[Rockhounds] Heat treating agate to get carnelian

[Rock Currier]

Don,
Don't go beating up on Axel too hard, I have been chewing on him a bit off
line too. Some how I don't think I changed his mind much or vice versa.

Carol,
I think that before you can heat treat agate and turn it into citrine, the
agate must have the appropriate iron minerals diffused into the agate
naturally or by man. Most agates are porous to some degree or another and
some layers are much more porous than others. The agate factories in Brazil
and other places commonly diffuse various chemicals into the agate and then
heat treat them to "develop" the color. There are various formulas for this
that have been used for more than 100 years. The oldest and used since
antiquity is to use sugar, originally honey, to saturate the agate and then
sulfuric acid or heat to carbonize the sugar and die the agate black. John
Sinkankas wrote a book that had formulas that are commonly used for this and
a wealth of other data as well. Some of the formulas contain a bit of
"magic" that have been derived over the years by trial and error. The people
who use them don't know why they work, they just do. A friend of mine in
Uruguay used to make green died agate and he said he got best results for
green by buying a certain kind of nails that the local blacksmith used for
shoeing horses and dissolving those in nitric acid. He said that other kinds
of iron he had tried didn't give as good results. He was an honest man and a
good friend, I bought a lot of his agates, and I am sure he was telling the
truth.

Some places have natural carnelian in abundance. In Artigas, Uruguay the
little town on the northern Border of Uruguay with Brazil, it is not
uncommon to find little pebbles of carnelian in the fields or in or along
the dirt roads of the area. A man I used to do business with would pay the
kids about five or ten cents a kg for the stuff which he would put in his
tumblers. In Kambay (Cambay) the little agate manufacturing town at the head
of the gulf of Kambat in Gugurat state in India (North of Bombay) agates
have been gathered for generations from the fields around town for the
manufacture of beggar beads and haj beads. This agate in its raw state is
very non descript looking, but after a heat treatment it often becomes a
very nice carnelian color. They put it into cheap earthen ware pots (or at
least they used to), placed in little rectangular low walled brick
enclosures and anything burnable was pilled on top of them and around the
pots. The whole thing was set on fire and covered with corrugated metal to
keep as much of the heat in as possible. When the fire went out and things
had cooled down, the agate was removed.

The agate formerly was allowed to dry out in the sun for a year or two
because this made it easier to work. All the agate there was knapped to
shape with the use of little pointed steel points that were driven butt end
down into the ground. The agate chunk was brought near the steal point, and
a little long handled wooden mallet was used to smack the agate down against
the point and spall of pieces. They could crank out various rough shapes
with amazing speed. Un dried agate was harder to shape, or so they told me
and heating the agate got rid of the water and improved the color. If you
have a piece of carnelian that had been heat treated to enhance its color, I
don't know how you could determine if it had been heated by a brush fire or
in a kiln by man. Some of the chemicals that are commonly used to die agate
red could probably be detected and those could be determined to be died by
man colors. In most of the agates that are died in Brazil, the agates are
first cut and shaped before they are died. The chemicals are diffused only
into the first two or three mm of the agate because that is all they need to
give the bookends etc the color they need to sell them. The Germans who were
the masters of dying agates would let the lapidary objects sit a much longer
time in the chemicals I think before they finished them, and the color
penetration of the agate was much greater than most commercial agate items
that are produced today.

Rock


Rock Currier
rockcurrier at CS.com
Jewel Tunnel Imports
13100 Spring St.
Baldwin Park, CA 91706
626-814-2257