[Rockhounds] heat treating for color

[Jim Small]

This one I can chime in on - yes, heat treating does improve agate 
color, where the coloring agent is iron (more specifically, one of 
the iron oxides). The way Hale Sweeney explained it was that the 
heating affected the valence of the iron in the iron oxide, and 
normally results in a brightening/deepening of predominantly red 
and/or orange body colors. For instance, much Oregon carnelian comes 
from the ground with a reddish skin, and is much paler inside; 
heat-treating improves the body color to a close match, especially 
after slabbing. Lake Superior agates travel great distances down the 
Mississippi drainage, and appear in large sizes/quantities as far 
south as Arkansas gravel pits - BUT, they are (for the most part) 
faded out. Slow, thorough drying, followed by heat-treating restores 
them to much of their original appearance. As far as I have been able 
to tell, the effectiveness of heat-treating on color is limited to 
improving the reds and oranges.

KOR,

Jim Small