Weeping Glass - a rare disorder

The ideal glass covering for a miniature is thin convex glass.  This

allows a good view of the miniature and protects it from dust and
damage caused by glass rubbing against the paint and possibly sticking
to it and lifting it  (as when flat glass is used). We have seen examples 

where flat glass has rubbed the ivory and damaged the painting.

Side view of a miniature frame showing curvature of the glass

 Flat glass is only used when there is a metal or cardboard 'slip' 

that separates the painting from the glass, 

usually when a miniature is contained in a travel case.

An example of a miniature with a mount and flat glass.

There is an uncommon condition called 'Weeping Glass' that is
caused by faulty manufacture of the glass.

 At first glance,
it might look as though the glass is 'misty' or very dusty, but on
closer inspection one can see that on the underside of the glass there
is a thin film of condensation! If old glass contains 20-30 percent

sodium or potassium, it may have 'glass disease' where the glass

weeps and begins to break down.

Amazingly, this fine mist of water droplets

 cannot be eradicated through cleaning, drying or any other
method that we know of!  And as it poses a huge threat to the
wellbeing of the miniature it has to be removed and discarded
immediately, however inconvenient.