II 
Field Museum of Natural History 
The methods of coloring depend upon the absorp- 
tion in the pores of the stone of some substance which, 
when acted on by another chemical or by heat, will 
give the color desired. Inasmuch as the layers of 
agate differ considerably in their porosity, there are 
corresponding variations in the amount of color they 
will absorb. This gives pleasing variety to the color 
effects. The agate colorists call layers which have 
great porosity and hence absorptive capacity, "soft," 
while those so compact that they take little or no color 
ai*e called "hard." 
For successful coloring, the stone must first be 
freed from any substances which would prevent the 
entrance of coloring matter into the pores. Thus, all 
oils or fats must be removed, as well as iron or other 
oxides of an undesirable color. Soaking in caustic 
soda solution will remove the fatty substances and 
treatment with nitric or hydro-chloric acids undesir- 
able iron compounds. For the latter process the stone, 
according to Dreher, should be placed in warm acid 
for two or three days, and the acid be finally brought 
to boiling. After cooling and washing, the operation 
should be repeated several times until the pores of the 
agate are thoroughly cleaned. 
For coloring red, a solution of iron nitrate is used. 
This is generally prepared, according to Dreher, by 
adding to about half a pound of iron nails, about four 
times their weight of concentrated nitric acid. A 
more or less slimy mass which is produced is allowed 
to settle and the clear liquid poured off, this procedure 
being repeated several times until the liquid has be- 
come perfectly clear. The stones or sections desired 
to be colored are immersed in this liquid at a luke- 
warm temperature and allowed to remain for a time 
determined by their thickness. Stones 3 millimeters 
thick, it is said, should remain 2 to 3 weeks, those 10 
[US] 
