Cooking Rocks

Introduction:

A long time ago, the Natives on this Continent learned a wonderful technique. They found that many types of their favorite tool making stones were much easier to work after being heated. How this was discovered is anyone's guess. Perhaps some chunks of flint were mistaken for cooking stones and placed in the campfire. Possibly a forest fire came through a flint rich area and burned over the rock. Who knows? What's important is that many stones that would otherwise be almost impossible to work are vastly improved by proper heat treatment.The stone usually turns glossy, more waxlike in texture, more brittle, and sometimes a dramatic change in color occurs. However, heat treating does have a down side. Sometimes rock can be overcooked, causing it to crumble, or pot-lid. It can also become so brittle that it is very difficult to work without breakage. Therefore, a fine line must be walked when heat treating. Depending on whether the stone is in chunks, spalls, blanks, or slabs, and whether the stone is to be used for percussion work or pressure flaking will determine the amount of heating required for a particular stone.

General Information:

Some stones, obviously, do not need to be heat treated at all or are ruined by it. Man-made Glass, Obsidian, Opal and it's variants, and the various Hornestone types are some examples. Others require varying degrees of treatment to obtain the desired results. Remember, it's better to undercook your stone and then have to re-heat it than to blow it all up with too high a temperature! There are no hard and fast rules when it comes to heat treating stone, but there are some general guidelines that can be followed that give good results without the heartbreak of ruined stone. First of all, for our discussion, a Kiln is a must. A special Kiln especially made for heat treating stone with an infinite temperature control and a digital pyrometer are my instruments of choice. There are several people who supply these types of kilns at reasonable prices. Next we come to some general guidelines for heat treating. First, lighter colored stone generally can take more heat than darker colored stones. The exception seems to be Silicified Ash. Also, thinner pieces such as slabs and blanks can take higher temperatures than chunks and spalls. This has to do with the stone getting too hot too quickly on the outside of larger pieces while the inside is still relatively cool. This can cause the stone to break apart of wose, burst. Also, a lot has been discussed about hold time, or the amount of time that stone is held at the target temperature. In my experience, hold time is more crucial for big pieces, where it may take a while for the middle to reach the target temperature. Hold time also seems to be a factor in color change. I have seem some dramatic changes in the color of stone by holding at the target temperature for as long as three days.

Recipes:

This next section is some of my heat treating recipes. These are the recipes I use with MY kiln. Yours may be a little different, but these guidelines should be helpful and get you in the ballpark. Remember, it's always best to start low and work up, rather than blow up that expensive rock!

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