FLOORING-PLASTERS AND HARD-FINISH PLASTERS. 



73 



" It appears that the gypsum heated at 300 sets more slowly than 

 that which has been heated at 200; after 23| hours, for example, 93 per 

 cent were set in the former case, in the latter after 21 hours only 56 

 per cent. 



" As these results rendered it probable that the floor-gypsum is not 

 a product of a temperature higher than the dead-burning temperature, 

 but that dead-burning ensues only after the formation of floor-gypsum, 

 we heated a floor-gypsum for 10 hours at 400. The setting capacity 

 had been thereby considerably decreased: 



Unheated sample, 2 gr. Capillaries: 1 mm. =0.00398. 



Heated sample, 2 gr. Capillaries: 1 mm. =0.00323. 



"Thus in the first case 67 per cent were set after thirteen days, 

 in the second case only 39 per cent. 



" On inquiring for a sample of dead-burned gypsum from the same 

 source, it appeared that it was a very finely crystallized half-hydrate 

 with the quantity of water corresponding to the hydrate (7.3 per cent 

 instead of 6.2 per cent). 



" A brief ignition of the commercial floor-gypsum in the platinum 

 crucible finally led to dead-burning, and the sample did not harden 

 even after weeks. 



" 2. Increase of weight on hardening. Exactly the same result, 

 that is to say, a gradual decrease of the binding capacity with increas- 

 ing burning temperature, was obtained on following the amount of 

 water absorbed in a moist atmosphere. For this purpose the samples 

 were placed on watch-crystals under a globe alongside of water. Well- 

 crystallized half-hydrate, obtained from gypsum with nitric acid, was 

 heated for ten hours at 200,' 300. and 400 respectively, and there- 

 upon the absorption of water was followed by means of weighing. The 

 result is contained in the following table: 



