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**painshill** · 04-08-2016, 05:18 PM

Posted by [painshill]:
Hi Wayne. When you asked: “Is this flaking done by heat treatment…?” I took that as asking whether the faceting itself was produced by the intentional application of heat. It’s an often-repeated myth that heating a stone and then dripping cold water onto it was used as a flaking technique… but there is no evidence for that and it would in any case be a process that’s almost impossible to control in any useful way.

There is however evidence in some places of heat being used as a quarrying technique, by building a fire against a rock-wall where desirable material was located.

Heat treatment of the material to improve its colour and/or knappability by conventional techniques is of course well established.

Apologies in advance if I’m being pedantic by taking you too literally.

Posted by [chase]:
I agree not heat treated. Color in jasper/chert can get exotic. Here is a example of material from one quarry.

Posted by [farmerwayne]:
Thank you Roger! I have several books with slim to no information on heat treatment, and you always come through with the info! I have heard of " heat treatment" and saw a post about it, but not exactly sure what it entails. So, is the quarrying technique the only heat treatment method used? I hope I'm not being a nuisance.

Thank You Chase for showing me that red color may not have anything to do with heat. Love all the color!

I would love to see some pictures of heat treated material if anyone out there has any.

Posted by [rmartin]:
Different materials react react differently when heat treated. Some show no signs of alteration at all. Burlington flint which is white turns a very light pinkish to red and will show different degrees of glossiness. A flintknapper friend of mine buries his in the ground and builds a fire over in and lets it burn for a few days. Here is a large blade of Burlington I found.

**painshill** · 04-08-2016, 05:19 PM

Posted by [painshill]:
Hi Wayne

No problem. Heat treatment is well established as a technique (on cores and blanks) to improve knapping quality and or enhance colour. You may find this useful… heavily edited down from Frank L. Cowan’s paper “Heat-Treating Experiments With Onondaga Chert”. These portions of his text relate to the techniques in general, rather than Onondaga material.

Heat-treatment in lithic technology refers to the controlled heating of quartz-rich, brittle, elastic lithic materials in order to reduce fracture toughness and improve knapping qualities. Successful heat-treatment reduces the point-tensile strength of many varieties of micro-crystalline quartz, including chert, without perceptibly altering any of the other properties with regard to fracture. This reduction in fracture toughness has tangible benefits to the knapper in terms of the relative ease with which fracture can be accomplished and controlled.

Heat-treated materials require the application of less force to produce flakes. Longer and, frequently, thinner flakes can be detached from heat-treated stone than can be removed with equivalent force from unheat-treated stone. The increased ease of flake fracture significantly enhances the knapper's control over the material. Failure rates in the production of chipped stone tools may be diminished. Greater latitude and flexibility may be possible in both the choice of retouching techniques, and in the variety of functional products that may be produced from a given lithic unit.

Heat-treatment is not, however, a necessary technological step in the stone tool manufacturing process. Many of the better quality lithic materials can be successfully knapped without thermal alteration. Heat-treatment is therefore an optional technological process in the manufacture of stone tools. It is a process which involves definite labor and time costs, and often certain risks, as well as variable potential benefits.

In most cherts (and other varieties of microcrystalline quartz and some quartzites), successful heat-treatment usually involves raising the temperature of the stone to approximately 275 degrees C., and maintaining that temperature for a period of several hours. The critical temperature at which thermal alteration occurs and the length of time the material must remain at that temperature is apparently quite variable with differing kinds of materials…. Arkansas novaculites require very high temperature curves, with thermal alteration only successfully occurring with sustained temperatures of 450-500 degrees C. …. color changes consistently took place in Florida chert heated to between 240-260 degrees C. when iron was present in concentrations of at least 1,100 parts per million. Critical temperatures for effecting textural changes were 350-400 degrees C. … Burlington chert underwent color changes (apparent iron oxidation) at approximately 230-290 degrees C., but textural changes were not apparent at less than 290-370 degrees C. … Burlington cherts may be successfully heat-treated at much lower temperatures. Sustained temperatures of 275 degrees C. appears to be sufficient, although higher temperatures will produce higher, more vitreous lusters. … generally more vitreous materials, and darker colored cherts and flints require less heat than lighter colored cherts, and, in fact, many high quality materials can not survive the high temperature curves suggested in some of the literature.

The upper temperature limit of successful heat-treating is definitely fixed at 573 degrees C. … this temperature does represent the apparent threshold beyond which microcrystalline quartz aggregates can not cross without disintegrative effects.

The controlled nature of successful heat-treatment becomes apparent with the consideration of temperature change rates. Rapid heating results in explosive fractures as excess moisture trapped in voids, fissures, pockets, and in the interstices between microcrystals vaporizes. Differential expansion of the brittle material results in characteristic "potlid" fractures. Overly rapid cooling results in reticulated fractures or "crazing". Rapid cooling and uneven mass shrinkage causes distinctive "reticulated" fractures which occur abruptly across the thickness of a chert blank and often leave a rippling surface across the fracture…

Conceptually and observationally, the distinction between heat-treatment and fortuitous burning is essential. Poorly controlled heat-treatment attempts can produce all of the fractures discussed above. Nonetheless, the abundant burnt and pot-lidded chert debitage and artifacts recovered from most archaeological sites pertains to … congruences of debitage and fires burning for other purposes… Post-depositional fires, whether humanly caused or the result of natural processes, may alter cherts, although the results are most likely to result in characteristc fractures of thermal shock, 'smoked' surfaces or structural deterioration to blue-black color and chalky textures. The distinctive textures of controlled thermal alteration for technological purposes occur on surfaces flaked after heat-treatment.

Posted by [farmerwayne]:
Roger
I just had a hunger for that information and like a big Thanksgiving dinner I'm finally satisfied. Thank you so much.

Ray
Thanks for the picture. Now I know what heat treated looks like. not what I expected!

**Tam** · 08-21-2019, 12:46 PM

Was reading this again to compare some of my other research and brought this back up for people who are asking this question .

**Quartzite Keith** · 08-21-2019, 04:06 PM

Looks like jasper, most of which start out that bron-yellow and turns red when heated. i think maybe this piece was accidently heated. I think it was flaked, r

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