3. Causes and phenomena of leather and parchment decay

Damage to leather and parchment

Deterioration in leather and parchment bookbindings can be categorised according to the cause and the associated external manifestations. A distinction is made between chemical, physico-mechanical, and biological damage and physical damage. The survey below covers the most important factors involved in the various forms of damage encountered. Several photographs illustrating typical examples of such damage are included.

Chemical damage

Leather
A serious form of chemical damage affecting leather bookbindings is the one in which the fibres break and, in the final stage, completely turn to powder; this may give rise to a brick-red colour (red rot). Photograph 1 shows a leather binding of 1891 where part of the grain has already been affected by this form of degradation. An advanced stage can be seen in another nineteenth-century bookbinding (photograph 2).

The discovery of high levels of acid and sulphur in leather exhibiting these symptoms had already led people over a century ago to point the finger at air pollution (combustion products from gas lamps and hearth fires). Their suspicions were confirmed by the fact that the spines of books that had stood untouched in a bookcase for a long time exhibited a higher acidity and a higher sulphur content than the boards that had been protected. Gaslight is no longer a source of air pollution but the degradation of bookbindings has nonetheless continued apace due to other sources of pollution (industry and car traffic). Nowadays, the absorption of nitrogen oxides and their conversion into nitric acid probably plays a greater role in the process of decomposition. Sulphur dioxide absorbed by the leather from the atmosphere is converted into sulphuric acid in the presence of an oxidant, for instance oxygen, ozone, and nitrogen oxide(s). The sulphuric acid thus formed catalyses the hydrolytic degradation of the leather collagen, thereby considerably reducing the strength of the leather. A high acidity and an increased percentage of sulphate are characteristic of this form of degradation.
Besides mediating the conversion of sulphur dioxide into sulphuric acid, oxidants also affect leather directly. The main sources of this oxidative form of leather degradation is oxygen in the air, ozone emitted by photocopiers or formed from oxygen in the atmosphere under the influence of sunlight, and free radicals - highly reactive fragments of molecules - that are also formed by sunlight in the atmosphere. Oxidative degradation manifests itself in the degeneration of the places in the leather fibres sensitive to this process. The resulting degradation products can only be detected by advanced analytical equipment. A characteristic effect of oxidative degradation of leather is an increase in the percentage of soluble ammonium compounds. Although they involve different reaction mechanisms, hydrolysis and oxidation of leather are closely related: both processes often act simultaneously and may reinforce one another.
A third mechanism also connected with the other forms of chemical damage, is the photochemical degradation of leather, where light plays an essential role. Light in general, and more specifically the UV part of sunlight, is an important factor in the ageing of organic material, hence also in the decay of leather. Organic material is composed predominantly of polymers, long, chainlike molecules that with ageing break down into ever smaller pieces in a process known as depolymerisation. In the early stages, this form of decay is extremely difficult to detect. It becomes evident only after the material has already suffered a noticeable loss of strength. The process of depolymerisation can be triggered off by photolysis, caused by light with wavelengths smaller than 400 nanometres (Lodewijks, 1963).
In photochemistry light acts as a catalyst in activating oxygen. Oxygen subsequently acts as an oxidant for the conversion of sulphur dioxide into sulphuric acid. The result is a breakdown of the links between tannin and leather fibre, followed by rapid depolymerisation of the leather and the chemical decay of the tannins. At the same time the activated oxygen can react with water, leading to the formation of hydrogen peroxide which promotes oxidative decay. These reactions, set in train by all wavelengths of the light, cause fading of the colours on dyed leather (photographs 3 and 4).

Not only is the degree of chemical decay strongly determined by direct contact with the environment (air pollution, light), but there are also specific local forms of chemical decay in leather. Deterioration of leather can, for instance, be caused by the action of corrosive substances associated with surface-etching techniques. The marbled decorations applied to many leather (especially calfskin leather) bookbindings in the seventeenth and eighteenth century were achieved with aggressive substances. Acid dyes as well as very alkaline products or a combination of both, were used for marbling, speckling and sprinkling. The damage caused in the course of time by acid dyes that were usually based on iron sulphate, can be recognised by black, corroded, 'burnt' patches on the binding (photograph 5).

In alkaline marbling, often executed with potassium carbonate (potash), the etching process damages only the grain, and the black discoloration so characteristic of acid marbling is absent (photograph 6).

