Leather deterioration

Stambolov, T. (1969), Manufacture, Deterioration and Preservation of Leather A Literature Survey, ICOM Plenary Meeting, Central Research Laboratory for Objects of Art and Science, Amsterdam. 

Dimethyl fumarate (DMF) is used by producers as a biocide to kill moulds that may cause furniture or shoe leather to deteriorate during storage and transportation in a humid climate. Placed in sachets, which are fixed inside the furniture or added to the footwear boxes, DMF evaporates and impregnates the leather, protecting it from moulds. However, it has been found to seriously affect consumers who were in contact with the products. DMF penetrated through the clothes onto the skin of many consumers. Cases of severe health problems have been reported in UK, Finland, France and Poland. Although it is normally not used by tanneries, there are examples where tanneries have used DMF. 

Measurement of pH is a potentiometric technique frequently used for measuring the degree of the deterioration of materials that are subjected to natural aging. The determination of pH levels is commonly carried out on ethnographic objects manufactured with parchment or leather, and it is especially relevant in altered paper due to the formation of acidic compounds from the decomposition of the woodpulps and other raw materials, which can induce the hydrolysis of the cellulose and then decrease the resistance and mechanical properties of the document [29]. 

Temperature is a significant factor friction may raise the temperature of the surface layers to the point where they become subject to chemical attack. Abrasion causes deterioration of many materials, especially of rubber (tire treads), where it can be offset by a high percentage of carbon black. Other materials subjected to abrasion in their service life are textiles (laundering), leather and plastics (shoe soles, belting), and house paints and automobile lacquers (airborne dust, grit etc.). 

Materials and Structures. Building materials have become soiled and blackened by smoke, and damage by chemical attack from acid gases in the air has led to the deterioration of many marble statues in western Europe. Metals are also affected by air pollution for example, S02 causes many metals to corrode at a faster rate. Ozone is known to oxidize rubber products, and one of the effects of Los Angeles smog is cracking of rubber tires. Fabrics, leather, and paper are also affected by S02 and sulfuric acid, causing them to crack, become brittle, and tear more easily. 

Bindings covered with vegetable-tanned leather are washed with pure toilet soap, saddle soap, or mild, high quality surgical toilet soap and a 7-10% solution of potassium lactate. 0.25% of p-nitrophenol can be added as a fungicide, but this is not necessary if the temperature and relative humidity are constantly satisfactory (see p. 5). The lactate, when applied to new vegetable-tanned leather, is thought to protect the leather from the deleterious action of sulfuric acid which usually forms in leather when sulfur dioxide is absorbed. It is. probable that it has little efficacy once chemical deterioration has started, and for some years doubt has been cast on its effectiveness, even on new leather, despite favorable results in laboratory tests. However, it is used, and its use is advocated just in case it is beneficial. 

To remedy the fault, Nazi experts first rub the diamond with genuine eau-de-Cologne and, while still damp, placing it in very fine sawdust where it is whirled in a rubber-lined container or shaker, driven by mechanical means, until the proper degree of brilliance and lustre is restored. The process ends with a final polishing with a specially prepared chamois leather. The treatment costs from one hundred to one thousand Reichmarks, and takes the best part of three months to complete, according to the deterioration in the gem that has taken place.65... 

Less recognized but also important is SO- damage to paper and leather. Paper products produced from about 1/50 on are embrittled by sulfur compounds converted to sulfuric acid which causes hydrolysis. Thus old books which are not stored in sealed cases undergo a gradual deterioration. Leather bindings are similarly affected. 

Products used for the preservation of fibrous or polymerised materials, sueh as leather, rubber or paper or textile produets, and rubber by the eontrol of microbiological deterioration. 

Leather objects are likely to show evidence of the wear and tear that they were subjected to before they entered the museum environment. This will include splits, tears, scratches and holes, as well as damage resulting from the effects of perspiration, urine and accidental spillage of other undesirable liquids. This type of mechanical deterioration does not automatically cease when the object is placed in a collection, as damage caused by inappropriate handling is not unknown. 

Leather can be considered as a relatively chemically-stable material. In general objects made from leather reach the end of their useful lives as a result of fair wear and tear rather than chemically-induced decay. If a pair of shoes lasts for 10 years or a wallet or briefcase for 20 years, the owner is usually satisfied. Books are different. Many leather-bound books spend the majority of their lives sitting unused on shelves. Libraries in historic houses, for instance, contain thousands of books which are rarely read but whose bindings remain sound after a century or more. It is not surprising, therefore, that when in the middle of the nineteenth century it was discovered that many new bindings were deteriorating rapidly, an explanation was sought. 

The results showed that while none of the books stored in the clean Welsh environment had deteriorated to any significant extent, nearly all the volumes subjected to London s acidic pollution exhibited evidence of decay, some within less than 10 years. In addition, the superior ageing properties of leathers prepared with hydrolysable tannins were confirmed. Of the various chemical analytical determinations undertaken, the only results to show any correlation with the degree of deterioration observed were those for the number of -terminal amino acid groups on the protein. This figure reflects the amount by which the collagen polypeptide chain had been broken and was considered to be evidence for hydrolytic deterioration. 

It can be concluded that chemical degradation of vegetable-tanned leather will occur in both polluted and unpolluted environments, and that both oxidative and hydrolytic reactions are always involved. The preponderant mechanism will depend on the exact conditions to which the leather is subjected. It should also be noted that the rate of decay of leathers exposed to the mainly hydrolytic effects of acidic urban atmospheres is much greater than the oxidative deterioration found with leathers held in cleaner rural surroundings. 

Conservation can be considered to be the application of a treatment or a series of treatments to an object of artistic, historic or even sentimental value to prevent, halt or reverse deterioration. The more philosophical and ethical considerations of the profession have been discussed, among others, by Pye and the practical application of these considerations to leather objects by Sturge. 

As has been stated, the most widespread form of chemical deterioration found in historic leather collections is the result of a combination of oxidative... 

A number of techniques using similar concepts were developed over the years to treat leathers that had already deteriorated. Aqueous solutions of potassium lactate were routinely applied to the surface of bookbindings and other leather objects. Unfortunately, in many cases, the deleterious effects of the liquid water solvent on acidic leathers took place before the buffer salt neutralised the acid, resulting in dark, seriously embrittled leather. In order to avoid the effects of aqueous solutions, methods were tried employing ammonia vapour. The object was placed in an enclosed chamber over an open dish of ammonium hydroxide solution. However, ammonia vapour is a strong alkali and there is a danger that even with the use of dilute solutions, leathers that were too acidic would be transformed into ones which were too alkaline. Another alternative was to use an organic base such as imidazole in a non-aqueous solvent. However, this was also liable to result in leathers that were too alkaline. 

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