Heat Treating

The flakability of some stones can be dramatically enhanced by heat treating. It will improve your capacity to thin and shape the piece using either soft-hammer percussion or pressure. Flakes will penetrate further and can be detached at more acute angles. Heat treating was practiced throughout the world in ancient times and is a necessity if you want to create sophisticated tools in materials other than obsidian. Heat treating also produces some aesthetically pleasing effects, including changes in luster and/or changes in colour.

Types of Stone

Heat treating seems to work by a variety of processes, and consequently critical temperatures are highly variable. The type of stones which can be improved by heating include most of the cryptocrystalline sedimentary silicates: flints, cherts, jaspers, opal, agates and chalcedonies. Most of these will undergo between 500� and 600� F. Temperatures of greater than 600� F will have unpredictable consequences, most of them unpleasant and some disastrous. Properly executed heat treating of these types of materials will increase lustre to a "waxy" finish. Dramatic colour changes are also possible, usually a reddening or darkening. Colour changes usually occur at slightly higher temperatures than lustre changes. Crazing and checking of the rock surface is an indication that excessive temperatures were reached. The stone will probably be unusable. Igneous rocks like obsidian and basalt will normally not be improved by heating to these temperatures, but they will sometimes benefit by heating to temperatures of 1000� F or greater. Temperatures of greater than 1500� F should probably be avoided. Such temperatures can be difficult to achieve in a controlled setting without a kiln. There are no visible changes in the appearance of the rock. Some metamorphic forms like slate, argillite and some quartzites may be improved by heating, but the vast majority will not benefit. Some types, most notably quartzites can suffer severe degradation at temperatures as low as 600 � F. Lustre and colour changes are unpredictable.

How to tell if a stone will be improved by heat treatment

It is almost impossible to predict how a stone will react to heat and it is equally difficult to predict what the critical temperature of any given stone is. The best way is to experiment by heating different rock types together, recording the maximum temperature reached and keeping notes.

How to do it (Modern Technique)

Reduce your rock to flake blanks. Pick a time when you can monopolize the oven for several hours without causing a family crisis. Get a suitably sized metal roasting pan, preferably one with a lid. Fill the pan with clean sand. Place your flake blanks in the sand in such a way that every blank is fully covered with sand, and no piece touches the edges of the pan or another blank. If you have a meat thermometer that goes to 600 � F or better, place the thermometer in the sand in the center of the pan. Place the pan in your kitchen oven. Turn the oven up to 150 � F (65 � C). If you have a range hood, you might want to turn it on particularly if your oven hasn't been cleaned for awhile or if your sand is not clean. Go away (don't go too far, it is unsafe to leave your oven on and unattended). After an hour (or when your thermometer says the sand has reached the same temperature as your oven setting) turn up the oven by 100 � F (about 50� C). Repeat this process every hour until you reach your target temperature (usually 500� to 600� F). If your oven dial doesnít go that high, you may have to set it on "Grill" for the final stage. Leave the oven on for about an hour at the highest temperature, and then turn it off. Let the pan cool slowly down to room temperature (it is often a good idea to leave it overnight). DON'T PEEK: flakes that suddenly cool down can break or explode catastrophically.

The following is an example of the procedure:

Time Oven Setting
7 PM 150� F (65� C)
8 PM 250� F (120� C)
9 PM 350� F (180� C)
10 PM 450� F (230� C)
11 PM 550� F (280� C) Might also be the "Grill" setting
12 PM Turn Off Oven
8 AM Remove from Oven

How to do it (Traditional Technique)

Reduce your rock to flake blanks. Prepare a bed of sand at least a couple of inches thick. (I do this in an old dish shaped barbecue, but you can do it anywhere, including a shallow pit in the ground). Place your flake blanks in the sand in such a way that every blank is fully covered with sand, and no piece touches another blank. Build a pyramid of charcoal briquettes over top the sand. Light briquettes. When the briquettes are going well, spread them out with a pair of barbecue tongs or stick in a layer over the entire sand surface. If you don't have enough briquettes, you can add more by inter-mixing them with the burning briquettes. Leave the fire alone. Don't knock off the ashes, just let them burn. Don't leave the fire unattended. Let sand cool overnight before removing your blanks. If you want to be completely authentic, use well seasoned firewood instead of charcoal. Keep the fire reasonably small and try to build it so you get a good bed of embers.

