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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.
初学者
Half a day
说明
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.
此步骤所需材料：
石膏（硫酸钙）2 公斤所需工具：
麻袋
大锤
研钵与研杵
细目筛
防尘口罩
安全眼镜2
2
Weigh it
Weigh it
Weigh the powder and spread it thinly on baking sheets.
所需工具：
数字厨房秤
烤盘3
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.
所需工具：
烤箱
烤箱温度计
木勺套装
耐热手套4
4
How much weight it should lose
How much weight it should lose
正在加载 Jupyter 笔记本…
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.
所需工具：
数字厨房秤6
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.
此步骤所需材料：
水0.05 升所需工具：
碗
秒表7
7
Store airtight
Store airtight
Seal the rest in an airtight container: damp air spoils it slowly.
所需工具：
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