Preserving history

Materials World magazine
,
3 Jul 2015

Harry Lovell takes a closer look at the crucial role anoxic packaging plays in the world of art and history.

The task of museums and art galleries is to preserve history. Superficially, cleaning, retouching, displaying and describing may be seen as all that is required. However, below the surface the scene can be quite different. Leading the charge against preservation are insects. Cigarette beetles, book borers, carpet beetles and webbing clothes moths are just some of those unwanted visitors. Buried in textiles or wooden furniture, they present a serious challenge to conservators. One method of prevention that is growing in use is anoxic packaging.  

Anoxic – meaning without oxygen – has provided a new approach for conservators. Enclosing the item in plastic packaging and replacing the oxygen with nitrogen results in the demise of insects, including larvae, eggs and pupae. Simple, clean and effective and with no damage to the artefact, anoxic treatment has been gradually gaining preference since its introduction in the 1980s.

Finding the right approach

The stage of the insect has to be considered – eggs, larvae and pupae require different approaches. It depends on their location and any protection that this provides – for example, in a piece of furniture, insects can bury themselves into the fabric. It also depends on the effectiveness of purging the atmosphere and replacing it with nitrogen. A major study supported by the Getty Conservation Institute, undertaken at the University of California Riverside, USA, demonstrated that the adult Firebrat (Therobia comestica) died within three hours, while the eggs of the cigarette beetle took 192 hours. This process revealed the need to reduce the oxygen level as much as possible. This could be achieved by repeated purging with nitrogen, but a more appropriate step would be the use of an oxygen scavenger. 

One such product is the oxygen absorber AGELESS, developed by Mitsubishi Gas Chemical Company. Used in the packaging of food, it is effective in preventing mould growth, colour changes, the oxidation of oils and fats and warding off harmful insects.  It is based on the use of a specially treated iron powder, which, when it rusts, absorbs oxygen in the process.

While AGELESS has become established as the oxygen absorbent for use in anoxic packaging, there are alternatives. In 2013, the Bodleian Library in Oxford, UK, began investigating anoxic treatments, with particular reference to the composition and fragile nature of illuminated manuscripts and a game board dating back to the 19th Century that showed some evidence of insect activity. 

The AnoxiBug approach uses ready-made enclosures in the form of bags, tubes and boxes, giving a comprehensive record of the conditions in the enclosures. It consists of specially designed plastic containers to accommodate items such as paintings and rolled-up carpets. Oxygen and relative humidity sachets are incorporated and the containers are monitored by an oxygen LED system. This can be further enhanced by the use of a Radiolog system, which provides a constant record of humidity and temperature. The oxygen scavenger is aluminosilicate clay contained in polyethylene sachet. The Bodleian included materials ranging from wool felt to leather, sheep parchment to paper, and lined silk to grey book cloth.

The selection of packaging films is of ongoing interest. The use of flexible plastic film is an obvious choice, as it can accommodate various shapes and configurations. However, displaying an item requires a high level of opacity and rigidity, which is why glass is commonly used. It also has to compete in terms of longevity. The choice of films has ranged from single films to composites, with the focus settling on PET and PVDC for short term protection and on more sophisticated materials, such as vacuum deposited ceramics (Escal), fluoropolymers or ethylene vinyl alcohol, for longer term use.

Escal is a heavyweight transparent barrier film with excellent resistance to oxygen permeation, designed especially for use with oxygen absorbers for the storage and treatment of museum and cultural artefacts. It is a multi-layer film with a polypropylene outer layer and PVA substrate with a vacuum deposited ceramic, which offers a barrier capacity approaching that of aluminised films. Oxygen permeability is 0.05cc/m2/24hrs and water vapour transmission is 0.01gm/m2/24hrs. As in most sealable barrier films, the inner layer is polyethylene. The integrity of the heat sealing is critical so an appropriate heat seal must be chosen.

Studying the problem

Research on anoxic treatments is steadily increasing. A recent study in Austria directed at developing standard procedures for the packaging of artefacts and art collections in museums and galleries revealed that a low oxygen level was sufficient to eliminate moths, which were the prime cause of damage in large items. 

Many inorganic substances change as a result of oxygen pyrite decay, such as iron sulphide  – commonly present in a wide range of rock types and, therefore, in mineral collections in museums. Variations in relative humidity can bring about breakdown in rock specimens containing pyrites that in the process release sulphuric acid as a vapour. Research at the National Museum of Scotland has shown that low-oxygen environments using oxygen scavengers are beneficial in preventing pyrite decay.

Anoxic packaging is not without its problems and, in the case of textiles, can cause colour fading. The University of Nebraska, USA, researched the impacts of the colour stability of dyed fabrics of nitrogen, oxygen stabilisers and light, showing that light had an effect on textiles in an oxygen-diminished atmosphere in a small number of cases. But whether this was due to the reduction in oxygen or the presence of light was unclear. 

The study centred on a comprehensively structured colour assessment and based on the use of selected dyes, including dyes known to be temperature sensitive, the researchers found that in a low-oxygen environment, incorporating an oxygen scavenger did not result in increased colour changes when compared with specimens in a nitrogen-only atmosphere. On that basis, it was concluded that fading is not necessarily slowed in all dyes – indeed, in some instances, it may accelerate colour changes. Anoxic treatments were recommended, but to be conducted in the dark. 

Anoxic packaging has proved to be a valuable tool in the preservation of historic artefacts and, while only introduced comparatively recently, its role has been established.