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Plaster of Paris from Gypsum
Gypsum is calcium sulfate with water locked in its crystals (CaSO4·2H2O). Heated to about 120–180 °C it gives up three quarters of that water as steam and becomes the hemihydrate, plaster of Paris. Mixed with water again it takes the water back and sets hard, starting about 10 minutes after mixing, warming as it sets and expanding slightly. The name comes from the big gypsum deposit at Montmartre in Paris.
Beginner
Half a day
Instructions
1
1
Crush the gypsum
Crush the gypsum
With a dust mask on, break the gypsum in a sack with the sledgehammer and grind it fine in the mortar. Sieve it.
Materials for this step:
Gypsum (Calcium Sulfate)2 kgTools needed:
Burlap Storage Sack
Sledgehammer
Mortar and Pestle
Fine Mesh Sieve
Dust Mask
Safety Glasses2
2
Weigh it
Weigh it
Weigh the powder and spread it thinly on baking sheets.
Tools needed:
Digital Kitchen Scale
Baking Sheet3
3
Calcine
Calcine
Heat it in the oven at about 150 °C, checked with an oven thermometer, stirring now and then. Steam comes off; carry on until a cold glass held over it no longer mists.
Tools needed:
Oven
Oven Thermometer
Wooden Spoon Set
Heat-Resistant Gloves4
4
How much weight it should lose
How much weight it should lose
Loading Jupyter Notebook...
5
5
Weigh again
Weigh again
Weigh the cooled powder. Turning the dihydrate into the hemihydrate loses about 16 % of the weight; much less means it needs longer.
Tools needed:
Digital Kitchen Scale6
6
Test a pat
Test a pat
Stir a spoonful into a little water in a bowl to a cream and time how long it takes to set hard. Feel it warm up as it sets.
Materials for this step:
Water0.05 literTools needed:
Bowl
Stopwatch7
7
Store airtight
Store airtight
Seal the rest in an airtight container: damp air spoils it slowly.
Tools needed:
Airtight Storage ContainersTools Required
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