CCI2014050710

The silver layer unrier the yellow varnish which produces the effect of gilt was also tamishing through the effect of the excess grease ćmd apparently light. The reason why light should be acting as a catalyst in this way has yet to be fully explained to me but clearly areas of the gilt leather, which were hidden behind shadows on the wali, remained still golden ooloured while brightly lit areas were tamished black.

Because the gilt leather hangings had to be removed from exhibition because of building renovations, we were able to give them a conservation treatment of which the removal of the excess grease from the morę than forty individual panels proved to be the greatest problem. No attempt was madę to reverse the tamishing on the silver layer.

Research

After establishing the grease content of the leather by the extraction with an organie solvent (in this case petroleum ether) it seemed that there was about 12.4% grease content in the leather, whereas 3% is considered to be enough. Because of the large amount of grease in the leather, the percentage of unbound moisture was reduced and therefore also the excess of fat had to be removed. With a fat percentage of 3% it is possible to manage the moisture content better which improves the preservation of the leather. It is not possible to remove only the excess fat and leave only 3%. Instead ALL the fat has to be removed and afterwards the desired level of fat has to be added to the leather.

The degreasing of the leather

Origionally we tried hexane and later trichlortrifluor-ethane to remove the linseed oil and the neat's foot oil from the leather but it seemed that odourless white spirits was the best available choice. People must be aware not to choose an organie solvent which removes all the moisture content from the leather which would completely dry it out. We were fortunate in the discovery of the use of non-aroma-tic white spirits which is less poisonous and flammable than hexane and trichlortrifluorethane. The use of hexane

10    degraase the leather would not: havo 1 * •< -i i |>onniasable I n the Rijksmuseum.

'Ihe degreasing process

We chose a stainless Steel basin for the degreasing process Ior several reasons:

It has a long life it could easily be sealed off it could be madę to our specifications the cost was not expensive.

After consultation with our officer and our electrical department, we provided the bath with a lid with a safety earth. We chose a bath of a size which could also be used for other purposes later but it was also important that the bath was madę large enough for the white spirits to work effectively. Ihe method we devised for stacking the leather panels in the bath with perspex laths worked very well and we were able to treat five panels at the same time.

During the treatment in the bath with the white spirits, gloves were wom which could withstand strong Chemicals,

011    and solvents. This was to protect the operator against the possibility of allergic reactions causing an eczema on the skin. In addition, a carbon filter mask was wom to overcome the tranąuilising effects of the white spirit fumes. Work was also carried out in a well ventilated room with an efficient fume extraction system. It was calculated at the Central Laboratory that the optimal ratio of grease to white spirits for efficient removal of the grease was 1:40. That is, 1gm grease to 40gm white spirits. One panel of the leather weighed about 800gm. Additional tests indicated that there was a 6% concentration of grease on the fibrę structure of the leather. There was therefore 6% of 800gm, that is 48gm, times 40 is 1920gms white spirits, or a total amount of 98.4 litres of white spirits for all 40 panels. In order to treat five panels in the bath at the same time it was necessary to use 130 litres of white spirits. There was therefore plenty of white spirits for treating all forty panels. We had calculated an extra 3 litre per panel (that is, 20 litres in total) for the solvent that would be absorbed by the leather or be evaporated during treatment. In fact only 10 litres was