Don Hanna
Calgary, Alberta

A recipe for cooking flint has 3 phases:

• Phase 1 - DRYING the rock by cooking off moisture from within the rock.
• Phase 2 - RAMP up the temperature from the drying temp toward the final hold temperature.
• Phase 3 - HOLD temperature is the amount of time to cook the rock at it's final temperature.

PHASE 1: the rock should be cooked at a "drying" temperature that is low enough to slowly cook out moisture, without cracking (pot-lidding) the flint. If the flint is "surface" found flint, meaning it is fairly dry, then you can dry it at 250 degrees for about 3 hours. If the flint is pulled out of the ground, then chances are that it may have a fairly high water content. Spall a small flake off the nodule, and see if it changes color in about 2 hours. If it does then chances are that it is pretty water logged. High water content flint should be dryed no higher than 200 degrees for about 6 hours, to reduce the chances of pot-lidding. The moisture must be SLOWLY.... cooked out to keep it from fracturing the rock during cooking.

PHASE 2: the rock should have been "dried", and is ready for the "ramp" up sequence. My kiln has a ramp rate of 100 degrees per hour, but 50 degrees would be a much better rate. The 50 degree rate reduces chances of pot-lidding. For example: if the final "hold" temperature that you will be cooking a certain type of flint is 460 degrees, ramp the kiln up from 200 degrees to 460 degrees at a rate of 50 degrees per hour:

The "final" temperature you use is dependent on the type of flint being cooked. Experienced knappers in your area can give you cooking temps for various local flints.

PHASE 3: the rock is now at 460 degrees at the end of the 4th hour, and should be HELD at this temperature anywhere from 4 to 30 hours. There seems to be no UNANIMOUS agreement of what the hold time should be, ask 3 different knappers and you will get 10 different answers. The argument is that the higher the hold time, the higher the gloss that the flint will take on. I believe that it depends on the type of flint. Experiment with the stuff in your area, and decide for your self.

OTHER THINGS TO CONSIDER:

bi-face the flint, don't cook big thick chunks. The bigger chunks don't dry out very well, and almost always pot-lid.
Put your spalls/bi-faces into a pot/bucket and fill the air spaces with sand. This helps even out temperature variations, reducing pot-lidding.

1) DISSIMILARITY OF MATERIAL WITHIN THE STONE

Dissimilarity of material within a piece will have different expansion rates during heat-treatment. So reducing the thickness of a piece helps to minimize the effects of expansion. Chert can have varying degrees of silicification from cortex on through to the center. J.B. Solberger used to refer to this type of potlidding as "dry spots". I particularly notice potlidding in chert that has a high silica content rind and cement-like core. When these two extreme types are together within a biface, the chances of pot-lidding are quite high. This is why taking the cortex off the material is usually recommended. The rind in many cases, has a higher degree of silicification then the inner core area. You see that when Freddie Flintstone would take a few large flakes off the outer rind area and throw away the rest of the rock. He was going for the high silica part to make some nice dart/arrow points.

2) NOT DRYING OUT THE ROCK ENOUGH

If moisture within the stone is not removed slowly enough, or not completely, during the first part of the heat treatment, then at the higher final cooking temperatures, it will fracture. So experiment with your particular lithic material to see what initial temperature and dry out time is needed. Some typical drying time/temps are 250 degrees for 3 hours for a lot of central Texas flint, 200 degrees for 6 hours would be more conservative.

3) RAMPING THE TEMPERATURE UP TOO FAST

Once the stone is DRIED, if the temperature rate is too fast then the material may fracture. Most knappers here in Texas use 50 degrees per hour. There are a couple of retired knappers that I know who claim that they don't do much bifacing, but put in large chunks of Ft. Hood and cook very slowly, something like 10 degrees per hour. This is an extreme on the SLOW end, but helps to illustrate one of the variables in heat treatment.

David McCracken ([email protected]) posted these observations and his traditional recipe on Knapper.

Some general guidelines that I have observed are:

This can be a very rewarding or a very destructive process. Trial and error will determine the proper amount. Good luck and don't be too disappointed if it doesn't work great the first time.

I've been heat treating Montana Moss agate at 450F for about 8 hours in a normal kitchen oven. I usually layer the slabs or preforms in metal cake pan with about two inches of sand on top and bottom. I usually step the heat up and down about 200F for an hour and leave it in the oven overnight to cool. The procedure seems to work for other agates, i.e. Brazialian, etc. Heating the agates usually results in waxy luster.

