Knowledge is worth it's weight in preforms...


Here are some heat treating tips and temperatures

Why do we heat treat? Heat treating alters knapable material that is otherwise unchippable and transforms it to a glass like characteristic. Heat treating will also improve the colors of some flints. Browns can become reds, grays can become blues yellows become orange and so on. It's an oxidation of the minerals in the stone. Soak heating (heating for 36-48 hours at a constant temp) is not necessary but will further enhance this knapability and color change. Not all materials can be heat treated. An example of this is the black and bluish Kentucky "Horn Stone".
      For the sake of simplicity, we will call all knapable stone "flints" regardless of what they are.
      Remember that there are different grades of all materials, So the chart below has ranges for each type and or grade. It is also important to understand that these temperatures are for spalls or pieces that are absolutely no thicker than 2 inches.
      Heating thicker pieces requires lower temps and a kiln with NO air leaks. Preforms, seem to be able to take hotter temps than spalls. This is likely because of there uniform shape. The preforms can be fairly thick. This is a nice advantage for knappers who have attractive flints that knap like concrete when there raw. They can be preformed and then heated with little risk of damage.
     If a material is not on the chart, experiment with it. If it seems high grade and you still want to heat it to get that glass like look or workability then start with low temps (around 350f.). A rule of thumb is this: white or gray flints take hotter temps than darker flints. Black flints or cherts.
      Moisture content is ever present throughout the stone in ALL flints. Some materials indicated below are very sensitive to heat due to this. They require a special drying process. With out this drying process listed below, your flint will be destroyed.
     Most of our Coastal Plains (including the Jaspers and Corals) flints need special care because of this. It is not total necessary but worth it! For example: on the chart below, Flint River" chert can be heated to 450-460f. This is with the drying process. If you don't want to go through the trouble of the drying, then the max temp must be lowered to 350f. Any hotter and it WILL become damaged. Now then, if you dry it, and following my instructions below, then the stone can be heated to 450f. This is fact.
      The final result for this drying process is better color and knapability with this particular type of stone.
      If you have heated your flint and it has not improved, you can always re heat the flint to hotter temps but you can never go back. Once you have over heated the stone, the damage is done.
      I have had experiences where I know I have heated flint just a bit too hot. The stone became very unpredictable and easily developed splits at the point of impact on the platform. After some tears, the rock was put in a bucket and forgotten about for over a year. I could not bring myself to trim through it and salvage what I could. Then, after it was re discovered a year later, hidden away in my shop, I tested the flint with a billet. The same pieces that had chipped uncontrollably now became very manageable. It was awesome I might add. This has happened to me twice now and I am certain that some of the stresses caused in heating had come out of the stone over time.

*Highly recommended drying as per directions below.
Any deviation from this will result in blown flint! That I guarantee!
First the flint must be dried…This takes about a week so be patient
Take the flint up slow and down slow, no faster than 50 degrees an hour!

Step 1. run up to 250 F and NO higher,. and allow to cool to room temperature.
Step 2. run up to 250 F. and NO higher, allow to cool to room temperature.
Step 3. run up to 250 F. and NO higher, allow to cool to room temperature.
Step 4. run up to 250 F. and NO higher, allow to cool to room temperature.

Step 5. The final heating will go to 450 for high grade material
For lesser grades 460-470. {Thin Slabs may go higher}

Do Not heat whole rocks or spalls thicker than ¾ of an inch.
Please remember…If I said it here, I mean it!
Basic flint heat treating instructions
Be sure the kiln is packed full with NO room for more stone. If you need to top the kiln off, get anything of poor grade to fill it up, even a brick will do! This insures that the flint will ramp up and cool slow with limited air currents within the kiln. (This causes temperature shock, which can blow up or crack your flint).
Take the flint up slowly and down slow, no faster than 50 degrees an hour.
Hold time at finished temps should be at least 3 hours. (No need to hold it longer unless your going for enhanced color)
Do Not heat whole rocks or spalls thicker than 2 inches.

