What is ‘weeping glass’? This is the phenomenon of droplets of water forming on a piece of glass. When that glass is a piece of convex glass covering a portrait miniature or a watch, it poses a serious threat to the condition of that object!.

Most recently, it affected three Victorian miniatures by a leading artist of his time, Edouard Lobo Moira (1817-1887), painted of three members of the same family around the same time of c. 1860. It therefore took about 60 years for the defect to show up. The composition of the glass affects its durability, and when the wrong proportions of ingredients are used, salts begin to develop on the inner surface, which attract moisture, forming into droplets of water. When deterioration starts, the glass can appear dull, clouding the vision of the miniature. This then develops into visible drops of water. Luckily, we are collectors who regularly view our works of art and so such happenings do not go unnoticed! On the first occasion, we noticed this occurrence on a late Victorian miniature. Having removed the glass, we tried to clean and dry it several times without success, monitoring it daily. We replaced the glass.

This piece of ‘weeping glass’ was taken out of a miniature frame and shows the

crystallization of the salts in the glass when photographed against a dark background

.

Miniatures and materials expert, Alan Derbyshire, Head of Conservation at the Victoria and Albert Museum, London, had this to say about weeping glass:
‘I have come across this more with continental miniatures. Very few examples here at the V&A but that might be because Jim Murrell* would have treated/ replaced them. Of course, as you know it depends on how (and therefore where) the glass was manufactured…It is due to poor quality glass and an imbalance of the salts used. Silica has a high melting point therefore alkalis such as sodium and potassium added – these weaken the silica bonds allowing it to melt at lower temperatures – sodium glass is more stable than potassium. Once manufactured the glass deteriorates at high humidity – in fact quite low RH is needed for stability – less than 40%. In practice you can clean the glass as a temporary measure or replace with better quality glass. Latter is often preferred since one of the time-consuming elements of miniature conservation is opening the lockets. So, it is often judicious to replace rather than clean the glass.’

The advice is clear. Collectors should regularly check their miniatures and promptly replace any defective glass.

*previous Head of Conservation at the V & A.