What hazardous material was used to create the 'snow' that falls on the characters in the poppy field scene of the 1939 film 'The Wizard of Oz'?

Asbestos

Despite a longstanding rumor, film historian William Stillman says MGM makeup artist Charles Schram recalled that the artificial snow was crushed gypsum, not asbestos. Asbestos was used elsewhere in the film, including in a fire-resistant version of the Scarecrow’s costume.