Sulfur is used for casting impressions in snow.  Dental stone, another material used to cast impressions, gives off heat during curing which typically will result in unacceptable loss of detail.   Sulfur is melted to liquid for pouring into the impression to be cast.  It must be allowed to cool sufficiently prior to casting or the impression can be ruined.  Adequate photographic documentation prior to casting attempts is necessary.  Utilizing various methods to provide additional contrast may assist in successful photographic documentation prior to casting.

Images below show (clockwise from top left) a shoe tread, an impression from the tread in snow, melted sulfur poured into the impression and the resultant cast of the impression.