Heat Treating Guide

with Temperature/Time/Thickness Table

This Heat Treating Guide and Time/Temperature Table is primarily, if not solely, the work of Master Knapper, Professional Geologist, and long time PSK member Jim Miller.  It has been widely distributed but he formally gave permission to us to include it on the website for FREE public use. No commerical reproduction or use is permitted. Most if not all of the information in included in Jim's copyrighted CD 'The Flintknapper's Guide to Rock' (available from Jim for nominal fee). 

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HEAT TREATING KNAPPABLE STONE

            Ancient knappers discovered that the flaking characteristics of many types of stone could be improved by applying heat in a controlled manner.  Some types of stone also can be flaked more readily by soaking the stone in water for extended periods of time.  Personally, I've had poor success with the soaking option.  Therefore, this book addresses only heat treatment for making rock more "chippy."  The descriptions of stone types presented later in this book provide general guidance on heat treatment for various kinds of rock that are commonly used by modern knappers.

WHY COOK YOUR ROCK?

Heat treatment has its advantages and disadvantages.  On the plus side, with proper treatment the stone becomes more brittle, the flake scars get smoother and glossier, it becomes easier to remove large flakes from a core, and pleasing color changes occur in some rock types.  A smooth flake surface also allows for creation of a very refined point with exquisite notching and serrations.  On the other hand, the increased brittleness induced by heat treatment reduces the durability and strength of the treated stone, making it easier to break a tool in manufacture or to dull its edge with use. 

Although heat treatment has been practiced for thousands of years, many prehistoric stone tools were fashioned out of untreated (raw) rock, even though the rock itself could have been heat-treated.  Apparently, many ancient knappers did not want to sacrifice tool durability in exchange for ease of manufacture. 

            Some knappers who try to faithfully reproduce ancient stone tools and methods work very little with heat-treated stone.  I certainly can't argue with those ambitions.  But personally, I really like the look and character of well-cooked rocks.  So I generally cook my knappable stone whenever heat treatment will improve the knappability or appearance of the material I'm working with.  Besides, my tired old wrists and elbows don't need the abuse derived from knapping raw rock.

            If you attempt to biface raw CCS materials and the resultant flake scars are rough or non-reflective, the stone is telling you what your wrists already know – this rock is pretty tough!  The highest quality stone for knapping has a satin-like or glossy surface on a flake scar.

            You may wonder what makes the surface of a stone flake glossy, as opposed to dull and non-reflective.  Glossiness is an indication of smoothness of the stone surface at the microscopic level.  Super-smooth surfaces reflect light uniformly (like a mirror or polished stone).  A dull surface on a flake scar indicates surface roughness; this uneven surface scatters light randomly and prevents a glossy reflection.  Heat treatment allows a flake to pass through, not around, the microscopic quartz crystals in the stone.  The result is a flake surface with a glossy or satin finish.  Obsidian has a natural gloss on flake scars because it has no tiny crystals of quartz or other minerals to scatter incoming light.

It's pretty easy to ruin good stone with heat treatment.  With the application of extreme heat, or if heating is applied too rapidly, the rock may break apart violently into sharp-sided pieces.  Overheating also may leave the rock largely intact, but the stone will be ruined by potlidding or crazing.  Potlidding occurs when disc-like spalls pop off the outer surface of the stone.  Crazing describes the interior of a stone that is full of intersecting cracks, making the stone unworkable.  In nearly all cases, stone that experiences potlidding will also have crazing on the interior of the stone.  So be careful.  Don't risk your best stone to heat treatment until you test small pieces of the material to determine optimum heating temperatures.  There are several ways to avoid ruining stone by overheating (more detail is presented in later sections):

1.     Raise the temperature slowly.

2.     Allow sufficient “drying time” at relatively low temperatures so that excess moisture is removed from the stone.

3.     Only treat slabs or bifaces that are less than 2 inches thick.

Many varieties of CCS (chert, flint, agate, chalcedony and jasper) can be "pre-treated" by slowly raising the temperature to about 300-350°F.  This moderate temperature generally will not damage the stone or give it a glossy finish on flake scars (Patten, 1999).  But this pre-treatment can remove some of the toughness of the stone, making it easier to spall and reduce by percussion.  In addition, the increased brittleness afforded by modest heat treatment will produce less wear on diamond saw blades. You may find that this relatively low heat treatment gives you all the additional workability that you desire.  If not, or if you want more gloss or color change, cook it some more!

HOW DOES HEAT TREATMENT WORK?

The short answer to this question is:  "Heck, I don't know.  It's a mystery and a miracle!"

