Susan Bradley

The British Museum, Department of Conservation

APPROPRIATE STANDARDS FOR CONSERVATION

ABSTRACT

I am often puzzled by what colleagues mean when they talk about standards for conservation. Do they mean a written document, drawn up through a full consultation process of the industry it will affect and validated by a standards body such as the BSI?. Such a document would include both a specification and a method of measurement to show compliance. Do they mean a standard approach. Scientists use many standard approaches. A 1M solution of hydrochloric acid is a standard solution, and spot test defined by Vogel are standard analytical methods. When these are mentioned in publications they are instantly understood around the world. They are procedures which can be reproduced.

There are few specific Standards for conservation. BS4971 'Repair and allied processes for the conservation of documents' and BS 5454 'Recommendations for the storage and exhibition of archival documents' are the best known. Latterly there has been a tendency for books on environment for the conservation of collections to use the word standards in their titles. However they lack the clear definition and protocols for measurement which would normally be associated with a Standard.

This is not surprising given the non standard nature of the materials and objects dealt with in conservation. Objects come in varying states from completely degraded or corroded in burial, through worn in use, to brand new. Some objects need high levels of control of RH to limit alteration, others survive well in an uncontrolled environment, some are not effected by pollutant gases, and others are.

The damage caused by the indoor air pollutants, acetic and formic acid, formaldehyde and acetaldehyde and hydrogen and carbonyl sulphide has been documented through the conservation literature since the early 70s. We know that to prevent, or reduce the rate of corrosion of metals, and the formation of mixed salts on porous objects containing soluble salts we needed to eliminate materials which give off these gases. The most common material of concern is wood, but varnishes, paints, adhesives and sealants can also give off organic acids and aldehydes. A corrosion test was developed at the BM in the early 1970s to screen materials for use in the storage and display of the collection. Using this test it is possible to achieve an organic acid/aldehyde free environment. Because the reduced sulphur gases, hydrogen sulphide and carbonyl sulphide, are present in the ambient environment, even if materials which give off these gases are eliminated, it is not possible to provide a sulphur free environment without the use of air filtration.

Another source of pollutants is objects which out gas. Everything, but the objects can be eliminated from showcases and stores.

There are two possible approaches to specifying maximum concentration limits for indoor air pollutants. One is to specify an acceptable maximum, the other is to specify an acceptable corrosivity level. There are several problems with specifying a maximum level. The rate of out gassing is determined by ambient temperature and RH. Some objects are more reactive towards the pollutants than others, and as yet we do not know anything about rates of reaction for the very wide range of materials which could be effected. There is no evidence to suggest that reaction rates are proportional to concentration, and nor can there be when at least three factors are involved in the promotion of a reaction, the pollutant concentration, the temperature and the RH.. Yet another problem is the potential for symbiotic reactions where more than one gas, or the combination of gas(es), humidity and temperature promote a reaction. Such reactions are not predictable from measuring the level of one gas. The final hurdle is a cheap, easy to carry out measuring technique. Measurements at the BM suggest that even at very high pollutant levels reactions do not occur on object surfaces when the RH level is low, below 45%RH. That other workers have reported reactions occurring at RH levels as low as 30% suggests that the reaction mechanism may be even more complex than our observations suggest.

The advantage of measuring corrosivity of the environment is that it is a measure which takes into account all of the potentially harmful parameters. There is an international standard, ISO 9223 (currently being revised) which describes a classification of the corrosivity of atmospheres from NOEL (no visual effect level) to saturation. This is an approach developed by corrosion scientists who spend their lives trying to unravel and understand complex corrosion reactions, and may be a more appropriate way of classifying environments for objects. However there will still be a need to seek convergence between the corrosivity level as defined by the test materials, and the actual effect on the object substrates. There is also the problem that organic acids affect a number of different types of substrates and a range of sensors will probably be required.

There are a number of options for the control of indoor air pollutants. These are not to use materials which give off pollutants, to install filtered air conditioning to clean the air, or to use absorbents inside showcases and cupboards. Of these options the cheapest and most effective is to not use materials which out gas. For some museums, and the BM is one, there are certain situations in which wood cannot be removed. In the BM the Grand Rooms such as the King's Library which once housed library books are to be developed as exhibition galleries. Because the rooms are listed the glazed wood book presses will be used to display objects. This will be a challenge, but with careful selection of the objects, and implementation of control measures in individual presses the situation will be resolved.

Air conditioning is expensive, and only really useful to remove external pollutants, and those pollutants which are given off by the materials used in the construction of the gallery space. Indoor pollutants generated within a showcase will not be removed, or even diluted, by a gallery air conditioning system especially when the showcases are very well sealed. Such systems are excellent for visitors, providing comfortable conditions in the cold of winter, and the heat of summer; and for providing conditions for objects on open display which all need the same RH level. However the systems are space hungry, and cannot always be readily accommodated in an existing building.

Absorbents are cheap and useful in well sealed showcases. If used passively they will not remove all pollutants, but they can substantially reduce concentrations, potentially to below danger levels. Another approach is to circulate the air in a showcase through a filter bed containing the absorbent which improves the efficiency of the materials. These materials are useful in existing installations where materials which out gas cannot be removed.

Considering the factors discussed above, it appears to me that defining standards for pollutant levels other than zero, is not appropriate. Providing simple guidelines for other to follow is achievable.

Susan Bradley
Head of Conservation Research Group
Department of Conservation
British Museum
London WC1B 3DG
United Kingdom
e-mail: sbradley@thebritishmuseum.ac.uk