I have been researching a piece of glass and came across a piece written in 1868 about the Rosenborg Castle collection as whole and where, under the Glass collection section, the author refers to a piece in the collection as 'the flat glass, intended to cut through the mustache, which must not be moistened;'
Does anyone know what this 'flat glass' might look like please?
I have had a number of moustache cups made in the late 19th early 20th century but they were porcelain or china and some two hundred years later
thanks

m