Summer 2003 Issue 44
Materials and their interaction with museum objects
All objects are subject to deterioration from chemical interactions to some extent. The agents of chemical interactions might come from the materials from which an object is made, from materials used to treat the artefact, or from corrosive interactions with emissions from materials in close proximity. The results of some of the degradation processes are obvious while others are less visible bit, nonetheless, significant.
In order to protect objects within their care, museums seek to minimize potential deleterious interactions by choosing appropriate materials to use in the storage and display of objects. A summary of the types of interactions affecting different artefacts may suffer is given below. This is followed by a short list of indicative materials that present minimal potential hazards. Lists of materials that have been tested for their corrosiveness, either within the V&A or by other organisations, and have been found to be suitable for use in short-term display or storage containers for artefacts, can further help selecting materials appropriate for use with museums’ collections.
Sensitivities of objects to chemical degradation vary widely depending on the materials from which they are made. General comments on common reaction for different artefact types precede a table indicating the effects of a range of common pollutants, below. The comments and table serve as an approximate guide only and consultation with a knowledgeable practitioner in this subject is highly recommended.
Metals and their alloys show a large range of corrosion reactions depending on the elemental composition of the alloys as well as the nature of the reactants. In general, metal corrosion is accelerated in the presence of moisture and acidic environments. Sensitivity to further corrosion is usually increased in the presence of existing corrosion patinas.
The large range in coloured corrosion products has been a rich source of inorganic pigments through the centuries and corrosion patinas on ancient artefacts are an important contribution to the aesthetic qualities and value associated with these objects. Nevertheless, it is generally accepted that further corrosion of metal artefacts in a museum should be avoided!
Materials such as cotton, cellulose, wool, silk, and leather are affected by highly acidic or alkaline conditions that can cause acid or alkaline hydrolysis, and by oxidation or biodegradation.
Minerals and rocks (calciferous materials)
Objects such as bones, shells, sandstone, limestone, and the like will be dissolved in the presence of strong acids (and, to a lesser extent, in the prolonged presence of weak acids). Mineral hydrates are very sensitive to changes in water vapour concentration and the reaction rates of many minerals increase significantly with water vapour concentration.
Ceramics and vitreous materials
The stability of glass objects is very dependent on their composition. Vulnerable groups can be particularly unstable even over short period (especially at high humidities and in acidic conditions). Ceramics tend to be stable but again the stability will depend on a number of factors including the nature of the body and the temperature at which they are fired.
These materials include natural rubber (isoprene) and artificial rubbers (chloroprene and neoprene). Rubbers deteriorate by oxidative degradation and would need to be stored in an oxygen free environment or coated with protective films for long-term preservation. They are particularly affected by strong oxidants (ozone, oxygen, peroxides, nitrogen oxides). They are also affected by exposure to high levels of acetates (natural rubbers) and (for chloroprene and neoprene) by a high levels of exposure to a range other pollutants including carboxylic acids, aldehydes, peroxides, phosphoric acid, sulfur dioxide and toluene.Vulcanised rubbers are a common source of sulfurous gases, released on their breakdown.
Many plastics leach plasticizers to become brittle and discoloured as they age. Several also undergo chain scission (saponification in the presence of moisture or oxidation/reduction in the presence of strong redox agents, heat and light) and chemical breakdown, releasing monomeric units from which they are composed. They are affected by exposure to a range of solvents (which can make them soft or tacky) and exposure to toluene diisocyanate. Many are degraded by acids and these reactions are often accelerated by the presence of metals (chiefly iron and zinc).
Ideally, museum storage and display environments should be free of all reactants that can interact with objects. This ideal is not (usually) a realistic expectation: many artefacts may be sources of corrosive agents that will interact with other objects. Anyway, increasing access to objects inevitably increases the rate of their degradation, if only by exposing them to radiation necessary for them to be seen and keeping them in climates that are appropriate for visitors.
Approaches to ensure that unacceptable rates of chemical interactions are not encountered in the storage and display of artefacts include:
- Use only inert materials in the construction of containers (avoid the problem)
- Use protective films to contain any sources of reactant species – by sealing materials that are known to be a source of corrosive agents or to provide a protective film around the artefacts themselves (keep the problem out)
- Use sacrificial materials preferentially to react with the corrosive species.
