
American Institute for Conservation of Historic and Artistic Works. 1979. Code of Ethics and Standards of Practice. Washington, D. C.: American Institute for Conservation of Historic and Artistic Works.
Beruldson, D. 1996. "Hides, skins and leather adding value to an important co-product." CSIRO Leather Research Center Web Site. http://www.wark.csiro.au/leather/dwtbroc.htm
Calnan, C. N. 1991. "Ageing of vegetable tanned leather in response to variations in climatic conditions." In: Calnan, C. and Haines, B. (eds.) Leather: Its Composition and Changes with Time. Northampton: The Leather Conservation Center, 41-50.
Color Plus. 1997. "About leather: Why leather goes bad and what to do about it." Color Plus Web Site. http://www.colorplus.com/book/book.htm
Ekornes. 1997. "Leather glossary." Ekornes Web Site. http://www.ekornes.com/stressless/leaglos1.htm
Haines, B. M. 1991b. "Mineral, alum, aldehyde and oil tannage." In: Calnan, C. and Haines, B. (eds.) Leather: Its Composition and Changes with Time. Northampton: The Leather Conservation Center, 24 - 28.
Haines, B. M. 1991c. "Natural ageing of leather in libraries." In: Calnan, C. and Haines, B. (eds.) Leather: Its Composition and Changes with Time. Northampton: The Leather Conservation Center, 66-74.
Thomson, R. S. 1991a. "A history of leather processing from the medieval to the present time". In: Calnan, C. and Haines, B. (eds.) Leather: Its Composition and Changes with Time. Northampton: The Leather Conservation Center, 12–15.