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With any leather object, a time comes when its historic value exceeds its utility. At that point, the type of care required to preserve it becomes very different from that which was applied when the artifact was in regular use. Indeed, continuing some practices (for example, saddle soaping, oiling) on aged leather actually will shorten the lifespan of an antique object made of leather.
This article gives a general overview of the structure of leather, the different methods of tanning, the causes of deterioration in leather, and optimal conditions and preferred methods for handling, storage, and display of historic leather artifacts.
Artifacts made of leather, or, more properly, of cured animal hide (from mammals, reptiles, birds, even some fish), most conveniently fall into four categories of tannage. Non-tanned leathers include rawhide, parchment, and vellum. Semi-tanned leathers are oil-tanned or alum-tawed, wherein the skin is simply soaked in either oil or potash alum. Native-tanned leathers include smoke-tanned and brain-tanned, and fully-tanned leathers are tanned with extracts of plants (vegetable tannage) or with salts of metals (mineral tannage). The purpose of tanning a skin is to make it impervious to rot and to render it useable; the type of tannage will determine a leather’s resistance to moisture, its flexibility, appearance, and longevity, and will vary with intended use.
It should be noted that this article is intended to be a guide for preserving leather artifacts that are in a collection of historic objects, rather than those articles that are still in use.
Leathers are made from the dermal layer of skins. The epidermis, the outermost layer of any skin, carries surface structures such as hair, feathers, or scales and generally is removed during leathermaking (except with furs). The dermis, or corium layer of skin, consists of connective tissue fibers (primarily collagen fibers), the cells that produce the dermal structures, and the ground substance (a viscous fluid surrounding the fibers, removed during tanning). The dermis also will contain some fatty tissue. The dermis has two layers: the papillary layer is the outermost; the distinctive grain pattern that one sees on a given piece of leather in the papillary layer is due to the way hair grows on different animals. The majority of the thickness and strength of a piece of leather is in the fiber network layer, which consists of long wavy bundles of collagen fibrils, arranged in a network perpendicular to the surface. Tanning is directed primarily at the dermal layer, specifically at the collagen fibrils: chemically, it fixes the ionizable side groups of the collagen fibrils by increasing hydrogen bonding between collagen molecules. This links the open network of fibers, leaving the leather pliable, and occupies all sites that otherwise would allow the leather to rot.
After tanning, leathers are frequently dressed, or treated with fatty substances, to improve their flexibility and resistance to water and wear. Heavy-weight vegetable-tanned leather is curried with cod oil and tallow, worked in mechanically. Alum-tawed leather is stuffed with flour, egg yolk, or oils. Chrome-tanned leather is filled by fat-liquoring with an emulsion of sulfated oil and water. As a result of tanning and dressing, leathers are acidic, the degree to which being determined by the specific tannage and dressing. Achieving the correct pH in the finished product is necessary to maintain the stability of the tannage and the collagen.
Leathers, being hygroscopic (readily absorb and retain moisture), are capable of being damaged by moisture loss and absorption. Control of the environment in which a leather artifact is being stored or displayed is essential. At a relative humidity (RH) level of 35% or below, leather becomes desiccated and can crack when handled. At RHs of 70% or above, mold growth can occur; molds will break down the very structure of leather as they grow and feed off the proteins in the leather and on the fatty acids in dressings. Additionally, absorption of excessive moisture can promote hydrolysis of the protein chains that form the collagen fibrils, resulting in shrinkage and the ultimate embrittlement of the object. (Hydrolysis is the decomposition of a chemical compound or structure by reaction with water.) Finally, leather may be sewn or riveted to other materials that will respond to changes in humidity at a different rate than the leather, leading to cracking or splitting of the leather. Therefore, a range of 35% to 70% RH is optimal. Monitoring and maintaining stable humidity levels is essential for promoting long-term preservation of leather artifacts.
Elevated temperatures also can have severe effects on leather artifacts. Besides drying the leather, temperatures above 68° F (optimal) can accelerate the chemical reactions just mentioned, causing deterioration to progress much faster. When a leather object becomes embrittled, the bundles of collagen fibrils stick together, resulting in hardening and loss of tensile strength.
