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Heat treating

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 Heat treating agate is a technique that has been used for thousands of years in India  and the heat treating of steatite , soapstone ,was a widely used technique in the mid east thousands of years before that development. Heat treating often is a direct expression of the regional culture  and as such can seem bizarre to many western stonecutters. Probably the best case of that nature would be the heat treatment of fine green  New Zealand jade to get the locally much more prized inanga jade which is white. The indigenous people of New Zealand , the Maori, prized the white jade because it represented an important food stuff the fish known as white bait.The Egyptians cunningly carved steatite first then heated it  to make it much harder and resistant to wear. Steatite is a fairly ugly stone so they coated  the unheated but carved steatite with  a glaze which we know as faience or Egyptian paste.Glazes in the mid -east are unique in that the chemistry of the rocks were quite different than in the pottery centers of Asia. The Egytians developed a glaze with which they could coat stone or ceramic and upon heating the mineral colorant would migrate to the surface.Copper blue is the classic example. Traditional eastern glazes are actually quite different being layered glass.Faience does not take fine detail so a certain simplified carving style evolved.In India the Indus valley was the production nexus of the world bead trade for many centuries. The famous Deccan trap rocks are full of agate filled vugs  and very early on they learned to heat treat the agates. This had a dual purpose in that they conveniently cherished red stones and the heat treating that produced red in iron filled agate also had the effect of softening the agate a bit making the work of shaping easier.Since heat treating often only changes the very outside layer of an agate the heat treating would be repeated 5 times or more as the shaping progressed. Techniques will be discussed in future posts.

wow, i'm sitting on the edge of my seat.   i look forward to reading more.  i must be a rockhound.    thanks, mary..

I always enjoy your geology and history lessons.  Professor Franks class is my favorite!  :headbang:

I acquired a kiln from an uncle who is a potter a couple years back.  I keep threatening to experiment with it but have yet to do so.

I have seen a few different protocols for heating agates and tigers eye but they all seem to have commonalities. I know some folks do it with charcoal on a barbecue but I will describe the standard method. You should plan to have the rocks slabbed to normal thickness as  the change from heating only penetrates  a bit normally. Plan to have the rocks buried in clean sand which keeps the air off the rocks lessening the shock of heating and cooling. The sand must be very dry as must the slab. Any organic matter in the sand will stink the place up upon heating so I think sand blaster silica sand is a good way to go. The rock must be very dry possibly put in the oven at 200 for a few hours with the door cracked a tiny bit to dry it out. When you turn off the oven close the door and let the oven cool to room temperature before removing the stone. Shocking the stone is not a great idea.Put your stone in a container buried in the sand and set the oven on 100 degrees F with the door closed. Each hour raise the temperature by 100 degrees. You can stop between 650 and 800 degrees , you will need to experiment with your rock. Let the oven "soak" at the ultimate temp for 15 minutes to an hour then shut off the oven and do not open it until the contents are cool.Perhaps overnight. You are attempting to change the valence state of the iron in the stone. The energy from the heat will do this if the proper iron  is available. Bright yellow or light red is the color you want to start  with. Agate normally has many porous layers and you can introduce iron to those layers by steeping the slabs in a mix of iron ( not coated ) nails and a mild acid. I use muriatic. Read up on the protocols for using acids and keep them away from pets and children at all stages. Proper safety equipment must be used and that includes protection for your lungs. It can take a month or so to stain the agate properly and possibly another month in a very dry place to get as much water out of it as you can. You can do this process to preforms as well and in many cases it would be preferable .Be careful.

Debbie K:
Years ago when I was fixing an old Amaco kiln, I noticed that they were offering faience glaze in blues and greens. I foolishly didn't buy any and when I inquired about it recently it had been discontinued. I wish I had; this would have been a fun thing to try.

Debbie K


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