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Evaluation of the correlation between natural and artificial ageing of vegetable tanned leather and determination of parameters for standardizationof an artificial ageing method

Objective

The objective is to develop a standardized method for measuring environmental deterioration of leather and thus provide a basis for conservation.
A project has been set up to identify and quantify the chemical and physical changes which occur in naturally deteriorated vegetable tanned leather due to exposure to atmospheric pollutants and to establish the parameters and conditions of an artificial ageing method which bring about the same changes in new leather as in naturally aged leather.

Naturally deteriorated historical leathers have been analysed from libraries in Denmark, England, Wales, France and Holland. The content of sulphate and the pH value reflected the degree of pollution and hydrolytical breakdown of the leather. The visual difference between the very deteriorated leathers from the British Library and those in far better condition from the National Library of Wales were very obvious and were closely related to the difference in their sulphate content and pH values. Analysis of hydrothermal stability gives a measure of deterioration and can be correlated with structural changes in the form of a melting or gelatinization of the fibres observed under the microscope. Analysis of moisture absorption indicated that drastic changes in relative humidity during storage would be likely to induce more stress on the more deteriorated leathers.

The oxidative breakdown of the collagen has been detected by means of amino acid analysis and naturally aged leathers are characterised by changes in the amino acid distribution of the collagen. In general these changes are loss of the amino acids arginine, hydroxylysine and lysine, and in many cases of the amino acids hydroxyproline and proline, which are all fundamental for the function and stability of the collagen. There are also characteristic changes of the vegetable tannins, which correspond well with the oxidative changes of the collagen. The amount of extractable tannins from leather decreases with increasing deterioration, and at the same time the amount of breakdown products in the form of monomers increases.

Ageing experiments on new leathers have shown that pollution with sulphur dioxide and nitrogen oxides at fairly low relative humidity and slightly elevated temperatures in cycles with dry heat at 120 C, can produce chemical and physical changes which are close to that of natural ageing.
The serious deterioration, known as red rot, of many vegetable tanned leather objects, in particular bookbindings, has been known since the last century. The cause of deterioration is ascribed mainly to air pollution. Many attempts have been made to develop ageing methods to examine the durability of leather, and to predict the effectiveness of conservation techniques. None of these methods has been recognized as an acceptable standard.

In this project changes of leather structure and properties in naturally deteriorated historical leather are evaluated and quantified by means of different analytical methods. These methods include high performance liquid chromatography(HPLC), gas chromatography-mass spectrometry(GC-MS), isoelectric focusing and other relevant chemical and physical techniques. The historical vegetable tanned leather samples are collected in the 4 participating countries.

Ageing trials are carried out in 2 different test plants. Materials used for ageing trials and as controls are vegetable tanned leather and untanned hide prepared specially for the project. Artificial ageing parameters include pollution with sulphur dioxide, oxides of nitrogen and ultraviolet light in atmospheric air at known relative humidities and temperature.

The establishment of the criteria and parameters for a standard method of ageing will be based on a comparison of the analyses of the natural deteriorated historical leathers and the artificially aged leathers and hides.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

Kongelige Danske Kunstakademi
Address
34,Esplanaden
1263 København K
Denmark

Participants (6)

Central Research Laboratory for Objects of Art and Science
Netherlands
Address

1070 AC Amsterdam
Koninklijke Bibliotheek
Netherlands
Address
Prins Willem Alexanderhof
2509 LK S'gravenhage
Københavns Universitet
Denmark
Address
2A,øster Farimagsgade
1353 København K
Leather Conservation Centre
United Kingdom
Address
34 Guildhall Road
NN1 1EW Northampton
MUSEUM NATIONAL D'HISTOIRE NATURELLE
France
Address
Rue Geoffroy-saint-hilaire 36
75005 Paris
ROYAL INSTITUTE FOR CULTURAL HERITAGE
Belgium
Address
1,Jubelpark 1
1000 Bruxelles