alum glass
now rare
noun
1A glassy form of alum produced especially by evaporating an aqueous solution (now historical ).
2A kind of glass containing alumina.
Origin
Late Middle English. Compare Anglo-Norman alum de glace, alun de glas, Middle French alun de glace, lit. ‘ice alum’ and also Anglo-Norman alomglas; compare further (after French) Middle Dutch aluun van glaetse.