120 University Geological Survey of Kansas. — 
Most of the gypsum material in Kansas is manufactured into 
the cement plasters, and the industry has been especially de- 
veloped in this state probably on account of the fine deposits of 
gypsum earth which make a plaster of most excellent quality. 
Improvements are constantly being made and the industry is 
rapidly growing. 
A cement plaster is a plaster which sets in two to twenty-four 
hours and reaches a considerable degree of hardness. Ordinary 
plaster of Paris, as we have seen, sets in a few minutes, and 
such material may have the set held back by the use of the 
proper retarders. 
In general, when gypsum plaster sets it goes back to its. 
original chemical composition, as has been indicated. Some 
dealers have taken advantage of this chemical principle, and 
have argued that this was also a physical law, and that the 
earth plasters on setting would go back to their original condi- 
tion of dirt, or, as they choose to call it, rotten gypsum. Now 
in our discussion of the secondary deposits of gypsum, it was 
clearly shown that the gypsum earth was in no sense rotten or 
decayed gypsum, but it was dissolved from the rock and pre- 
cipitated again, so that physically its structure was the true 
crystalline gypsum structure. 
There is no basis whatever for this claim against the earth - 
plasters, and the sooner dealers cease to use that argument the 
better it will be for their own trade. Whether such plasters 
are better or worse than the ordinary retarded rock plasters is 
for the trade to determine, mainly by long experience. Labo- 
ratory experiments do not solve that problem, and the plasters 
have not been used long enough to settle the problem. As far 
as the writer has been able to determine by actual experiment 
and by an examination of the plasters in buildings, and per- 
sonal conversation with practical plasterers, the gypsum earth 
plasters and properly retarded rock plasters are both good, and 
are both popular. 
Sometimes mistakes have been made in the burning of both © 
kinds of plaster, and the resulting work with them was unsat- 
isfactory. So if poor plaster is found on the walls of a build- 
