21 Apr 21

Sheep conservation: A hard slide


Long term conservation work has been completed on Hard slide by the prominent Australian sculptor Les Kossatz. The sculpture is a construction representing five sheep on a wooden slide and is made of cast bronze heads and feet with real sheep skins sewn around a padded armature. The unusual material choice by the artist, with the use of unwashed sheepskins in this sculpture, has had implications for the long term stability of the work and provided an opportunity for interesting conservation research and treatment to stabilise the fragile sheepskins.


About 10 years after the creation of Hard slide, significant degradation to the sheep skins were noticed and in consultation with the artist, the sheep in poorest condition was refleeced in 1993. The sculpture has since intermittently been on display with temporary repairs to the remaining sheep. It was evident that major stabilisation work was necessary to the remaining fleeces to reduce further deterioration. There were many conservation options to reinforce or ‘repair’ degraded leather but before commencing any conservation work it was important to confirm how the leather was made. In contrast to other raw materials, leather cannot be regarded as just a single material as the processing and tanning of skin give varied final results and can result in the object being susceptible to certain types of degradation.

There are also external factors that influence deterioration of leather, including the environment in which it is stored and displayed, and agents like moisture (water), heat, light and pollution which get absorbed into the leather and causes harmful results. Ideal storage conditions for leather is 18 degrees celsius with a humidity range of 45-55%. Avoiding fluctuations in humidity is paramount as leather responds to even the slightest change. Leathers are also sensitive to light, and overexposure will cause bleaching and embrittlement. Leather also has a great ability to absorb sulphur dioxide from the atmosphere, second to only cotton and wool, transforming it into sulphuric acid which attacks the leather. This is particularly common in vegetable tanned leathers. This degradation process irreversible and is known as red rot, as the leather is turned into fine reddish powder.

While the tanning of raw skins is only one step in the production of leather, it offers the most issues affecting long term preservation of the skins. Tanning is the process of creating soft and flexible leather from the skin of an animal which will also resist the natural process of decay. Untreated, the skin is subject to rapid bacterial deterioration and the un-tanned skin will become stiff and un-flexible, making it unsuitable for many uses. The most common methods of tanning are vegetable and mineral tanning- the former producing tough durable leather used in harness and saddlery while the latter produces softer leathers used in the production of wallets, shoes and automotive interiors.

The NGV conservation labs are equipped with a range of chemical and analytical equipment. Specific established methods to identify tanning processes range from Colorimetric spot tests to X-Ray Fluorescence (XRF). We were fortunate to have small reference samples of the different fleeces that could be used for testing. Colorimetric spot tests are carried out by applying specific chemicals to a sample to produce a chemical reaction which causes a visible colour change. To see if vegetable tanning had been used, the reference samples were tested using the ferric and vanillin tests. A negative result for both indicated that the sheep were not tanned using this method.

Colorimetric spot tests

Burn tests were also carried out to small reference samples of the original fleece, placing the samples into a flame and using the colour of the ash as an indication of the tanning method used. These tests confirmed that the skin was not tanned using chrome tanning as the characteristic green colour was not present in the ashes.

Further analysis of the skins was undertaken using X-ray fluorescence spectroscopy (XRF) to identify the specific elemental composition the artwork. XRF is a non-destructive analytical technique used to determine the elemental composition of a material by using a high energy X-ray beam to interact with the atoms which measure the characteristic fluorescent X-rays it emits. The results from the analysis confirmed that the composition of the 1993 replacement skin was different from the original four sheep. The replacement skin contained aluminium (as you would expect in an alum tan} while the original skin showed neither aluminium nor chromium confirming the results from the chemical and burn tests.

With analysis excluding alum, chromium and vegetable tanning, research was carried out into which home-tanning method were available. Kossatz had been involved in all aspects of the creation of this work, indicating that methods used in ‘home-tanning’ could have been used. Research revealed that sulphuric acid tanning was a likely option, as this method was in common use. Sulphuric acid residue can degrade skin over time and lead to the type of degradation seen in the Hard Slide fleeces. However, this could not be confirmed using XRF as the fleeces naturally contain sulphur and this would also be the natural degradation pathway for the fleeces.

Due to the fragility of the fleeces, conservation work to improve the structural stability was a necessity. By lining the sheep’s interior, additional strength could be imparted while still allowing for movement and flexibility. Reemay®, a random spun-bonded polyester, was chosen and applied in strips. As the sheep fleeces were very water sensitive, a solvent activated adhesive film was used to adhere the strips into place.

In areas of loss, a map of the position of the original pieces was made before removing the original pieces from the existing support materials. To make the fills blend in a custom made textured silicone stamp was used, made by taking an impression from the surface of the belly of the sheep using cling wrap as a barrier.

New fill material and original pieces on Reemay® support

The stabilisation and fills on the Kossatz sheep significantly improved both the structural stability and the appearance of the work. Due to the fragility of the sheepskins, a meticulous approach was needed as the sheep had to keep their original shape at all times during the interior lining to avoid causing damage to the fleeces.

The conservation work on Hard slide was challenging due to the unusual material used in the artwork and the condition of the sheepskins. Despite using multiple different methods to identify the tanning method, it is still unknown how these fleeces were processed making it difficult to predict how the materials will age. What we know however is that the original fleeces are very fragile compared to the fleece that was replaced in 1993.

Replacing the remaining fleeces is an option that might be revisited in the future as the methodology from 1993 treatment is well documented. However, at present Kossatz’s sheep were still able to be stabilised and their condition has improved significantly.