“The main priority of present-day conservators is to ensure the long-term stability of the works of art in their care. The most important way of doing this is to control the surrounding environmental conditions, keeping light, temperature and humidity at steady levels appropriate to the types of materials the artist used. At the same time, safe methods of display and storage protect against accidental damage: for example, particularly fragile or vulnerable paintings are usually exhibited behind inconspicuous, low-reflecting glass.

These ways of preserving paintings are passive – they prevent things happening that might cause deterioration. The term preventive conservation is used to describe this sort of approach and it forms the basis of everything a modern conservator does. In an ideal world, paintings would be kept stable by 'preventive conservation' measures alone but, unfortunately, many are made of inherently unstable materials or have deteriorated in the past from neglect or mistreatment. Conservators therefore sometimes have to intervene and carry out structural treatment on paintings in order to repair or stabilise them.

Over the years, many remedies for treating damaged or deteriorated panels and canvases have been devised, some of which were sensitive and sympathetic, some drastic and even disastrous. The modern approach is to do as little as possible: if a painting can safely be left alone, nothing is done to it. This is a quite different philosophy from that of the nineteenth or earlier twentieth centuries, when canvases and panels were routinely subjected to major structural treatments, whether or not they needed them.”  (David Bomford, Jill Dunkerto, Martin Wylid 2009)