If we add airborne pollution to the mixture of high RH and temperature, resulting deterioration can be noticeable in a few years. Airborne sulfuric acid, formed when sulfur dioxide (generated by manufacturing plants and burned fossil fuels) combines in the atmosphere with oxygen and moisture, will attack vegetable-tanned leather in a reaction known as red rot. Initially the leather becomes softened and swollen, followed by cleavage of the protein chains in fibrils, the leather ultimately becoming dry and powdery and easily crumbled by handling. This most often is seen (for example, on leather bookbindings) as reddish, raw areas of loss. Red rot is not reversible; damage from it is permanent. Additionally, all acids, whether atmospheric or residual acids left in the leather from the tanning process, will accelerate the hydrolysis of collagen. It goes without saying that liquid water will have the same effect on partially degraded vegetable-tanned leather. Water is also detrimental to untanned hides, such as parchment, vellum, and rawhide. Contact with water will cause these materials to cockle and/or shrink, and the deformation is for all purposes permanent.
Alkaline conditions (relative to the natural acidity of leather) also can be damaging to leather. A pH* greater than 6.0 will result in hardening and embrittlement of vegetable- and chrome-tanned leathers, whose normal pH varies from 3.0 to 5.0. As an example, buffered tissue (as opposed to acid-free or unbuffered) is impregnated with an alkaline salt and should never be used to wrap a leather artifact.
*pH is a logarithmic scale to measure percent Hydrogen, as an indicator of acidity or alkalinity. 1.0 is the most acidic, 14.0 is the most alkaline; water is neutral at 7.0.
Light can fade dyes and pigments on or in leather and, given sufficient time, will break down virtually any organic material. In leather, strong light accelerates oxidation of organic materials (collagen) and causes the formation of peroxides from tannins and oils in the leather, which catalyze further hydrolysis. Damage from exposure to excessive levels of light is cumulative and cannot be reversed. Avoid exposing leather artifacts to direct sunlight, spotlights, fluorescent lights, and even bright indirect sunlight for prolonged periods.
Mold growth, as mentioned above, can permanently damage leather, but can be controlled by lowering the humidity in which the artifact resides. Mold can appear as a white, gray, or green powdery deposit, or as black spots on leather. Mold can be removed by vacuuming the artifact or by gently brushing the mold off, but the artifact should be removed to a well-ventilated area away from other objects during cleaning to avoid depositing spores on them. If the artifact is fragile, the vacuum nozzle should be covered with screening, and the nozzle should never touch the leather to prevent pulling off sections of the surface. A dust mask should be worn during mold removal, as many different mold spores have been proven to be a respiratory hazard. Removal of the "fruiting bodies" (the visible portion of the mold) from the surface still can leave the tendrils and spores inside the leather. Solutions designed to kill the remainder of the mold can discolor the leather, and some are toxic to humans. It is safer for the artifact if the owner takes advantage of the fact that mold remains dormant at lower humidity and keeps it suppressed by denying mold the humidity it needs to grow. Additionally, improving air circulation around an artifact will reduce the chances of deposition of new spores. Finally, it is possible to confuse fatty bloom from leather dressings and salts from perspiration with white molds; bloom will have a waxy feel, and salts will be crystalline.
There are certain insects for which leather is a feast. Some beetles, such as the hide beetle, the carpet beetle, the leather eater, and the museum beetle, will eat the leather itself. Other insects are attracted to the oils in the leather or to materials attached to or painted on the leather. Artifacts should be inspected regularly for signs of insect activity, which may include droppings, larval cases, holes in the leather, or damaged surfaces, especially on the darker or hidden surfaces of a displayed object.
Dust can be very difficult to remove without damaging a fragile or decorated leather surface. It is a fact of today’s world that airborne pollutants are found everywhere. Dust, in addition to being unsightly, is hygroscopic and contains pollutants; the combination contributes to the degradation of leather. Storage and display enclosures for artifacts should be designed to exclude dust.
