Conservation challenges
Over time – some gilt leather can be 400 years old – leather ages and degrades. Chemical bonds within the leather fibres break down, causing the fibres to shorten. This results in a weaker structure. The natural characteristic of leather to absorb moisture from its environment change also changes as leather ages. Gilt leather responds continuously to its surrounding climate by shrinking or expanding. With ageing these dimensional changes pose more of a risk.
Hanging systems
There are many locations where gilt leather hangings are hanging for a very long period of time and still function well. When hangings begin to fail and leather starts to distort or tear rehanging will be an issue. If gilt leather has to be rehung then the hanging system chosen should allow for the appropriate movement of the leather.
Varnish
The varnish on canvas or panel paintings is there to enhance and protect the painting. In time varnish on paintings is replaced as it ages and darkens. In contrast stands gilt leather. The varnish layer on the silver of gilt leather is an essential part of the decorative surface which should never be removed or altered. Over time this layer may darken slightly and its natural ageing should be accepted. Touching the protective layers of the silverleaf risks tarnishing or -as the silver leaf is so extremely thin- the complete removal of the silver leaf.
Painted surfaces
In contrast to paintings the painted decoration sits on top of the varnish. These paint layers can be covered by later coatings of varnish layers. In general, the same lighting conditions apply to gilt leather as for historic paintings. Therefore light-exposure should be controlled.
Light and UV – use filters or
sunscreens
Prolonged exposure to light, especially the UV-range of the spectrum, will lead to damage to both the leather, the varnishes and the paints. The ‘gold varnish’ contains light sensitive colorants and will bleach under prolonged exposure. Organic pigments, such as indigo blue, red madder, etc. – which are often used in the paints on gilt leather – can be prone to colour change when exposed to light. Cracking of the paints and varnishes is a second sign of light induced deterioration. Light is also an accelerator of corrosion processes, and poses a risk to the silver foil, even if it is covered by varnishes.
Direct sunlight (infrared range of the spectrum) can cause a high increase in temperature locally. These temperature gradients cause local changes in relative humidity, which have an effect on both the leather and the decorative finishes. The leather will show deformations as a result of RH differences. Changes in RH and temperature may cause mechanical as well as chemical damage to leather. The leather fibers will become dry and brittle, and mechanical strength is reduced.
Direct light exposure on gilt leather should be avoided. Preventive measures to be taken are: placing UV-filter foils on the windows, closing shutters when a room is not in use and placing sun screens over windows.
Temperature
For leather materials one can follow the rule of thumb that the rate of deterioration will double when the temperature is raised by 5C. In Moritzburg Castle (Germany) conservators observed that gilt leather wall hangings in heated rooms had a worse condition than the ones in unheated rooms. Temperature is of as much importance as relative humidity for the conservation of gilt leather. For gilt leather it is advised to keep temperatures moderate. And localized rises in temperature, either by sunlight or by heaters / radiator should be avoided.
Silver
The protective layers on either side of the silver leaf are essential to prevent tarnishing as unprotected silver will react to hydrogen sulphide present in ambient air. This tarnishing process will darken the appearance and give gilt areas a dull and sometimes even a black surface. Highly humid conditions (> 70% RH) will accelerate tarnishing processes, as will elevated temperatures. Extreme values should be avoided.
B Indoor environment and preventive conservation measures (MPo)
Most physical damage to gilt leather can be prevented by ensuring that the environment remains as stable as possible and by implementing interventions in the environment.
Relative Humidity in the air
Leather is a hygroscopic material that absorbs and desorbs water in relation to the relative humidity (RH) of the surrounding air. Its flexibility depends to a large extent on the amount of water that it has bound in its fibres. Over time, leather loses much of its bound water and continuous humidity fluctuations cause a gradual dehydration of the leather fibres. This shows as a loss of flexibility, an increase in brittleness and shrinkage. A low RH may cause mechanical damage, such as tears and brittleness. A high RH may lead to expansion causing deformations, and may create tension in the decorative layers, leading to delamination. High RH-values (>70%) increase the chance of mould growth. Sudden changes in RH are a high risk for aged leather, as degraded fibers absorb and desorb water at a very high rate. Sudden water sorption induces high temperatures and causes chemical damage to the fiber.
For leather in archives and museum collections it is advised to keep the RH as constant as possible. In the case of gilt leather hangings in historic buildings maintaining a constant RH is often not realistic. Gradual seasonal changes are usually not problematic for the conservation of gilt leather. Sudden changes can be reduced if there are humidity buffering materials (such as wooden panelling) behind the wall hangings.
Use of heat and moisture in conservation treatments
Aged leather fibers are extremely sensitive to moisture and heat. Before using heat or moisture during a conservation treatment the condition of the leather should be carefully assessed. Test should be done to assess whether heat and moisture can be used at all. The combination of heat and moisture should be avoided all together. When the leather is wet or humid no heat should be applied. And vice versa, when ambient temperatures are high treatments with moisture or humidification should be avoided.