Gray tin (or α-tin ) is an allotropic modification of tin that crystallizes in a cubic diamond- like structure and has a lower density than metallic (“white”) tin. At temperatures below 13.2 ° C, white tin turns into gray, an increase in specific volume by 25.6%, and the metal crumbles into gray powder. This transformation is called the "pewter plague." The tin affected by the “plague” needs to be melted, then it will again turn into a white tin.
Gray tin is a narrow- gap semiconductor . Below 3.72 K, α-Sn transforms into a superconducting state.