Conservation and restoration of leather objects
The conservation and restoration of leather objects is the process of investigating causes of deterioration followed by the best practices of cleaning and restoration to ensure the future protection of leather objects for use or display.
- 1 Properties of leather
- 2 Causes of deterioration to leather
- 3 Principles of care, storage and display
- 4 Conservation and restoration treatments and techniques
- 5 Conservation and restoration materials
- 6 References
Properties of leather
Leather is a highly sought after material due to its resistance to tearing, flexing and puncture. It is also a good insulator of heat and prevents the passage of air flow. In order for skins to be turned into leather they must go through the process known as tanning to stabilize the collagen for the duration of the manufacture. However, leather is unique in that it contains more than just the hide itself. It also contains all of the materials used in the manufacturing process which must be known for the proper conservation and preservation of leather objects.
Manufacture of leather
In Prehistoric times, the use of skins or leather in the early history of man goes well beyond food to include clothing, shelter and other purposes. The first use of animal skins is attributed to Australopithecus habilis who is noted as roaming East Africa some two million years ago and possessed a diet with significant meat consumption. Artifacts from this time indicate the use of a chipped stone used cutting through thick hides. This tool is use is attributed to the increased use of additional stone tools that will be used throughout the history of man. It is suggested from the bones found at the sites of Australopithecine that the joints of larger animals were butchered with this tool from the carcass and dragged on the skin to their dwelling places for multiple uses. Evidence from the hominid, Pithecanthropoid, shows the use of the a coup-de-point tool for butchering and skinning, and that groups would live in large, tent shelters made from spreading skins over wooden frameworks. Skins are also believed to have been running along the sides of shelters in rough bundles. The skins used in these structures are believed to have been warmed by fire which created a curing effect by drying the skins slowly. It is suggested that special skinning and scraping tools were created after witnessing this natural progression of the skins, which spread the skin's fats across the hide. This process is considered by some the first creation of leather nearly a million years ago.
During the Medieval era tanning was considered a skilled trade and was second to wool as a textile fabric. During this period the first job of a tanner was to wash hides free of blood, manure and other curing materials before being rehydrated. To accomplish this task a nearby river or stream was often used. Other methods for this task would include trampling on the hides with feet or pounding on the material with hammers. Upon this task's completion, the hides were then set with the hair side up to remove the hair without damaging the surface. To loosen the hair it was common to either let the leather become putrid or by speeding up the process with liquors such as stale beer, fermented barley or mulberry, dung or bryony leaves. Soaking skins in liquors prepared with wood ash or lime would accomplish a similar effect. When the skin was deemed adequetely prepared it would be stretched over a beam and scraped. The hair side would be scraped with a blunt, single edge unhairing knife and the flesh with a sharper, two-edge fleshing knife. The process would continue with another soaking and scraping with a blunt scudding knife. The next step in the process was to wash the skin further and then add an alkaline bating to remove excess lime and create a softer, finer grain. After repeating this step, the skin would be laid over the beam once more to remove any of the waste products that were released in the process often referenced as slime. After the skin was considered cleaned and prepared it was ready for the tanning process. This would occur by placing the skin in a pit on top of ground tanning material, then covered by additional ground tanning material, and the process would be repeated until the pit was nearly full. Tanning materials were dependent on the local landscape and were most often tree matter (oak, birch, willow, spruce, etc.). Upon filling the pit with the skins, the pit would be filled with cold water or another mixture and the skins were left to sit for close to a year. When a tanner judged that the leather was sufficiently tanned, the skins were removed, rinsed, and smoothed with a knife before being slowly dried in a dark space. At the point that the leather is dried, it is sold to a currier for additional processing.
Causes of deterioration to leather
Acid Hydrolysis is the breakage of bonds by hydronium ions. Hydronium ions are contained within the liquid form of water and cause the breakage of bonds within ionic structures. When an acid is dissolved in water it separates the hydrogen ion from the other ions of the acid and the hydrogen reacts with the water molecules to form the hydronium ions causing the aforementioned hydrolysis. With the presence of an acid in this reaction this process is known as acid hydrolysis. The most commonly agent of acid hydrolysis found in historical leathers is attributed to sulfuric acid, which is absorbed from industrial pollution as sulfur dioxide. In the presence of sunlight, the sulfur dioxide is converted into sulfur trioxide and then absorbed by the tannins in the leather. The trioxide is then hydrated into sulfuric acid, dissolving in the present moisture of the leather creating the hydronium ions. It is then from these hydronium ions that internal links are broken among the leather molecules. Breakdowns include a reduction in bonds between amino acids and collagen, and additional breakdown may occur from the tannins and acids inherently present in the leather which may increase the risks and severity of acid hydrolysis. Heat, moisture and a low pH are also contributing factors in this process.
Oxidation is the loss of an electron among leather, most often attributed to oxygen as the oxidizing agent or molecule of electron loss. Oxidation may also occur due to the effects of light and heat.
