Hi
I have done a lot of serious trials & experiments with heat treating british rocks. There are plus sides to it but it does make the rock more brittle, so flakes/pieces can snap more easily. I have tried many types of rock and with a few exceptions, it does work. Temps needed are easily achieved in a domestic oven but we used a computer controlled kiln as a base so we could be exact and replicate conditions.
Use a metal container filled with sand. This stops dangers from any that may go 'pop' and also the sand evens out the rate at which heat reaches the rocks. If you use a baking tin, ensure there is a good layer of sand between tin and rocks (inch plus) and all gaps are sand filled. Do not use preforms more than an inch thick - they will often appear as OK on the outside but the rock seems unable to cope with the expansion etc. You start knapping a piece and then get to the middle and only to find 'crazing paving'.
First thing is to dry everything, so a soak period on heat, maybe 80C for up to 8 hrs to dry sand and get rid of moisture, and then slow temp raising. About 25C per hour max and then up to ceiling temps. This we found to be best for most of the rocks we tried to be about 220C and then hold there for about 4+ hours. If the container is big, used longer times to ensure the heat gets through.
Some US info suggests ramping down the temps as well. We did not find this necessary. Turn off the heat and then leave to cool for 24 hrs. Do
not open the oven - cool too fast and they
will crack. Then once cool, knap away
These are a few points I have knapped out after treating but this group does not include any true flint. It does work for flint though if you have good flint to start with, I am not sure of any real benefit (a lot of effort for little gain). In other rocks, escpecially some of the cherts agates, it does make the stone work more easily (or even workable at all), often (not always) creats a glassy appearance and it can change the colours, especially iron oxides in the rock. On true flint, it has varying effects on the ghosts in the flint - the grey areas which are probably casts of burrows. Sometimes it will help, but often make no difference. You can get a good idea if it will do any good on the raw flint: if flakes tear across the ghosts, then it is unlikley heating will achieve anything.
Many of the British cherts and agates do improve considerably and I can recommend the effort but trouble is, you have to try it first with your local rock to see whether it works in the first place
.
Hope that helps.
PS I may be able to 'pdf' a paper I wrote on this and post it but it is on the work PC if you are interested. (technicalities allowing!)
