Athelflaeda, the material is called foamex over here. It is a fairly hard, dense, foam board. It is a very sturdy material used in temporary signs. The local authority that I work for had a bunch (around 200 sheets of 3mm signs) that were no longer valid as we had split from another council so my manager let me have them. From what I can tell, it is denser than balsa foam if that helps.