REPORT OP THE DIRECTOR AND STATE GEOLOGIST 1903 127 



to the size of an egg and spread out on the sole of the oven. 

 Such an oven had no separate fireplace, but a fire was kindled 

 on the sole, and, when the oven reached the proper temperature, 

 tke coals and ashes were removed and the gypsum was thrown in, 

 and the door closed. From time to time pieces were taken out 

 for trial, and, when they exhibited on the fractured surface only 

 a few shining crystalline particles, the charge was removed and 

 ground. 



At Paris, where the manufacture of calcined gypsum, or plaster 

 of paris, was for a long time carried on more extensively than 

 in any other one locality, special plaster kilns or ovens were 

 employed for baking the gypsum. The kiln was rectangular in 

 form, covered with an arch and inclosed with side walls 

 and also a wall at the back in which the chimney was erected. The 

 front was left open for convenience in discharging and regulat- 

 ing the fire. The fireplaces were always constructed afresh in 

 these ovens, and the largest lumps were used for this purpose, 

 and smaller pieces were thrown on above. Wood fires were used 

 in this kiln, and the smoke escaped through openings into the 

 chimney. 



In such an oven a certain percentage of gypsum was of necessity 

 ruined, a part being overburned and another part being changed 

 to calcium sulfid. A kiln very similar to the foregoing was 

 used with coal as a fuel, but a special chimney was not used, 

 and a grate of brick was built so as to permit the removal of 

 ashes. The front was left open, but the gypsum was introduced 

 through a door at the rear. Beneath the arches of gypsum were 

 brick arches on which the coal was placed while the ash fell through 

 the perforated bottom. The burning occupied from 12 to 16 

 hours when the lower layers were free from water. This type 

 of kiln was open to the same objections as the earlier type, and 

 a great proportion of the plaster was dead burnt, so that it would 

 no longer set. 



The kiln used by Scannegatty, though similar in some respects, 

 is far better than the previous types, because the coal is not in 

 contact with the gypsum, and the liability of forming calcium 



