90 University Geological Survey of Kansas. 
Set of Plaster. 
LAVOISIER’S THEORY. 
Lavoisier in those early experiments (1765) on gypsum, 
quoted before, considered the problem of the setting of plaster 
of Paris when water was added to it. His early explanation, 
which has been verified many times since, and which is without 
doubt the true explanation of the set of plaster, is seldom men- 
tioned in our reports on gypsum; and it is not very generally 
known by those who are in direct contact with the plaster in- 
dustry. 
As this explanation of Lavoisier is the first recorded discovery 
of the principle of set in plaster, it will be interesting to quote 
the original account: 
‘‘T took the calcined plaster, as has been described before, and which hardens 
readily with water. I threw it into a considerable amount of water, in a pan or 
ina large dish. Kach molecule of plaster, in passing through the liquor, seized 
its molecule of water of crystallization, and fell to the bottom of the dish in the 
form of small brilliant needles, visible only with a strong lens. These needles, 
dried in the free air or with aid of a very moderate heat, are very soft and silky 
to the touch. If placed on the stage of a microscope, it is perceived that what 
was taken under the lens for needles are also parallelopipeds, very fine, so they 
are described as thicker, or many thinner, and many more elongated. The 
plaster in this state is not capable of uniting with water, but if it is calcined 
anew, these small crystals lose their transparency and their water of crystalliza- 
tion, and become again a true plaster, as perfect as before. One may in this 
fashion successfully calecine and recrystallize the plaster, even to infinity, and 
consequently give to it at will the property of seizing water.”’ 
This explanation of the formation of fine crystals during the 
process of set in plaster was generally accepted, and it was 
given by Payen in 1850 as his first principle.” 
LANDRIN’S THEORY. 
Landrin, in the paper quoted above, which was written in 
1874, gives the results of an elaborate study on plaster which 
40. Quoted by Landrin, Annales der Chimie, 1874, pp. 434, 435. 
4'. See page 87. 
