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Many types of flints and cherts are difficult to knap in their raw form, and should be heat treated. If properly done, heat treating can greatly enhance the knapibility of the flint. Many types of flints and cherts will have a glossier finish and turn beautiful colors during heating.
THE OLD WAY:
Dig a shallow pit and put flakes, preforms, and/or
sawed slabs into the pit. Put the thickest pieces towards the bottom, with
the thinner flakes covering them. This is to prevent heating the thicker
pieces to rapidly and causing them to fracture or explode. Make sure all
the pieces are also dry to prevent exploding. The layer of flint fragments
shouldn’t be more than 5 inches thick. Cover the flint with a layer of
dirt that is 2 inches thick all over. The dirt acts as an insulator, and
lets the flint heat up slowly, and also blocks most of the hot flint fragments
that often shoot up from the top layer during heating. Build a fire over
the layer of dirt and keep it burning very strong for at least 15 hours.
Let it cool off for 2 days before digging it up.
THE MODERN WAY:
Many modern day flintknappers heat treat flint in kilns with pyrometers. A jeweler’s burnout oven also works if you have time to tend to it (that’s what I use). You can also buy kilns now that are designed especially for heat treating flint. See below for link for heat treating kilns. A lot of people use turkey roasters and conventional ovens, but these do not get hot enough for novaculite and a few flint and chert types, though they do work on most.
Put the flint flakes, preforms, and sawed slabs into a metal can or bucket, with the thicker pieces in the center. The can will shield the red-hot heating elements from being damaged by chips that may fly off. Put sand in the can or bucket all around the flint. The sand acts as an insulator and slows the heating and cooling of the flint and protects it from fracturing. Put the can or bucket in the kiln or oven, shut the oven lid, and start it at 200 degrees. Hold it at that temperature for at least 2 hours. Then raise the temperature at a rate of 50 degrees per hour, until you reach the desired temperature (refer to Heat Treating Temperatures on the inside of the back cover of this book). Hold it at the correct temperature for 10 hours, then reduce the temperature by 50 degrees per hour until it reaches room temperature.
Do not open the oven lid at any time during the heating process, as this will cause a sudden temperature change that will ruin the flint. When the flint reaches room temperature, it is ready to knap.
Thicker pieces need to be heated and cooled much slower,
and should be held at the maximum temperature for much longer. This can
take up to several days to complete the process.
HEAT TREATING KILN SOURCE:
http://www.thenaturalearth.com/
HEATING TEMPERATURES:
Alibates Flint - 425° -500° F.
Boone Chert - 650° F.
Brazilian Agate – 500° -600° F.
Burlington Chert - 750° -800° F.
Carter Cave Chert - no heating required.
Chalcedony - 300° -350° F.
Coral - 425° -650° F.
Coshocton-Upper Mercer Chert - no heating required.
Deepkill Chert – no heating required.
Delaware Chert - no heating required.
Dover Chert - no heating, but works easier when soaked in water for a long time.
Edwards Plateau Flint & Chert - 350° -550° F.
Flint Ridge Chalcedony – no heating required.
Flint Ridge Chert - 700° F.
Flint Ridge Flint - 500° -600° F.
Fort Hood - 400° -550° F.
Georgetown Flint - 425° –450° F (can also be worked raw).
Grimes Cave - 400° -475° F.
Holland Chert - no heating required.
Illinois Dongola (Cobden) Chert - no heating required.
Indiana Hornstone - no heating required.
Jackson County Chert - no heating required.
Jasper - 350° F.
Kanahwa Black Chert - no heating required.
Kaolin Chert - 600° -700° F.
Kay County Chert - 500° -600° F.
Kentucky Hornstone - no heating required.
Knife River Chalcedony - no heating required.
Mexican Agate - 500° F.
Montana Agate - 550° F.
Mozarkite - 550° F.
Normanskill Chert – no heating required.
Novaculite – 775° -900° F.
Obsidian - no heating required.
Onandaga Chert - no heating required for higher grade pieces.
Pedernales Flint - 400° -450° F.
Peoria - 550° -650° F.
Petrified Wood - 300° -480° F.
Sunset Jasper - 450° -475° F.
Tequovas - 425° F.
Uvalde Cobbles - 425° -500° F.
Zaleski Chert - no heating required.