Discussion
A Plaster of Paris Cast can't be prepared in snow because of :
Answer: A
Plaster of Paris cannot be used directly in snow because of its exothermic nature.
When plaster of Paris (calcium sulphate hemihydrate) is mixed with water it sets by an exothermic reaction, releasing heat as it hydrates to gypsum. Poured into a footprint or tyre track in snow, that heat melts the snow and destroys the detail of the impression.
Snow impressions are therefore first sprayed with a protective layer such as Snow Print Wax, and casts are made with dental stone mixed with cold water, or with sulphur, which melts at a low temperature. Cost, setting time and expansion are secondary concerns compared with melting the impression.
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