The Quarry

The Art of Heat Treating

The marvelous mystery of changing the crystalline structure of rock.
Let’s start by why we would want to heat the rock and my journey of trying to understand the process.
As a boy of about twelve I started looking for arrowheads in our local fields on a regular basis. I would find a few but I would also find waste flakes. There in the Rogue Valley of southern Oregon they were mostly red jasper. Some bright red but many more of a burnt or brick red. There were a few agates and a rare obsidian but mostly jaspers. The waste flakes were what I first started to learn to knap. Because of their small size and weird shapes it didn’t take long to see why they were discarded.
As the nearest sources of raw material was only about 20 miles away and we had friends with property there I was soon looking for bigger pieces the knap. The trouble was that every thing I found was yellow, some very smooth and some quite grainy. All so tough I couldn’t get to flake. As I was total self taught I now know that part of the problem was not just the rock but my tools and technique.
As I grew older and was able to drive and widen the range of where to look for rock I still had the same trouble. Yellow Jasper! Sometimes I would find some red but it was always still real tough or so brittle that it was unusable. This continued until in my early 30’s when I finally got into the knapping circle.
What??? The native Americans heated the rock to make it more workable, and most yellow jaspers turn red when they are cooked!! What a revaluation and a pile was questions. Some I got answered and some would take many years to be answered.
So I asked how they did it and this is what I was told at the time.
First you have to break the rock into usable flakes or spalls less than 1” thick then you bury them in the dirt or sand and build a fire on top. Let the fire burn all day and add more wood so it will burn into the night. Then wait until the ground is cool and dig up you heat treated rock. Ha Ha Ha!!
It was a starting place but took a lot of rock and wood to understand
what you really needed to do for a better success rate.
What the natives really did is watch someone who, had watched someone, who had watched someone. They knew how big of pieces they could use of the type of rock in their area. They then also knew how to place them in the hole and how deep. Then how big to make the fire and how long to keep it burning.
But I had to start from scratch. So I broke the yellow jasper some slick some grainy into 1” or thinner pieces made a shallow pit put in one layer of rock covered with about 1.5 inches of sand. But being a white guy that is in a hurry I did have to put one bigger thicker piece in there. Then I built my fire small and as the day went on I would add more wood trying to keep an even heat that would slowly get hotter. Stoked for the night. Could hardly wait for it to be cool enough to dig up the next night. It wasn’t cold but cool enough to touch..
WOW!! What a difference. The slicker jasper was bright red and the grainy jasper was the burnt red like I have found while artifact hunting. The trouble was that it was all OVER COOKED AND BLOWN TO HELL. I think I had one real thin piece of the grainy jasper that was workable. Without telling you the whole journey, I will tell you where I ended up. A computer controlled kiln. Well back to the fire.
I realized that I had gotten it too hot and the bigger the piece to start with the more it broke into smaller pieces. So in the end I would dig a hole about five inches deep and put the bigger pieces in the bottom and the thinnest pieces on top covering with about 2” of dirt. I don’t make the fire as big but held the same amount of time. The grainy jasper takes a little more heat than the smoother so it will be a little closer to the heat. I might lose a little from overcooking and I might have some not cooked enough in the bottom but the layer in the middle was nicely done.. just like the natives I can’t tell you to temperature I just know it worked.
Undercooked can always be recooked, overcooked is just trash.
Because I got more and more different kinds of rock from all over the county it quickly became clear that they don’t all cook at the same temperature. Time to get a kiln to control temperatures better. This is when more and more tests were done on all kinds and sizes of rock. What I realized was that the whole rock when cooked at the same temperature as a flake, it was just overcooked. So I started trying cooking whole rocks but at lower temperatures and found that I was getting closer to a good product and on some types of stone very good results, but it was still kind of hit and miss. I shared what I was doing with a few people at the Texas Maxdale knap-in. Then I get a call from a geophysicist from Texas. He had been talking to a number of knappers from Texas about heating whole rock who told him that is wasn’t and couldn’t be done except one who told him what I was doing, so he gave me a call. When I started telling him what I was doing, he stopped me and said.
I feel like the dumbest man alive, I have had 9 years of schooling and should have know this..
It takes 3 things to change the crystalline structure of rock either in nature or man made. It takes time, temperature and pressure. There is a lot of water in the stone both free flowing and locked in. For every silica molecule there are 4 locked on water molecules.and in the spaces between the crystalline structure there is free flowing water. If we look at opal that has no crystalline structure all of the molecules are lined up in rows like a checker board with only the locked on water attached. If we look at cherts, jaspers, flints, agates and other crypto crystalline stones they also have other elements put into the mix. When we heat the stone first the free flowing water escapes and as the temperature increases then the locked on water releases this creates enough pressure and space for the silica molecules to move, changing the crystalline structure. The silica will always try to move as close to the opal checkerboard pattern as the other molecules will let it, this is why it look shinier. Notice that on heated rock the color may change but the outside of the rock still has the texture, a flake has to be removed to see what or if the texture has changed. In fact on some rocks I have found that there is no change until you go as much as 1/4” deep for temperature I cooked it at. The surface vents the pressure to fast to change.
So back to time, temperature and pressure. Since in nature there is lots of time, which we don’t, we have to work with these there things our way.