Florida coral needs to be heated, as do our cherts. Paleo points were made from high grade cherts but very few from coral as it is extremely hard to flake in it's raw state. Paleo points were not heat treated in Florida. From my observations, depending on silicon content, Florida cherts and coral vary considerably. The same qualities of each heat the same. High grade, translucent 550 degrees. Low grades to 700 degrees. This is 1/4 to 5/16 inch slabs or thin preforms. Thicker preforms will invariably take less heat. The longer the soak heating the more intense the color.

I also use vermiculite (fertilizer at Wal-Mart) instead of sand as a packing medium for my stone. Vermiculite is high fired mica. It weighs nothing and insulates as well as sand. I fire several hundred pounds of rock at a time and don't need the added weight of sand in my containers.

I use a lot of stone from central Missouri. This is mostly various types of chert. Some of it heat treats very well but some of it needs to get very hot.

I use a round wash tub with about a foot of sand in it. I then lay the flakes on the sand with the hard-to-heat peaces in the center and the easy-to-heat peaces round the edge. I then place about two to three inches of sand on top of that. With that in place I put about 10 to 15 pounds of charcoal on top and light it up. After the charcoal is ashes on top I place another 10 to 20 pounds on it and just let burn. After about a day and a half I dig out the flakes.

This works great in the early spring and late fall when the weather is cool. Using less charcoal or spacing out the times I place charcoal on the fire in the summer is required. As long as I have the hard-to-heat stone in the center, the flakes come out fairly nice. If I heat treat agate I will place these flakes on the very outside edges. Agate does not require much heat, in fact if too much heat is applied you will destroy the flake.

We have been experimenting with heat treating some jasper and chalcedony from Rhode Island lately and find that it heat treats at 300 - 350 degrees. 450 made it absolutely shiny and UNUSABLE.
If conserving your material is important give the lower temps a try first.

John Cianfarani Jr. [email protected]

Here's Steve Ashley's [email protected] Recipe for Ft. Hood Flint. Steve has also had some very good results lately with multiple heat treats. He heats and reduces the blank two or three times and achieves a marked color change from white to red!

Just cooked up 7 large coffee cans filled with spalls and preforms using my bachelor technique of: turn on the ventahood, put the cans of flint in the oven, preheat slowly starting at 250o F and going up 100 degrees an hour up until I get it up to 550o and then I cooked it for 10 hours, turn the oven down in 100 degree increments until it is cooled down. Let it sit for a few hours until I feel it is ok.

The Ft Hood flint cream colored flint turned a pinkish hue on the outside and when knapped has a waxy look. This is just the right amount of heat I think for the Ft Hood flint as it works so great! The heat treating enhances the fossil inclusions.

I added some heavy metal rust to one can, and the rust imparts a red oxide to the stone, only skin deep though. I am amazed at how great the flint works and I chipped out several nice projectiles and 2 blades in about an hour.

The following recipe was Ken Goff's [email protected] preferred method for heating novaculite.

The only material I have done any heat treating on is novaculite. there are different grades I have worked with but I have heat treated them all the same.

I used what was called a china painter's kiln. It plugged into a 110 volt outlet. The maximum temperature was about 1200 degrees or so. I always started with the kiln loaded full and set it on it's lowest setting. It didn't have a thermostat, just a dial that went from 1 to 10. However, I did have a pyrometer so I knew what the temp was. I would wait for the temp to come up and level off. after it leveled off I would wait about half an hour to let the rock get evenly heated and then bump the dial up to the next number. I would repeat this process until the temp got up to about 900 degrees. Then I would shut it off and wait about 24 to 36 hours for it to cool. I've been told that this is a fairly high temp for "hillbilly obsidian," but I liked to get it as glossy as I could.

Going slow was the secret. I rarely had any break, even at the high temps.

Here are Ralph Conrad's [email protected] specs for cooking Burlington Chert and a number of Oklahoma Cherts.

Cook Burlington (MO chert) at 580 for 1 to 18 hours after reaching 580, if using a kiln that has been rewired to 14 ohms or less of resistence and an infinite oven swith that acts as a reostat, thus bring the temperature up very slowly. My kiln takes 12 to 18 hours to reach 500.

Cook Florence A (Kay County OK) 600-650 using the same method as above.

Cook Gray Boone (OK chert) 650 using the same method as above.

Cook Peoria (OK chert) 580-650 using the same method as above.

The longer cook time seems to color the chert up more when using Burlington and Kay County. Otherwise, I don't find much difference in the workability of 1 hour cook time and 18.

Steve Behrnes of Baton Rouge, Louisiana, makes the No-Babysit Kiln specifically for heat treating chert. These variable temperature control kilns are available from:

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