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date: 02 December 2022

Conservation of leatherfree

Conservation of leatherfree

  • Christopher Calnan
  • , revised by Rosie Bolton

Leather is a complex organic material made up of skin proteins, tanning compounds, and lubricants (see also Leather). The character and, to a large extent, the type of deterioration is determined by the nature of the tanning compounds.

Historically, leather across Europe has most commonly been made using various vegetable tannins. This leather is to be found on saddlery and harness, military accoutrements, upholstery, and items that require tooling and embossing, for example screens and panels for wall hangings. Other common types of tannage found in Europe include alum tawing, which produces a white leather used principally for items such as gloves and furs; and oil tanning, which produces buff leather from which gloves and jerkins were made. Oil tanning (brain tanning) was also historically the primary tannage throughout the Americas.

Common tanning processes throughout Asia used alum, oil, and smoke. Natural lacquers, native to Eastern Asia, were commonly applied to the leather to provide a hard-wearing finish (see Lacquer). Gilt leather techniques involving the application of metal foils and paints to achieve a highly decorative finish were common throughout the East and West.

1. Evaluation.

Conservation of leather requires a multitude of techniques and a flexibility of approach, due to the number of variables present in each leather object. Leather conservators use a range of analytical techniques to identify leathers and diagnose common deterioration patterns. These include spot testing and chemical analysis, pH testing, Zooarchaeology by Mass Spectrometry (ZooMS), DNA testing, microscopy, Fourier Transform Infrared Spectroscopy (FTIR), X-Ray Fluorescence (XRF), Scanning Electron Microscope (SEM), shrinkage testing, and X-rays (see Technical examination and imaging).

One of the commonest and most damaging forms of deterioration is the chemical breakdown known as “red rot.” This has only been reported in vegetable-tanned leather, primarily on objects from the late 19th and early 20th centuries. The deterioration is characterized by the reddish coloration of the leather, delamination, embrittlement, and, in extreme cases, a total loss of strength of the leather fibers, which turn to powder when rubbed. It is caused by the action of acidic gases from the atmosphere (principally sulfur dioxide), which are absorbed and converted into acids within the leather. Once the deterioration has begun, specialist help must be sought to stabilize the leather. Acid-deteriorated leathers may be in a very fragile condition and, if this is the case, consolidation and support treatments will need to be carried out after stabilization. Red rot is commonly found in 19th-century leather bookbindings as a consequence of air pollution and of the use of sulfuric acid in the leather production process. Various alkaline reagents have been proposed as treatments for red rot, but none has proved effective in the long term. A similar form of deterioration is caused by contact with iron; it is known as “black rot,” as badly affected leather fibers turn black. It leads to embrittlement, delamination, and loss of strength.

Leather objects can be very hard and inflexible. Sometimes this is an intentional property, as in cuir-bouilli, but often is due to low moisture content or low fat content. Low moisture content is more common, and in this case the leather should be gently humidified under controlled conditions to reintroduce water. Low fat content can be rectified by the addition of a lubricant containing oils and fats; however, lubricants can be damaging to historic leather so advice should be sought about their suitability. Excess oil or fat can be almost as harmful as not enough.

Contact with water can cause many types of historic leathers to stiffen and sometimes blacken. Water and heat combined can cause irreversible shrinkage and stiffening.

Breakdown of oils within the leather can lead to a white bloom forming on the leather surface; this is termed “fatty acid spue.” It is easily removed with organic solvents, but the potential for further spue formation will always be present.

2. Treatment.

Common conservation treatments for leather include cleaning, humidification using controlled ambient humidity, ultra-sonic humidifier or Gore-Tex sandwich (see Conservation: Preventative techniques), and consolidation of friable surfaces (see Consolidant). Where leather needs patching or supporting, it should be carried out with non-woven materials or a suitable leather, and conservation grade adhesives. Woven textiles can impose a directional pull on the leather, causing stress on the fiber structure and encouraging splitting.

In common with other organic materials, leather is affected by extremes in temperature and relative humidity. Long-term exposure to heat and high/low humidity cycles can lead to a cumulative hardening and loss of flexibility. Surface cracking may also occur, and this may eventually develop into deeper cracks and splits especially if the leather also suffers from chemical deterioration.

If leather is stored at too high a relative humidity (above 65% RH), mold growth may occur. When this happens the leather items should be immediately removed from contact with other objects to prevent contamination and placed in a dry environment. The mold can be removed by brush or with a HEPA filter vacuum cleaner and potential further regrowth prevented by attention to environmental conditions. Too low a relative humidity (below 40%) can cause leather to contract, stiffen, and warp, tearing away from fixings. Overly fluctuating relative humidity can cause surface damage, splitting and tearing due to expanding and contraction of the leather.

The best conditions for the storage or display of leather are a relative humidity of 55–65% RH and moderate temperature. Room temperature in itself, unless extreme, should not affect the leather, but as it is so closely linked to relative humidity care should be taken to keep it stable. Direct sunlight and high light levels should be avoided as these can lead to fading, cracking, and embrittlement. For painted leather surfaces (e.g. screens and wall hangings), recommended light levels should be similar to those for oil paintings. Leather should be kept free from dust. Non-buffered acid-free tissue or archival boxes are preferable; however, silicone release paper should be utilized if the leather may have a tacky surface, as a result of spue formation or excessive use of leather dressings. Leather is very prone to deformation: if it is not correctly supported it may stretch and distort, and if folded it may develop crease lines. This applies particularly to leather used in clothing, so costume accessories should be padded with acid-free tissue and garments hung on padded hangers or laid flat without folding (see Conservation: Preventative techniques).

As with all historic objects, care should be taken when handling historic leathers as there may be toxic materials present. In particular, historic fungicides which can be very damaging to health were applied in the past to many leather objects. There may also be toxic pigments present in surface decoration, and many molds can be hazardous to health.


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