We understand time and temperature easy enough but what about the pressure. Pressure is controlled by mass. Let’s look at a thin flake off of a larger stone. Both heated the same length of time and temperature and the flake can be cooked just right but the whole rock blown to pieces. Because of the thinner mass the flake can vent its internal pressure faster and easier than the mother stone. So the mother stone vents rapidly by breaking apart.
Over the next year or more my new friend when he had time did all kinds of heating tests. With his kind of work where he is looking at the rock on a molecular level for rare earth elements. He was seeing thing that you just can’t see with naked eyes. When each test was done I would get a call and some lasted for hours.
Simple conclusion was this.... when heating the rocks always try to heat the same size rock. You never want the outside of the rock to have more than a 7 to 10 percent temperature difference than the inside. This applies to both the heating up and cooling down. If there is a bigger difference then stress cracking will start.
If you get a new rock and ask what temperature do I cook it, you will usually get quite a range of temperatures. This is for several reasons. First you only ask for one part of a three part equation. You need temperature, how long and size of material. I will use novaculite as an example. When I first got it I ask two guys what to cook it at. This first guy said I cook to 900 degrees. The other guy said what? I would blow mine up if I cooked it that hot, I only go to 825 . Turns out with more talk they were both cooking 1/4” slabs but the first guy took his up and as soon as he reach 900 turned his kiln off. The second guy raised temperature slower and when he got to 825 held that temp for 8 hours before turning off.. The lesson is that you can get the same results
With a lower temperature held for a longer time. This is also a much safer way to heat with less overcooked rock.
Now there is also the preference difference. I once saw three points made by three different people all from the same rock. The first thing I noted was even though the pattern on each was the showing that it was from the same stone the color and shine of each was way different. The first had been heated to the maximum that the stone would take and had a high gloss and purple in color.
The second had a shine and was more red in color. The third was flat almost no shine and had yellows. All produced by heating at different temperatures and holding times. Some want stone just workable and the next wants his easier than glass which is opal.
So not only does the heat change the crystalline structure but also can change the color as in my first heating test. This can be a fun thing to play with. Most yellow jaspers will turn red with heat, but if heated just right some will become very workable but only have a red skin and still be bright yellow inside. You can then do one of two things. First take two side by side pieces so you can tell they are sisters and cook and knap both. Then take one and recook to then have one red and one yellow sister points. Or heat one slab a little longer or hotter so the red color is all the way through then knap both for the same red/yellow result. Only have one slab red outside yellow inside knap a Clovis and get ready to flute. Recook just hot enough to turn red then flute. So you have a red point with yellow flutes. I have been know to make a batch of small west coast gem points and decide they were all to much the same color. And take halve and recook just so there was a wider range of colors.
I have found that for some material that you will want to heat treat twice, first to get it to percussion nice second cook is for pressure. Because of the thickness it is heated at a lower temperature for the percussion, then after the pice has been thinned down it is reheated to a higher temperature to give it a texture you want for pressure flaking. This technique is used fair amount especially on bigger wider pieces. It is like those who might precession something like good Georgetown raw and then heat a little to get those pressure flakes to run a little farther on something wide.
Most of the agates and jaspers that are found in this country will heat treat at 500 degrees or less but may range in hold times depending on what you like. But beware that many of the stones that are called jasper will not heat treat at all. That is because these are usually really a colorful form of rhyolite. Bruno and Willow Creek are a couple but there are many more.
I buy and cook thousands of pounds of Brazilian agates. There is no one temperature that they all cook at. Now that I have make a number of trips to Brazil to buy I understand better why. The simple reason is they come from all over the place. At the cutting houses as people bring them in from wherever they are sorted but several ways but mostly by size and color.
If I take a batch of slices and cook about 560 I will usually overcook about 10% to 15% and trash them. I will also have about the same amount that will not be cooked enough.
So here is how they are cooked. First cook is to 470 +- then every slice has a small flake taken off an edge. If it is slick and glossy it is done. Most of them will not be and they will get cooked to 510+-. Then they have another flake removed. Slick and glossy done. Rest recooked to 560+- repeat test flakes up 50 degrees each time until all cooked. Some of the real dark brownish reds will need 800 degrees.
BUT BUT BUT BUT this happens a fair amount as a warning.
Say I have a 7” round slice and no matter the temperature I cooked to get that slick and glossy flake, does not mean the whole slice is cooked the same every where. The more the color varies or the more banding the more true this is. You can’t tell by looking you will only know knapping it. I once had a 6”x2” blade that one face knapped great but on the other face 1” from the base was a dime size spot that the flakes would not travel across. First I shortened to get closer,still no go. Then shortened right to the edge, grunt as I might,
less than a 1/4 flake then just had to cut off to make a 4” point.
So now let’s say I take that 7” round slice and cut it into 1.5” wider slices to preshape. I hope the whole slice is cooked but it could be that the two on outside edges are cooked good but the middle piece is only cooked right at the ends. Even if I take a test flake off each one it is at the end not the middle of the slice, so it is possible for me to sell a piece that is very hard to work or unworkable. This doesn’t happen often but it can.
One last comment the rule I live by is this. THE MORE THE COLOR, THE MORE TO PROBLEMS.
-Craig Ratzat