Incorporated materials, especially metals (rivets, brass eyelets, buckles) and their salts, can be corroded by leather and can be corrosive to leather. Iron and its compounds seem to be most reactive with leather, followed by copper, brass, and tin. Copper and brass also react with the fatty acids in oils used as leather dressing, forming waxy green accretions of organometallic salts, generally copper stearate, at all areas where leather contacts metal. Wherever it is possible without damage to the leather (such as inside the loop around an iron buckle, behind a brass button), a barrier of thin mylar® sheeting should be inserted between metal and leather, preventing contact.
Ample studies have proven that leather dressings and saddle soaps, rather than preserving aged leather artifacts, actually hasten their deterioration. Oils in dressings are intended to provide internal lubrication for leather that is still in use; the oil allows the bundles of fibrils to slip over each other as leather is flexed, keeping it supple. Historic leather artifacts in a collection no longer need to be flexible, since they are no longer functional objects. Research has shown that many oils and fats used in leather dressings (neatsfoot oil, mink oil, lanolin) oxidize and harden over time, causing the leather to become even stiffer and brittle; oils also will darken with time, thus darkening the leather. Saddle soap originally was developed as an emulsified dressing for leather. Its ability to clean a surface is dubious, as the "soap" in it is employed to emulsify the oil/water mixture, leaving little reserve cleaning power. Saddle soap is also alkaline and leaves residues that cause degradation of the leather.
Obviously, one should never attempt to use a historic artifact, or attempt to reverse the effects of age with the intent of restoring it to usable condition.
The greatest danger to leather from improper handling, especially with aged, brittle leather, is from insufficient support, both when moving it and in storage. If a brittle section is allowed to bend far enough when it is lifted, it may crack; thin leather straps and belts (for example, a sword belt), are most susceptible. Artifacts being lifted should be completely supported by a tray or stiff board that has been covered with polyethylene foam (Ethafoam ®) or unbuffered, acid-free tissue. One will want to provide extra support for heavier, attached elements to take strain off the point of attachment. Cotton or nitrile gloves (many people are allergic to the latex in examining gloves) should be worn when handling objects to avoid leaving dirt, oil, or perspiration on and in the leather. When in storage, artifacts should be wrapped in cotton fabric or acid-free tissue. Leather artifacts that are still flexible (baby shoes, dress gloves) can be stuffed with polyester batting or unbuffered tissue paper to maintain their shape and to provide internal support. Inner corners of sharp folds should be padded to prevent cracking and splitting as the leather settles over time (leather pouches). Artifacts that already are hardened and brittle may be damaged in trying to regain their shape and should never be overstuffed to try to restore lost shape. Wrapped and padded artifacts should be kept in cabinets or in unbuffered, acid-free cardboard or corrugated polyethylene or polyester boxes to exclude light and dust. (Never use any type of vinyl or PVC.) Storage containers should not be completely air-tight to reduce the chances of mold growth within a sealed enclosure. Storage locations should avoid high temperatures (attics) and high humidity (basements), for reasons already enumerated.
Similarly, artifacts being exhibited should be subject to the same limits of light exposure, temperature, humidity, and support as those in storage. A schedule of regular monitoring for insect activity should be followed.
Special problems with a historic leather artifact may be beyond the ability of the owner or collector to repair, such as a cockled parchment document; an exceptionally dirty, painted buckskin shirt; or a torn and red-rotted child’s shoe. In such special instances, the owner may wish to contact a professional conservator, who is trained to perform treatments that neither damage the artifact nor contribute to its long-term deterioration. The conservator will examine and analyze an object to determine original materials used and cause of deterioration. The conservator will propose a minimally intrusive treatment and will document the treatment performed. For help in locating a conservator in your area, contact the American Institute for Conservation of Historic and Artistic Works (AIC), 1717 K Street, N.W., Suite 301, Washington, D.C., 20006; (202) 452-9545.
Disclaimer: Due to the potential fragility of leather artifacts, aged leather can be damaged easily by incorrect treatment. The Museum of Florida History does not recommend that the owners of leather artifacts perform their own treatments and assumes no responsibility for damage incurred by owners based on information provided in this article.