Several factors can accelerate chemical decay of leather. The principle external factors, those connected with environmental conditions, are temperature and relative humidity. An increase in temperature serves to speed up all the degradation processes men-tioned. High humidity also has a catalytic effect, particularly on hydrolytic decay. In this context we might also mention the detrimental effect of moisture that, with or without undesirable chemicals, was introduced into leather bookbindings in the course of earlier attempts at conservation (chapter 2).
Another environmental factor which may accelerate leather decay is the presence of minuscule dust particles. Industrialised urban environments have particularly high concentrations of dust particles which can penetrate almost anywhere and can be very reactive. Because they are also hygroscopic, dust particles may cause a damp film to form on a dusty surface. The consequences, once the particles have settled on a surface, are several. Physically they act as an abrasive, chemically they may, when combined with moisture, behave like a chemical reagent. This may be alkaline (cement-clay particles) or acid (certain metal salts).
The acceleration of chemical decay in leather is also affected by a number of internal factors. These are closely connected with certain constituents of the leather that are a direct consequence of the leather manufacturing process:

• Sulphur compounds (sulphate, sulphide, sulphite) present in the leather and whose influence is not yet quite clear. They appear to be present partly in the form of soluble compounds and partly bound to the fibres. The sulphur compounds derive from the leather manufacturing process where sulphurous chemicals are used at different stages to accelerate or facilitate certain processes (chapter 1). These compounds are largely washed out during the following steps of the process, but still remnants remain, bound to the fibres. There is evidence to suggest that these compounds are released during the ageing process of the leather and converted into sulphuric acid. This accelerates the degradation of the leather.
• The use of less suitable vegetable tannins. It appears that leathers tanned with pyrocatechol or condensed tannins (e.g. mimosa) are generally in a worse state than leathers tanned with pyrogallol or hydrolysable tannins (e.g. sumac). This is probably connected with the fact that leather tanned with tannins of the latter group appears to absorb less oxygen and sulphur dioxide from the air over a given period of time.
• Traces of specific metals such as copper, iron and manganese that might have been present in the rinsing water used and that sometimes remain behind in considerable concentrations. These metals act as catalysts in the conversion of the sulphur dioxide absorbed by the leather into sulphuric acid.

Parchment
Although parchment is on the whole more durable than leather, especially with regard to hydrolytic deterioration, it too is susceptible to some chemical decay processes. In general, degradation of parchment is often directly or indirectly the result of the manufacturing process (chapter 1).
From the early nineteenth century chemicals such as calcium oxide, calcium carbonate, and sodium sulphide have been used in parchment manufacture. They were added to the lime bath to accelerate the unhairing of the skins. The disadvantage of these fast working baths was that they often removed too much tissue material from the skins, resulting in an inferior or even poor quality skin.
Parchment thus manufactured is therefore often less durable than parchment that was made with lime only (i.e. before the nineteenth century). The effect of other chemicals, used to counteract the damage caused by the fast working baths, was even worse. The parchment acquires a stiff structure and loses its elasticity. One chemical used a lot in this context was formaldehyde which has a slight tanning effect. The problem with sulphurous chemicals is that they are difficult to rinse off and remain behind in bound form.
In parchment, too, the sulphur compounds are converted into sulphuric acid by the catalytic action of iron and copper ions present in the skin. Sulphuric acid reacts with calcium carbonate present in the parchment to produce calcium sulphate (gypsum). A dullish grey colour in parchment is an indication of this type of deterioration. In conditions of fluctuating humidity, the calcium sulphate thus formed will repeatedly dissolve and recrystallise. In the course of this process dirt and dust particles from the atmosphere may become trapped in the parchment so giving the surface an increasingly grey appearance.
The UV radiation in daylight and artificial light plays an important role in parchment degradation. Parchment, like leather, is affected by photochemical reactions in which hydrogen peroxide is formed (photochemical degradation). The parchment is broken down in this process and gelatinised. This seriously damages the cohesion of the fibres and the parchment becomes brittle, fragile and liable to split. Gelatinised parchment brought into contact with water, for instance during conservation treatment, will disintegrate into loose flakes. The water balance of parchment that has been partially converted to gelatin is disturbed. Unable to absorb sufficient moisture, the parchment turns hard and shrinks. The hard gelatinised mass prevents the fibres in the parchment from moving freely.
As with leather, environmental conditions (temperature, relative humidity and dust particles) can accelerate chemical decay in parchment.
Discolorations, which usually appear earlier on the spine than on the boards, are caused mainly by light. Photographs 7, 8, and 9 (arranged in order of increasing severity) show the results of chemical deterioration in parchment.

Physico-mechanical damage

Leather
One of the causes of physico-mechanical damage in leather bookbindings is the frequent stretching and shrinking of the leather due to fluctuations in relative humidity. Cracks may appear and the grain may separate from the fibre network layer. This applies particularly to books bound in sheepskin; the weak chain is the layer of fatty cells between the grain and the fibre network layer.
As the leather continues to degrade, its capacity to absorb or relinquish moisture decreases. In an environment with great fluctuations in temperature and relative humidity this may lead to desiccation of the leather, a process that becomes irreversible in heavily aged leather. Photograph 10 shows a leather bookbinding covered with cracks caused by fluctuations in atmospheric humidity.