Each of these approaches has its own drawbacks and limitations:
- The number of truly inert materials is limited and they do not, on the whole, offer enough flexibility fully to meet the design requirements. Restricting use to only such materials may also impose a severe burden on resources
- Protective films on artefacts may significantly reduce their aesthetic qualities (although the use of such films, for instance, for polyurethane or rubber artefacts,may be the only way of preserving the artefacts against rapid oxidative decay). The performance of barrier films used on materials known to be a source of hazardous corrosive emissions is dependent not only on the material from which they are made but also on the quality of their application. Many materials used as barrier films are themselves potential sources of corrosive reactants. The performance of any seal will deteriorate significantly if it is punctured (for instance, by pinning objects through it) and the effectiveness of any protective film may deteriorate with age and wear.
- The dynamics of interactions with sacrificial absorbers or adsorbers will affect the success with which these materials reduce the concentrations of reactive species near artefacts – especially if they need to be placed in discrete positions so as not to interfere with the display.
Acceptable strategies will balance the risk to artefacts with access to them, in keeping with available resources. This will normally involve a combination of approaches, such as:
- Restricting materials as far as possible to ones that are inert
- Testing all materials not known to be inert to ensure that they do not pose unacceptable hazards to the artefacts in their vicinity
- Appropriately sealing materials that have been identified as potential sources of corrosive species to which artefacts in their vicinity are sensitive
- Giving appropriate consideration to sealing artefacts themselves (for instance, lacquering silver objects or varnishing artefacts) if they are made of materials particularly sensitive to decay
- Allowing sufficient time for any materials to dry and off-gas fully before installing artefacts (offgassing times are dependent on the mechanism by which barrier films are produced and range from one day – for powder coating baked on to metals – to four weeks or more for films on wood product substrates produced by solvent evaporation, catalytic polymerisation, or coalescence)
- Segregating display contents to avoid interactions between artefacts
- Including general adsorbers (for instance, charcoal cloth or molecular sieves) in display cases contain artefacts that may be emitting corrosive chemicals
- Regularly inspecting the displayed artefacts so that problems are detected at their onset and appropriate action can be taken.
Other mitigation strategies will involve the control of climates (for instance, most metals will not corrode at relative humidity below 40%). Guidance on acceptable environments (climate and radiation) is outside of the scope of this paper but forms a crucial aspect in providing the best care for artefacts in a museum’s keep.
Materials for display cases
The materials listed below can be considered as inert in terms of their chemical interaction with museum objects. They are safe for use in constructing tightly sealed environments providing good protection against externally generated pollutants and variations in external humidity.
- Glass and other vitreous systems
- Rigid plastic materials such as acrylics (Perspex™, Plexiglas™) and polycarbonates (Makrolon™). Unplasticized polyvinyl chloride (uPVC) structures are also likely to be stable for the periods in question although there is a small risk of the emission of HCl over longer time periods
- Unbleached, undyed cotton
- Acid free paper and card (“archival quality”)
- Metals – but beware of the possible problems due to electrode potentials between different metals
- Powder coated metals (if appropriately baked)
- Alcoxy (non-corrosive, alcohol curing) silicones
- Silica gel
- Polyester films (Mylar™ D, Melinex™, etc.)
- Polyethylene, polypropylene, and other “virgin grade” polymer films not containing plasticizers etc
- Carboxy methylcellulose adhesive.
Other materials should be tested for corrosiveness and pH before being used in the vicinity of delicate artefacts. However, it is unrealistic to expect each exhibition to undertake a full materials testing program before commencing the fabrication of displays, both because of the significant resources such testing programs require and also because of the delay this would necessarily introduce to the commissioning of an exhibition.
Many institutions now hold lists of materials that have been tested in the past and some, like the British Museum,will sell lists of materials found acceptable. Strictly speaking, the test results are only valid for the batch of material as tested as deterioration mechanisms involved in producing corrosive agents are often complicated and not elucidated in detail. Manufacturers may change composition and formulation processes without notification. Even very small modifications might introduce chemical components that will over time release corrosive agents, turning a material that had been tested as safe in to a potential hazard for artefacts.
An acceptable balance between the reliability of historical data from materials tests and the resources and time available for commissioning an exhibition might be to accept only results from test that have been performed in the last few years (minimizing the risk that the product formulation will have changed). If the evaluation of a materials is based on test results that are more than, say, five years old, exhibition teams will need to get reassurance from the manufactures / suppliers that the formulation and processing of the materials in question have remained unchanged since the testing was undertaken.
A final word of caution is warranted. Material degradation processes are sometimes not linear in that the evolution of pollutants may not be constant with time. Manufactures frequently use adsorbents and chemical modifiers that have a defined lifetime when in use. When these modifiers are exhausted there can often be a rapid increase in pollution emission.