Gypsum 石膏粉

CaSO4 · 2H2O. Dihydrate 

生石膏粉

现在药房出售的生、熟石膏均为白色的粉末状，不太好鉴别。简易的区分方法为：取石膏粉末10克，加水约15毫升，搅拌均匀。放置10分钟后，若呈干性黏团块状者是熟石膏，而呈湿性散渣状者，则为生石膏。

硫酸钙化学式CaSO4。熔点1450℃，相对密度2．96，难溶于水???。它的二水合物CaSO4•2H2O俗称石膏（或生石膏）二水硫酸鈣。gypsumchina

半水合物CaSO4•1/2H2O hemihydrate称熟石膏（或烧石膏）。(TB 最常買到的是熟石膏，甚至把熟石膏當作生石膏賣）plaster of paris. 二水 vs 無水

石膏加热至128℃，失去大部分结晶水，变成熟石膏；163℃以上，结晶水全部失去。熟石膏粉末与水混合后有可塑性，但不久就硬化重新变成石膏。此过程放出大量热并膨胀，因此可用于铸造模型和雕塑。

硫酸钙和石膏可用作联合制造硫酸和水泥的原料，还可做油漆的白颜料、纸张的填料

熟石膏粉用作豆腐的凝结剂。

Gypsumchina

Gypsum(dihydrate)is moderately water-soluble (~2.0–2.5 g/L at 25 °C)and, in contrast to most other salts, it exhibits retrograde solubility, becoming less soluble at higher temperatures. 

When gypsum is heated in air it loses water and converts first to calcium sulfate hemihydrate (bassanite, often simply called "plaster") 

and, if heated further, to anhydrous calcium sulfate (anhydrite). 

As with anhydrite, the solubility of gypsum in saline solutions and in brines is also strongly dependent on sodium chloride (common table salt) concentration.

Factors that affect solubility

Temperature:

Gypsum becomes more soluble as water temperature increases. At around 30°C, gypsum is about twice as soluble as at 0°C.

Other factors 

豆腐做法：

另一做法，大同小異

原料不同，口感不同：内酯最嫩，石膏其次，卤水最老

内酯（葡萄糖內酯GDL）【适合做内酯豆腐，日本豆腐，豆腐脑】

石膏【适合做嫩豆腐，南豆腐，豆腐脑】

卤水（氯化鎂MgCl2)【适合做盐卤豆腐，老豆腐，北豆腐】

Gypsum from LD Carlson is dihydrate. MSDS .

How those ppm figures are calculated?

Brewersfriend water calc is calling for gypsum dihydrate 

Analyse water content of gypsum : heat @60 deg c, and measure weight loss.

Reference 

Wiki

Solubility 

Homebrewtalk ajdelange was actually referring to hemihydrate , aka plaster ( 1/2 H20), but said as gypsum ( 2H20 dihydrate)