Trouble shooting problems with breakage
1. Pot lid fractures: Due to flaws i.e. Water pockets, fossils, inclusions and crystals to name a few or general overheating.
2. "S" shaped fractures across spalls or thicker stones: Air leaks in kiln, ramping temps (up or down) faster than 50 degrees per hour.
3. Crazed gritty appearance when chipped: Over heating.
4. The "...My kiln is full of sand..." affect. Severe overheating.
5. Rock keeps cracking when trying to knap it. Overheating, or excessive moisture in the stone. It is also possible that there are color bands in a stone that have different compositions and therefore one stone could have stresses in the rock from the heat treatment process. One stone could have such a diversity that, for example, one half might heat perfectly at 550 and the other half is on the verge of being over cooked because of mineral diversity. The answer to this is experimentation to find the best possible temp for that stone. Also, excessive drying helps with these problems!
Basic Kiln safety
     Operate your kiln safely!
1. Never operate kiln on a wooden or flammable surface. Use cement blocks.
2. Place kiln elevated on concrete blocks with holes in them or a proper metal stand. Do NOT set directly on any floor!
3. Keep kiln at least 20" from any walls or other objects at all times.
4. Use cement fiberboard on near by walls for an extra-added protection!
5. Use a "dedicated" outlet for each kiln switch on a 15-amp breaker.
6. Never heat-treat large blocks of stone. Stones could violently break apart. This could knock the lid right off a kiln ejecting VERY      hot Fragments, creating a serious fire hazard!
7. Keep your kiln out of the weather, Damage to electrical parts will result.
7. Never operate kiln with flammable fumes, liquids or vapors present.
8. Avoid heat-treating in your home or living space. Heating rocks can produce poisonous or harmful vapors, even if not cut on      rock saws!

Below I have classified the materials into three grades:
"A" = High grade, sometimes very knappable in the raw state. A slight to good gloss.
"B" = Medium grade, a dry texture. Gritty when you scratch it with your fingernail. No gloss
"C" = Quite dry in appearance. Coarse and very difficult to knap.

All Temperatures are Fahrenheit
Some common but not all of North America's lithics...

*Coastal Plains
Corals found in water
Rivers or oceans
A 490-515 North Dakota
"Knife River"
Fossil cattails
A 350-380
B 515-540    
C 540-600    
*Coastal Plains Fla.
Corals found on land
A 450-550 Arkansas
B 550-600 B  
C 600-630 C  
*Coastal Plains Cherts
found in water
A 480-500 Ohio "Coshocton" flint A  
B 500-550 B  
C 550-650 C  
*Coastal Plains
Cherts found on land
A 520-530 Ohio "Flint Ridge" flint A  
B 530-575 B  
C 575-650 C  
Coastal Plains
Chalcedony found in water pronounced (kal-sed-ni)
A 350 Ohio "Flint Ridge Chalcedony" A  
Coastal Plains Jaspers A 450-500 *Midwest "Burlington" chert A 500-540
B 500-575 B 540-580
C 575-750 C 580-630
*Coastal Plains
"Flint River" Chert
A 440-450 Texas "Georgetown" A 350-360
B 450-480    
C 480-515    
*Florida Cherts A 500-540 Texas "Alibates" A 450-530
B 540-600    
C 600-700    
*Southern Alabama Coastal Plains
A 500-530   A  
B 530-600 B  
C 650-700 C  
Texas "Ft. Hood"
fossil stramatolites
A 500-520   A  
B 550 B  
Edward's Plateau
River cobbles & tabular cherts
A 350   A  
B 350-400 B  
C 400-550 C  


A 450   A  
B 500-520 B  
C 600 C  

Some flints that will not heat treat:

North Dakota
      "Rainy Buttes" Fossil wood
      "Horn stone"
      "Ft. Payne Chert"
      "Dover Chert"
Pa. NY. Ontario