Metals that act as catalysts of oxidation are the greatest concern in leather deterioration. Iron is the most common contaminant found in the manufacture of leather and can act as a catalyst for oxidation and eventually lead to breakdown of molecules by hydrolysis.
Heat is the transference of kinetic energy to the leather. When this occurs, the internal molecules of the leather increase in speed and begin colliding with one another at a rate so fast that the bonds of the leather molecules are no longer capable of remaining intact and thus break. The effect of heat is most often associated with long term exposure and rapid fluctuations in temperature on the leather. When exposed to heat in either condition, leather may lose the ability to absorb water from the air and the result is a hard or brittle state, this is referred to as hysteresis. This is due to the interconnected nature of external heat and moisture content in leather. Water: see the above relationship of heat and moisture content in leather
Red rot is the degradation of leather when it 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 color that becomes progressively darker as the decay progresses. The degradation and disintegration of red rot cannot be reversed. In order to preserve the object in this altered state the two most significant steps for preservation are to store the object in a controlled air environment and to place where minimal handling will occur.
Principles of care, storage and display
To ensure the longevity of leather objects these are the proper steps that should be taken toward preservation
Display and storage
When the primary focus of an object's future is storage, the first concern is to ensure that the object is in a stable condition where it can be handled, with aesthetics as a low priority. The handler of the object should be aware of the conditions for display and storage and changes, if any, in location needs should be addressed. The relative humidity of any storage location should be monitored and kept below 65% to prevent mold growth as well keeping good circulation and temperature. Most leather conservators focus their efforts on identifying the proper methods for the storage and display of leather objects because it is reversible and causes no direct physical harm to the object.
Levels of treatment
The first principle in determining treatment is to require that all conservation be reversible, appropriate and the least invasive possible. It should be the intent of the conservation to place the object in a stable condition with the least amount of future deterioration. The simplest form of this process can be found in moving an object to a more stable environment. An example of a minor concern would stabilizing loose sections of leather or a flaking surface of the material. Several solutions could be utilized from application of surface finishes or inpainting of damage. Careful consideration should be given to ensure the selected method is the best conservation practice. On the more severe side of the spectrum of conservation is full restoration. This would mean that a most if not all of the leather covering of an object would need to be replaced. The context in which an object is to be displayed is an important factor in determining what degree of repairs should take place. A used workman's pouch or bag would not require a new condition look as it would alter the object's interpretation, dependent on location and history.
The degree of handling and access will play a large role in the decisions of care and treatment, as well as display locations. If an object is in a position that is highly susceptible to touching or handling by the public, it should either be moved away to a newer location away from open display or staff should be prepared for the object's frequent repair or eventual replacement. In most cases, if an object's condition is to a point where it may be lost forever due to continued exposure, it should be properly stored away in the proper environment and have a minimal future handling as possible.
A finish or dressing is most often applied for aesthetic reasons rather than conservation. When a dressing is applied it may brighten an object and give a new sheen to the leather without ever contributing to the preservation of the object, in some instances this may lead to conservation problems. Therefore, any applied finishes should be carefully considered and studied before placing on an object for conservation. Dressing or finishes may absorb dirt creating a tacky surface. Some may contain inappropriate ingredients or over application may lead to spewing. Spewing is the rise of white deposits of free fatty acids to the leather surface which is often mistaken for mold. Dressings should never be applied to painted leather of any kind. The oils and fats can permanent discoloration and softening of the varnishes. Application of this kind may not happen immediately and should not lead to a belief that it will be successful in the following months or years. Previous removal of over-varnish utilized the same materials as the original varnish, which led to stripping the entire varnish coat. In modern leather preservation the original surface coating may be stripped completely off to provide a uniform replacement finish. It is not advised for this method on historical pieces but is highly appropriate on reproductions and modern leather-works as they may prolong the life of the material without compromising the historical value or integrity.
Conservation and restoration treatments and techniques
There are a number of very specific techniques used to conserve and restore leathers.
- Consolidation - Consolidation is an option for leather that has suffered extensive degradation and the leather surface needs to be reestablished to create a firm surface for repairs and/or cleaning. Common consolidants include Paraloid B67 in white spirit, Klucel G, Pliantex (no longer commercially produced, SC6000, Lascaux wax-resin, and Gelatin.
- Dry cleaning - Removal of dust, which may cause tiny abrasions, should be done by using a brush vacuum. If more extensive dry cleaning is necessary brushes or sponges maybe used. Brushes should be used with care as they may cause similar abrasive issues.
- Humidification - Humidification is the process of adding the appropriate level of humidity to the leather, without wetting, in an effort to reshape and/or restore the object to its desired condition. During humidification special care should be given to ensure that the object never reaches a wet state, as well as, making certain that objects are not left in areas that may contribute to a quick change in temperature due to the shrinkage of leather and its potential damage. Because damaged leather may be more sensitive to heat, objects should avoid any exposure to heat which may be present near windows. Because leather is more susceptible to mold, it should be regularly monitored during this process. Heat treatments should always be avoided while leather is in the humidification process.
