Agate— Physical Properties and Origin 
15 
millimeters thick, 3 to 4 weeks. Stones thicker than 
this can rarely be colored throughout. For the best 
results repetition of the treatment is recommended, 
the stone being dried after the first bath and then re- 
immersed. The agate is now saturated with iron 
nitrate and this must be changed to iron oxide by 
strong heating. First the stone is dried at a gentle 
heat, the time required being from two to ten days, 
according to the thickness of the stone. Then, with- 
out being allowed to cool, it is put in a closed crucible 
on a warm hearth and heated until the hearth be- 
comes red hot. It is then allowed to cool very slowly, 
care being necessary both in heating and cooling, that 
no sudden change of temperature will occur which 
would fracture the stone. 
For coloring green, as much chromic acid or potas- 
sium bi-chromate as possible is dissolved in a quart of 
water. The stone desired to be colored is placed in 
this bath and if thin, allowed to remain from one to 
two weeks, according to the degree of absorption. 
Stones one-half inch in thickness often must remain 
two months before they absorb sufficient liquid. When 
this treatment is finished the stone is placed in a 
closed, flat vessel containing lumps of ammonium car- 
bonate. It is then allowed to remain exposed to the 
fumes of this volatile substance for about two weeks. 
It is then dried and gradually and strongly heated 
until the desired color is obtained. 
For coloring black, about 13 ounces of sugar are 
dissolved in a quart of water and the stone allowed 
to remain in this solution for from 2 to 3 weeks, ac- 
cording to its absorptive capacity. Water must be 
added from time to time during the immersion of the 
stone in order to replace that lost by evaporation. 
The stone is then transferred to a vessel containing 
concentrated sulphuric acid and then slowly warmed 
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