Several varieties of knappable stone (typically CCS materials) can be improved by heat treatment.   I have yet to see a definitive explanation of how heat treatment improves the flaking characteristics of stone.  Nevertheless, in many cases the differences before and after heat treatment are dramatic.  For example, raw Montana agate can be so tough that my wrists and elbows hurt just thinking about flaking the stuff raw (unheated).  But after effective heat treatment, Montana agate will behave like top quality flint or chert.

            Not all knappable stone can be improved with heat treatment.  In fact, some varieties of stone, such as common opal, will disintegrate with the application of even moderate amounts of heat.  Other varieties of rock do not respond at all to heat treatment. 

Some people believe that heat treatment causes partial melting of the impurities in the stone, thereby "fusing" the stone together to make a more solid and homogeneous mass.  The temperatures typically employed for heat treatment are far lower than the melting temperature for all types of knappable stone.  So, from my perspective, partial melting is not the likely explanation for why heat treatment improves the brittleness and glossiness of knappable stone.

            From my own observations, I think the ability to improve stone with heat depends on a number of factors.  First, the stone must consist entirely, or almost entirely, of cryptocrystalline silica (such as flint, chert, agate, chalcedony, jasper and other CCS rock types).  Also, the stone must have tiny pore spaces that are occupied in part by water.  It has been suggested that heat causes internal pressures to develop in the rock pore spaces, with resultant micro-fractures distributed through the stone (Flenniken and Garrison, 1975; Patten, 1999).  The micro-fractures help percussion or pressure flakes follow a smoother surface around and through the microscopic quartz crystals that comprise the rock.

            Similar rock types can vary considerably with the temperature needed for heat treatment.  I think this depends on the size and shape of the microscopic quartz crystals as well as the size of the pore spaces in the rock.  Also, I believe that a minor amount of water is needed in the pore spaces of the stone to develop enough internal pressure for beneficial treatment.  Regardless, each stone type has its own secrets when it comes to finding a good cooking recipe.

            To make things even more frustrating, the same piece of rock may have different responses to heat treatment.  The highest quality stone is often found on the outer surfaces of flint, jasper and chalcedony nodules.  These high-quality rinds often require little or no heat treatment.  But the more opaque interior portions of these nodules are often more coarsely grained and may require high temperatures for heat treatment.  This can make heat treatment very difficult.  The outer edges of a stone may be very knappable following a moderate application of heat, while the interior remains tough.  Higher temperatures may treat the interior of the stone effectively, but create potlidding in the outer portion of the rock.  It’s hard to win under these circumstances!

            The key variables in the heat treatment process are: 1) the size or thickness of the stone, 2) temperature, and 3) holding time at maximum temperature.  Greater stone thicknesses produce greater internal pressures as the stone is heated.  Consequently, a thin piece of stone typically will require higher treatment temperatures to achieve the same degree of workability improvement as a thicker piece of the same material.  On the other hand, if you treat a thin biface or slab successfully at a specific temperature, and later try to treat a chunk of the same material at the same temperature, the chunk of rock may simply break apart and be destroyed.  Experience has shown that lower treatment temperatures with longer holding times can produce the same results as higher temperatures with shorter hold times.  My personal experience has shown little advantage to a cooking recipe with lower temperatures and longer hold times.  But some knappers feel that better flaking characteristics result from longer holding times during heat treatment.

WHAT IS THE PROCESS FOR COOKING KNAPPABLE STONE?

            Ancient knappers apparently built sand-lined fire pits for heat-treating spalls and bifaces.  The stone was buried in a layer of sand at the base of the fire pit, a roaring fire was started, and after things cooled down the rock was treated (if everything went well). This time-tested method of heat treatment is certainly an option for you, particularly if you like traditional approaches to knapping.  But modern knappers have lots of other more convenient options to consider.

            I have successfully treated thin spalls and slabs of stone on top of a wood stove.  I ruined a lot of rock in the process because it is very difficult to control heating temperatures with this method.  But sometimes the results were very pleasing.  Temperatures of up to 550°F or higher can be attained on top of a wood stove.  Do NOT place slabs or spalls directly on top of the stove (they will only break).  My method is as follows:

1.     Place a half-inch of sand along the bottom of an old iron frying pan.

2.     Put the spalls or slabs on the sand.

3.     Place an inch of sand above the spalls or slabs.

4.     Put a frying pan lid or aluminum pie tin over the top of the sand.

5.     Position the frying pan in the middle of the flat surface on the top of the stove.

6.     Start a fire in the stove and gradually bring it up to a full temperature.

7.     Let the fire die and the contents of the pan slowly cool. 

Some people treat stone in their kitchen using the family oven.  This is bound to win the cheerful support of interested spouses, particularly when the rock starts giving off stinky odors or popping off spalls from over-cooking!  Second-hand ranges can be purchased quite inexpensively by the dedicated knapper.  Although this method of heat treatment is feasible, typical kitchen ovens only attain a maximum temperature of about 500-550°F.  And long cooking times in the family range can use a lot of electricity!