- Molding - Molding is the process of adding a fill to the a spot in the leather to return it to its original state. A common fill known as Beva 371 can be painted to match the tone of the original.
- Proprietary leather cleaners - Commercially produced cleaners are commonplace with leather care but can have negative effects if not closely researched or familiar with the ingredients.
- Wet cleaning and solvent cleaning - The primary purpose of any wet cleaning on leather is to remove surface soiling and not to soak the material. By soaking the leather in water, additional issues may arise such as distortion, discoloration, hardening, movement of salts and tannins, and tidemarks. Using cotton swabs with common solutions is a safe and regular practice in cleaning leather. Common solutions used are mixtures of water with ammonia, white spirit, alcohol or detergents. White spirit or mineral spirits is also a suitable solution for wet cleaning. Dependent on the material and its construction, any mixture used should be prepared with caution and the appropriate ratio.
Conservation and restoration materials
There are a number of very specific materials that are used in the conservation and restoration of leather.
- Aluminum alkoxide - 1% in white spirit, used as a chemical restablizing retanning agent for red rot leather.
- Bavon ASAK ABP - leather lubricating compound. Alkenyl succinic acid derivative that is soluble in white spirits and Genklene
- Bavon ASAK 520S
- Bedacryl 1225
- Beva 371
- British Museum leather dressing or Pliantine
- Sodium Carboxymethyl cellulose or CMC
- Connolly's Leather Food - a dressing for hide upholstery, leather goods and clothing
- DDT - now banned
- p-dichlorobenzene - mothball
- Disinfectant 1473 - used as a fungicide
- Dowicide A - water-soluble fungicide made of sodium salt of orth-phenyl phenol
- Ethylene glycol
- Ethylene-vinyl acetate
- Facteka A - granular cleaner for suede or leather with abraded surfaces. Rubber like and made from rape oil.
- French chalk or talc
- Fuller's earth
- Genklene - non-flammable (1,1,1-trichloroethane)
- Invasol S - Synthetic anionic oil
- Isopropanol or Isopropyl alcohol
- Japanese tissue paper
- Lanolin anhydrous
- Lipoderm Liquor SA
- Lipoderm Liquor LPK - synthetic anionic oil free of natural fat
- Lissapol N. - non-ionic detergent
- Magnesium carbonate
- microcrystalline wax
- Neutralfat SSS - stabilized olein soap which on drying loses emulsifying property so that it no longer promotes absorption of water
- New leather
- Paraloid B-72
- Plexisol - consolidant for leather affected by red rot. Polyacrylate resin preparation containing 25% solids. Must be diluted with Genklene.
- Pliancreme - cream form of British Museum leather dressing, emulsified with water, containing a fungicide.
- Pliantex - same as Plexisol (see above)
- Polyester sailcloth
- Polyvinyl acetate
- Preventol L - fungicide; sodium salt of chlorinated phenol
- Opodeldoc recipe,
- PEG 400 or polyethylene glycol
- Renaissance Wax
- Rubber cement
- Saddle soap -
- Santobrite -fungicide for leather, Pentachlorophenol
- Silicon leather wax
- Soluble nylon
- Spun-bonded polyester fabrics
- Tannic acid
- Vulpex - potassium oleate soap, soluble in water or white spirit
- White spirit BS245
- Woven textile
- "Leather Properties and Characteristics". muirhead.co.uk. Retrieved 25 June 2016.
- Dirksen, Vicki (1 November 1997). "The Degredation [sic] and Conservation of Leather". 3. doi:10.5334/jcms.3972. Retrieved 25 June 2016 – via www.jcms-journal.com.
- Kite, Marion and Thomson, Roy. 2006. Conservation of Leather and related materials. Oxford: Elsevier. 73-77
- Waterer, J. W. 1971. A Guide to the Conservation and Restoration of Objects Made Wholly or in Part of Leather . New York: Drake Publishers, Inc.
- "Removing Mould from Leather - CCI Notes 8/1". Cci-icc.gc.ca. 2013-09-17. Retrieved 2016-06-25.
- Canadian Conservation Institute. 1992. "Care of alum, vegetable, and mineral tanned leather." CCI Notes, 8(2), 1-4.
- Kite, Marion and Thomson, Roy. 2006. Conservation of Leather and related materials. Oxford: Elsevier. 126
- Kite, Marion and Thomson, Roy. 2006. Conservation of Leather and related materials. Oxford: Elsevier. 124-126
- Storch, P. S. 1987. "Curatorial care and the handling of skin materials, Part 1: Tanned objects." Conservation Notes, 17, 1-4.
- Kite, Marion and Thomson, Roy. 2006. Conservation of Leather and related materials. Oxford: Elsevier. 121-123