The crystallization and the hydration of the calcined plaster during setting was examined to recognize the conditions of “initial setting” which was provided in the JIS R·2119 testing method.
The crystals which was accumulated under the measuring needle at the initial setting was mainly fine needle shaped dihydrate crystals and small amount of grane hemihydrate crystals.
About 1 grams of the slurry under testing was taken and put into unhydrous ethanol to stop the growth of the crystals. And these crystals was seperated by decantation into the coarse one which is mainly hemihydrate and the fine one which is dihydrate and hemihydrate.
The fine crystals at the initial setting time was observed by electronmicroscope to be 0.1-1μ X 10-20 μ needle crystals and assembled crystals. The secrystals can be supPosed to be the seedcrystals on setting.
The size of these seed crystals was varied from large to small by agitation. The compression and bending strength of the set gypsum were increased by vigorous agitation, by reasen of that the seed crystals become smaller by agitation so that the grown gypsum crystals become smaller needle one.
The hydration rate at the initial setting was very slow and it was proceeded at the time between apparent setting and true setting. These hydration was also observed microscopically by colouring the hemihydrate crystal with Congo Red dyes, and was recognized that considerable amount of hemihydrate remains after true setting. And it was supposed that this remained hemihydrate contribute to increasing the strength afterward.

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