With such a long history, reverse painted pictures and examples of glass gilding cover the spectrum from museum quality heritage pieces to amateur artworks of minor interest. The usual curatorial issues of when to conserve and when to restore must be made according to each individual case after evaluating its heritage value.
Leaving these curatorial deliberations aside, let's look at the actual requirements
of older and ageing works. The major problems usually encountered are:
Delamination of paint (peeling) occurs for two common reasons. Firstly because the paint oxidizes over time and becomes very brittle. If it is subject to cyclic changes in temperature and humidity, the expansion and contraction will cause it to separate from the glass as its flexibility is lost. Another reason for delamination is if the painting was mounted with a backing panel that touches the paint. Over time, the paint adheres to the backing and becomes detached from the glass.
Preventative measures to delamination include preventing or slowing oxidation rates, keeping in a stable environment and ensuring nothing is in contact with the paint surface (contact at the margins may be difficult to avoid).
In some cases it may be possible to seal the painting with a reversible coating. This will greatly reduce the rate of oxidation and slow the ageing process. I'm reluctant to specify suitable materials on a general basis. The choice of sealant depends on several factors.
Broken and cracked glass can be repaired using epoxy adhesives made specifically for the purpose such as Hxtal. These products have similar refractive index to glass, and low viscosity to enable them to wick into cracks.
Paint fading occurs with exposure to UV light. Even clear glass will block out some UV light but the simple prevention method is to keep the work out of sunlight.
This is where we face some difficulties. Reverse work on glass is very difficult to repair because a paint failure like delamination or fading occurs at a layer that is within the painting, between the paint and the glass, not at an exposed surface. It may be possible to re-adhear the peeling paint to the glass. This is a job for an experienced restorer.
If an attempt is made to repaint sections of the painting a good edge must be established, where the old paint isn't curling up, and the color must be matched exactly. Again, not an easy task. Matching gilding can be even more difficult. You not only have to match metal color, you also have leaf texture, reflectivity level and patina.
With old gold leaf window signs, if it has water-gilded gold leaf that is peeling off, this can sometimes be re-gilt with little visible difference. Oil-gilding tends to become milky or discolored over time and is very difficult to match. It will usually need re-doing. Repairing a section or patching has the added problem that the older work that was failing will probably continue to degrade faster than the new work. The repair may become obvious later as they age differently.
With most pieces of limited heritage value, broken glass usually means producing a new matching piece. A suitable piece of matching glass can be obtained and the original work is used as a guide for an exact match of the design and color. The result can be far more satisfactory than a painstaking and possibly compromised restoration of an original.