      Another popular device for cooking stone is an electric turkey roaster.  If you can find them, electric turkey roasters are fairly affordable (particularly at second-hand stores or garage sales).  They can also hold quite a bit of rock at one time and the heat can be controlled with a dial gage.  But the maximum temperature attainable for most turkey roasters is about 450-500°F.  This is good for some types of rock, but other types of stone need more heat.

Many modern knappers now use kilns for cooking their rock.  Some kilns are even made specifically for knapping and are programmable.  Once the basics are learned, rock wastage from over-cooking can be minimized using a kiln.

Some knappers place their stone in vermiculite or sand during the treatment process.  This allows for slower heating and cooling of the stone and spreads the heat more uniformly within the material being heated.  I have found this process to be unnecessary when a kiln is used for cooking rock.

There is a learning curve for success with any heat treatment process.  Personally, I use a programmable kiln for treating a wide variety of stone.  I often put a variety of materials into my kiln at the same time.  The process I use for treating mixed types of stone at one time is as follows:

1.     Place slabs, bifaces, spalls or whole rock into a wire frying basket (the kind used for deep-frying food).  The basket is slightly smaller than the interior of the kiln.  I support the basket on three ceramic posts so that heated air has direct access to the bottom of the basket, as well as the sides and top.

2.     Raise the temperature slowly to about 205°F.

3.     "Dry" the stone at 205°F for anywhere from 4 to 40 hours (longer drying for thicker pieces).

4.     Raise the temperature to 350 degrees at 25 degrees per hour.  Hold this maximum temperature for 2 to 4 hours (longer hold time for thicker pieces) and then turn off the kiln.

5.     After the material has cooled, remove it from the kiln and detach test flakes from each piece of stone.  The materials with satin or gloss finishes are done.  Put the others back in the basket for treatment with more heat.

6.     Raise the temperature of the re-treat material at 50°F per hour until reaching 350°F.

7.     Increase the temperature by 25°F per hour until reaching a temperature of 400°F.

8.     Hold this temperature for 2 to 4 hours (longer time for thicker pieces) and then turn off the kiln..

9.     After cooling, repeat Step # 5.

10.  For materials needing more heat, repeat Steps #6 through #9 at higher temperatures until all of the stone is successfully treated (or until you discover that the stone does not respond to heat treatment).

COLOR CHANGES DURING HEAT TREATMENT

Heat treatment will sometimes cause stone to change color.  Typically, the color change is from white or brown to pink or red.  The pink and red color change occurs from oxidation of trace amounts of iron in the stone.  The red coloration results from the mineral hematite, which is one variety of iron oxide.  If you are trying for a color change with heat treatment, try small pieces of rock first so that you don’t ruin your favorite material by accident.  The next paragraph explains why…

Most color shifts improve the look and character of the stone, in my experience.  But there are some exceptions.  I’ve had some varieties of silicified sediment (Biggs jasper) change from mainly brown to gray or black during heat treatment.  Also, I’ve had some fairly colorful varieties of Texas chalcedony change to a dull gray-brown with heat treatment.  I suspect that these reverse color shifts result from rock types that contain trace amounts of organic carbon or hydrocarbons (petroleum) within the stone.  As the carbon is heated, it robs oxygen from the stone itself to form carbon dioxide gas.  In the process, trace oxide minerals in the stone are altered to a non-oxidized state and loose their coloration. 

Fairly long hold times at maximum temperature may be needed to get a color shift in some types of stone.  Sometimes the color change from heating goes all the way through the stone, and other times the color shift is only skin deep.  Oxygen is needed to produce a red-shift color change.  For dense stone with little porosity, oxygen can only extend a short distance into the stone and the color shift is only skin deep.  Stone that is more porous may allow relatively deep exchange of oxygen within the stone, with a resultant color change that extends all the way through the stone. 

If you like the color change, but the coloration layer is removed by detaching flakes during manufacture of a point, all is not lost.  You can typically re-cook the finished point to get the desired color.  Just don't exceed the original cooking temperature for the stone, or else it may break on re-heating. 

