Conservation in action: Glazing in conservation framing for exhibition

Conservation mounting and framing is carried out at London Metropolitan Archives for both internal exhibitions and exhibition loans. Today the glazing material we use is of high quality as it has very good ageing properties, protects the artwork from environmental conditions and doesn’t damage the item itself. It took centuries to develop such high-tech glazing and here Assistant Conservator Julia Roberts broadly outlines the evolution of the use of glazing in conservation framing.

Before glazing was invented and used, varnish was the earliest method employed to provide a protective layer to paintings. Dirt would be trapped in the varnish layer which could then be removed together with the dirt. It was also common practice in Europe in the second half of the sixteenth century to use silk curtains of various colours to cover paintings.

The increase in industrialisation in the nineteenth century led to the use of glass to protect framed exhibition items. Tate Gallery was one of the first to use it to protect works of art from pollutants, environmental changes and accidental damage.

At this time, large sheets of glass could be made by blowing cylinders and cutting them open, and so its use in museums and galleries increased. The subsequent development of rolled glass, where molten glass was drawn through rollers, meant even bigger sheets could be produced. However, this left roller marks on the panel and the process was gradually replaced with float glass, which is made by floating molten glass in a liquid metal bath.

The two types of float glass are ‘regular’, which has added iron to increase strength but because of its green tint is not suited to framing, and ‘water white’ which has fewer impurities, so its colour does not impair the experience of viewing artworks.

The invention of acrylic in 1933 offered a new option for protecting artwork as an alternative to glass and its use for framing began in the 1970s.

Nowadays both glass and acrylic are used to protect artwork. They offer, for instance, a shield against UV lights. This is very important as light accelerates the degradation of the artwork resulting in discolouration and brittleness. Also, they can absorb all the reflections and protect against heat sources such fires. However, there are some disadvantages in using them and the conservator usually decides which one to use based on several factors such as the exhibition area conditions, the transport methods and the condition of the artwork itself.

Glass is rigid and does not warp like acrylic, so that larger sheets will not bow towards the surface of the artwork. It is virtually impermeable, which is good for creating a microclimate and helping to keep the artwork at a consistent temperature and humidity level. It has a low electro-static charge, which is more suitable for friable material than most types of regular acrylic. It is however more fragile and twice the weight of acrylic.

Acrylic is more easily scratched and has a higher electro-static charge, which attracts dust and disturbs friable material, like pastels and graphite. Water passes through acrylic as vapour, meaning that eventually relative humidity will reach the same level inside the frame as outside. Heat causes its outward expansion which can lead to frame corners splitting open. However, it is much safer to work with and lighter to handle and transport.

Acrylic will bow outwards towards a source of heat. Glass is more heat resistant but also more heat conductive, so it will not bow as much but will transfer the heat to the artwork more easily.

It is important to achieve a balance between protecting the artwork (or indeed any other flat item such as maps or single sheet documents), minimising any compromise to the quality of viewing and the practical aspects of working with the material when choosing which material to use.