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.
Definition of alum glass in US English:
alum glass
noun
rare
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.