Mistakes of the past: damages from inappropriate treatments
Lack of understanding of the structural components has led to many mistakes and inappropriate restorations. Gilt leather can’t be treated as a painting, nor can it be conserved as a piece of leather. It’s unique layered structure demands its own appropriate treatments.
Surface protection
Most gilt leathers show traces of old restorations. In the 18th, 19th or 20th century during restoration a varnish or a wax coating was applied to saturate the colours. Varnishes were applied using solvents. Often the original ‘gold varnish’- which is extremely sensitive to polar solvents – is damaged. One usually observes corrosion of the silver leaf when the gold varnish that covers the silver is damaged. Varnishes that have been applied more than 100 years ago have usually darkened and yellowed causing a discoloration of the paints. To remove these discoloured varnishes one needs solvents that will also dissolve the original gold varnish. Varnish removal is an intervention with a high risk of damage to original materials.
Waxing will cause dulling of surface: white bloom.
Overpainting
Irreversibility; altering the original outlook; toning down due to changing tastes; ethics
Consolidation
Can alter the natural physical nature of the leather fibres and can lead to hardening and stiffening, and lastly to darkening and irreversibility; epoxy-materials to strengthen the leather fibres
Treatments for Leather are often in conflict with the varnish & paint layers;
Lining
Inflexible structures; irreversibility; the importance of flexible mounting systems
In the past – 19th and beginning 20th century – restorers found that the leather of gilt leather wall hangings had weakened. The seams with which individual panels were kept together were often damaged, torn or ripped. This was the result of repeated re-sewing or of the shrinking of leather. Therefore strips of a textile (linen or cotton), leather or paper, were adhered to the edges. Sometimes it was even found necessary to back the entire hanging to ensure stability. A variety of materials has been used – textiles, leather, paper, that may not be compatible with the hygroscopic behaviour of old leather. Often natural adhesives, such as starch paste, animal glue, or wax-resin mixtures, were used, and these proved too stiff or too strong.
=> examples of old linings causing troubles/problems.
Nowadays the lining of an entire leather wall hanging is rarely done. Strip linings at the borders are applied, mostly to be able to mount the wall hanging on a flexible mounting system. A variety of natural or synthetic materials may be used. Compatibility with the aged leather – strength, elasticity and hygroscopic behaviour – should be carefully assessed.
The application of a lining is not without risk. To adhere the lining two types of adhesive can be used: either a water-based adhesive, or one that needs to be activated by heat. Two recent cases have been reported where the leather has been damaged by the use of heat in humid conditions. When degraded leather is humid or wet and is heated at the same time chemical bonds within the leather fibres start to collapse. The leather turns black, shrinks and stiffens. The leather is permanently altered and cannot be revived again.
Removals & cuttings
Original seams (even when weakened) should never be cut off. The removal of (part of the) original materials should never happen. Present conservation-ethics prohibit such interventions.
Oiling / oil-dressings
Irreversible disruption of the paint surface; darkening of the paint surface; stickiness; effecting the silver layer; mould growth
The animal glue with which the silver leaf is adhered, and the gold varnish on top of the silver, act as natural barriers for sulphide containing gasses in the air. These layers however become get porous over time. Specific restoration interventions (materials), such as the use of certain solvents, oils or emulsions may damage the integrity of the layers underneath and on top of the silver and should be used with caution by conservators.
Up until the last quarter of the twentieth century it was common practice to “feed” old leather with oils or fats to restore some strength, to lubricate and soften the leather, or to improve the surface appearance. In some libraries and museum collections. It was even standard procedure to apply oil dressings or emulsions to historical leathers. Institutions, such as the British Library, the British Museum, and national libraries in other European countries (France, Netherlands, Belgium (check?), Spain (Check?), all had their own recipes for these leather dressings. They are usually composed of oils or fats, lanolin, or wax dissolved in either a solvent (dressings) or in water by means of a surfactant (emulsion).
By the end of the twentieth century these same institutions began questioning the practice of “feeding” the leather as they did not see or detect any positive effect on the condition of the leather. And in many cases they were even confronted with negative side-effects, such as stiffening of the leather, discolouration and staining and softening of the original finishes and decoration.
Past conservation treatments with oil dressings are a serious concern for gilt leathers. In the Netherlands and Germany there are cases where repeated oiling of gilt leather has caused irreversible damage. The oils have migrated through the decorative layers of the gilt leather and have plasticized and partially dissolved varnish and paint layers, resulting in a sticky surface. Recently research has demonstrated that certain oils and oil dressings cause corrosion of the silver leaf, which results in a darkening of the surface. The use of oils and fats is strongly discouraged in current conservation practices.
Mounting: nails, staples & frames
Causing irreversible damages of all kinds: tears and distortions
Nails & staples
Glue/adhesives to s solid support (wooden panels / stucco walls / …)
The applications of heated adhesives, and or heated instruments on gilt leathers might cause sudden and immediate reactions of shrinkage and irreversible distortions
Gilt leather that has been fixed with nails to battens on a wall, or to a frame is likely to tear at some point. To mount gilt leather wall hangings in a safe way, the hanging system chosen should allow for the appropriate movement of the leather. Conservators specialized in gilt leather design flexible hanging systems that are customized to the specific requirements of the location and the climate the gilt leather will be hung in.
In the past newly produced gilt leather hangings were mounted in various ways. Some were hung loosely on small leather loops. Others were fixed to battens using nails. Fixing new leather is not problematic as it is still elastic and does not shrink and expand that much. Aged leather however shrinks and expands enormously in response to humidity changes. Especially large surfaces of wall hangings have been damaged by being fixed with nails or staples. Tears and holes at the borders can be repaired by a restorer, but it is a labour intensive task. A mounting system that allows for movement of the leather in all directions is indispensable for historic gilt leathers.
In museums quite a few gilt leather panels have been mounted onto boards or have been fixed with nails to a strainer. After half a century of time (check!!) the same problem as mentioned above occurs: tears at the borders.
Storage
Inappropriate storage: damp can lead to mould; iron nails can lead to rust; radiator and overheating will lead to shrinkage!