Dyes and other finishing layers applied to leather can have different effects on the physico-mechanical damage. Photograph 11 shows an alum-tawed sheepskin bookbinding of which the red layer of paint, often used for such bindings, is severely cracked. The leather itself, however, does not exhibit any signs of damage.


This is different in the example of the sixteenth-century bookbinding in photograph 12, where the alum-tawed leather is coated with an albumen layer, usually identifiable by the smooth, shiny surface. This layer has hardened the grain, causing cracks.


Physico-mechanical damage to leather may also be due to earlier conservation treatments which have disturbed the leather's natural water balance. Examples are the thick wax and resin layers applied to leather or an excess of fat introduced into the leather. But too little fat can also cause physico-mechanical damage. Certain hygroscopic substances formerly added to bookbindings to increase the water absorptive capacity of the leather may end up having the reverse effect, because these substances may start to absorb the moisture in the leather fibres themselves.

Parchment
Parchment is far more sensitive to heat and less resistant to fluctuations in temperature than leather. Irreversible changes may already occur at temperatures over 30 °C. Relative humidity also plays an important role in the development of physicomechanical deterioration in parchment. In an overly dry environment, parchment will relinquish moisture, causing it to dry out, split and warp. An environment that is too humid may lead to rippling and often irreversible distortion of the parchment (photograph 13).

Biological damage

Fungi play a much smaller part in the degradation of leather and parchment than in paper. Under proper hygienic and climatological conditions leather and parchment bindings will not be much troubled by this form of biodegradation. The basic form of leather, the animal skin, is preserved by the tanning process, and thus made practically invulnerable to micro-organisms, but parchment, prepared from an untanned skin, is in principle a culture medium.
A conditio sine qua non for deterioration by fungi is a combination of a high level of humidity (RV 70%-100%) with a high enough temperature (>22°C). If leather or parchment bookbindings do get mouldy, it will often be the later finishing layers that serve as a substrate. But conservation methods applied at an earlier stage can also promote fungal growth if they make the leather (or parchment) too fat and consequently liable to retain dust and dirt.
If bookbindings have become moulded, this will be manifested mainly in a clammy feeling (if the infection is recent), various forms of discoloration, a powdery deposit or woolly growths (photographs 14 and 15), and a characteristic smell.

The insects found in book and archival collections, such as woodborers and silver-fish, first attack the paper (or the cardboard and the wood) of the text block, and the boards (they are cellulose eaters). But in many cases this also entails damage to the covering material. The borers eat tunnels through the entire book. Silverfish graze on the surface and are particularly interested in the starchy adhesives.
Insects that feed on animal matter, such as carpet beetles, hide beetles and larder beetles (= bacon beetles), are generally less frequently found in libraries and archives. Omnivores, such as cockroaches, can do considerable damage to parchment, leather and paper. They like to sample the exterior of the objects.

Physical damage

Books can be damaged by (thoughtless) use. Headcaps may be damaged, even torn off, when books are inexpertly removed from the shelves. Boards and spine covers are often pressed open too far and thus strained. This puts so much pressure on the hinge points that they break. Clasps and bosses may leave deep impressions in the covering material of books placed next to them. The grain may eventually wear out if a book is frequently slid across bookshelves and tables. Parchment, because of its smooth, solid upper layer - often finished with soap, shellac and egg white - is less sensitive to sliding and chafing than leather.

Literature

Frey, R.W. and Clarke, I.D., in: Journal of American Leather Chemists Association, 26 (1931), pp. 461.
Goddijn, P.A. and Leeuwen, I.M. van, 'Schade door leermarmerverven', in: de Restaurator, 23 (1993), 2. pp. 58-64.
Haines, B.M., 'Deterioration in Leather Bookbindings - Our Present State of Knowledge', in: British Library Journal, 3 (1977), pp.59-70.
Lodewijks, J., 'The Influence of Light on Museum Objects', in: Recent Advances in Conservation: Contributions to the IIC Rome Conference, London 1963, pp. 7-8.
O'Flaherty, F., et al. The Chemistry and Technology of Leather, 4. New York, 1965.
Protein Chemistry for Conservators, Rose, C.L. and Endt, D.W. van, (eds.) A.I.C., 1984.
Sierpapier & marmering. Een terminologie voor het beschrijven van sierpapier en marmering als boekbandversiering. The Hague/Brussels, Koninklijke Bibliotheek, 1994.
Soest, H.A.B. van, Hallebeek, P.B. and Stambolov, T., 'Conservation of Leather', in: Studies in Conservation, 29 (1984), pp. 21-31.
Vlimmeren, P.J. van, Chemie van de lederbereiding. Waalwijk, 1955.
Waterer, J.W., 'Leather', in: A History of Technology, 2. Oxford, 1957.
Young, G.S., 'Microscopical Hydrothermal Stability Measurement of Skin and Semitanned Leather', in: 9th Triennial Meeting Dresden, German Democratic Republic 26-31 August 1990, pp. 626-631. (ICOM Committe for Conservation 1990).