You can also use a propane torch to apply heat rapidly to part of a point to create a color shift.  But be careful with this one!  Rapid application of heat can result in uneven thermal expansion within the stone, leaving your hard work lying in pieces on the floor.

TYPICAL HEAT TREATING TEMPERATURES

            All rock is different and considerable experimentation is needed to determine the optimum heat treating temperatures for different varieties of knappable stone.  Based on my personal experience, typical temperature ranges for treating different varieties of stone are listed below.  More detailed information is presented under the descriptions of stone types in the Appendix of this e-book.

·       Flint and hornstone:  350-500°F (typically 400°F)

·       Agate and chalcedony:  400-600°F (typically 450°F)

·       Jasper:  400-750°F (typically 550°F)

·       Chert:  400-700°F (typically 600°F)

I’ve done a fair amount of flint collecting in central and southeast Texas.  A huge variety of chert and flint exists in this area.  In my experimentation with heat treating the flint and chert from the Edwards Plateau, I noted an interesting general trend – the darker the color of the stone, the lower the temperature needed for heat treatment.  For instance, the translucent dark brown or dark gray flint generally cooks very nicely at 375°-450°F.  Light tan or light gray Texas chert, on the other hand, typically requires temperatures in excess of 500°F for improved workability.  THIS OBSERVATION FOR TEXAS MATERIALS IS NOT UNIVERSAL!  So don’t try to apply it to rock types from other areas.

HEAT TREATING HINTS

            If you only have one type of knappable stone in your area, heat treating can be pretty routine.  But for those who may be treating a variety of sizes and types of materials, here are a few pointers:

·       Your life will be simpler and your power bill will be lower if you treat similar materials in batches.  For instance, treat a load of Burlington chert all at one time.  And for you next effort, load your kiln (or turkey roaster) with agate and jasper.

·       Although I may place large chunks of rock in my kiln for pre-treatment at relatively low temperatures, I generally do not attempt to reach final treating temperatures on slabs or bifaces thicker than about 2 inches.  Thick pieces of stone taken to maximum temperature often results in potlidding on the outer surface of the stone, or crazing of the interior of the stone, making it unusable for knapping.

·       If you are treating sawed slabs, place them loosely in the kiln at odd angles (not stacked like poker chips).  This will allow even heat distribution to all of the rock slices.  I put small chunks of obsidian in my wire frying basket when I’m cooking slabs.  I place the chunks of obsidian among the slabs to keep them from lying flat together in the frying basket.  Obsidian will not melt or degrade at typical temperatures for heat treatment (after all, obsidian was once molten).

·       Clean all sawed slabs thoroughly before treatment.  Saw oil smokes and smells terrible once the cooking temperature exceeds about 450°F.  I wash my rock slices with soap and water before the stone goes into my kiln.  Some people even wash their slabs in their dishwasher!

·       Some rock types from volcanic areas can contain mercury, particularly if the raw stone has a reddish color.  Apply heat to this stone only in very well ventilated areas, as mercury vapor may be liberated during cooking.  Breathing mercury vapors is a bad idea – it is highly toxic in vapor form.

Heat Treating Time/TemperatureTable    
 Updated February 3, 2014
 Lastest update October 10, 2017

This table was created from an accumulation of data from different books, and heat treating experiences of other knappers who were generous in sharing their knowledge and experience.  Much of the content was contributed by Jim Miller

For additions, corrections, comments, email  

Heat Treating China Hollow - courtesy of Jim Miller

China Hollow produces a wide variety of interesting knappable stone, ranging from translucent agate to tough jasper.  The agate is typically found on the outer portions of jasper nodules, or as small nodules, and requires temperatures of 400 – 450 degrees for effective treatment.  Use bifaces, slabs or spalls less than 1 inch thick for the agate.

The interior portions of nodules typically contain tan jasper that needs 500 to 650 degrees for effective heat treatment.  Some of this material changes to a purple color with heat treatment.

The major difficulty with China Hollow material is that the rock varies from agate to tough jasper within the same nodule.  Effective heat treatment for the agate barely makes a difference to the jasper.  And good treatment of the jasper will be too hot for the agate.  So you need to try to keep like kinds of rock together and treat each different type of rock to its optimum temperature.

NOTES:

1)    Hold at 200 F for 24 hrs to release moisture.

2)    Hold at 200 F for 48 hrs to release moisture.

3)    Cortex, if present, should be removed (skinned) before heat treating on all flint.

4)    Soaking in water after heat treating improves workability.

5)    Added 3/8/15 JCK ref PP.