Bottles of this curious and appealing colour (this one dating from c.1710) are something of an enigma.
However, the opaque blue is probably the result of impurities in the mix (chlorides and sulphates of potassium and sodium) that would occasionally produce a surface scum known as glass gall (or sandever).
Glass gall is unheard of in modern glass production when ingredients are carefully measured but it was commonplace in the Stuart and Georgian periods and could be used to quirky effect.
Handful of survivors
Only a handful of these bottles have survived and they certainly make a striking addition to any serious collection of black or green glass.
One came for sale at Dreweatts (24/20/12% buyer’s premium) in Newbury as part of the collection of barrister Phillip Lucas, owner of a silk merchant’s house in Spitalfields, London.
The best performing lot in the December 2 sale, it was guided at £800-1200 but sold at £11,000.