Rock Recipies from The Tarp

(This is slightly modified from a file posted to The Tarp)

In the knapping community there are several ways to heat treat stone. The following is a brief discription of three of these cooking methods: abo techniques, use of a kiln, and use of a conventional roaster oven. Following this brief outline is a listing of various materials, as well as their cook and hold times. Enjoy!

Abo Style

The Abo way is in a pit, burying the spalls, bifaces and flakes under sand or dirt, from several to about six inches deep. A campfire is built. The next day the coals are scraped away from the surface and  dried out soil is removed.  The spalls or bifaces are layered in and the soil is replaced. Then the coals are placed back over and more fuel is added so that the coals remain for up to twelve hours. After a day or two the stone is dug up and checked to see if it has been heat treated to satisfaction. Temperatures range depending on stone thickness. A  good place to start is that a 1" biface buried 1" gets about to 600 - 650 degrees. For each 1/2" deeper temperature drops 50 degrees. Thicker stone needs less heat, thinner stone needs more to reach the same 600 - 650 degree results. Don't discount the use of charcoal. It makes a great abo fire!

Kiln Cooking

Kiln cooking rock is the most predictable method, especially if you have a computerized control device as you have the most control. Ramp times vary depending on material. A general rule of thumb is to dry your rock out at 200° for 8 hours, then ramp-up in 50° increments (spalls and smaller stone) until you reach the desired temperature, then hold for the desired "soak" time. For larger stone reduce this ramp-up and down times as larger stone does not absorb the heat as quickly as smaller sizes. Additionally, there are several varieties of stone that should also be ramped-up a bit slower: Harper, TX tabs, Rootbeer, Knife River, and Mozarkite(20-30° per hour). Frisco, Pedernales amoebas and some of the darker rocks should only be approached at 5° per hour. And it appears that ramp-down times are just as crucial as ramp-up times. In most cases 30° per hour is a good measure. Fuller kilns will ramp down slower than than emptier kilns. Also, one should test their rock to see if it cooks better in whole or slabs. Frisco, Montana agate, and Pedernales amoebas can only be cooked as THIN spalls or as slabs up to about 5/16 inch, while Burlington, Flint Ridge, Mozarkite, Knife River, and Kay County can easily be cooked whole. Care should be taken to asure yourself that there are not any enhydros (water pockets) in your rock as disasterous results can be obtained and you may have to hunt far and wide for your kiln lid or garage door! Doug Kries recommends ramping all rock up 5 degrees per hour and down 30 degrees per hour, except for Burlington, Flint Ridge, and Kay County.

Roaster Oven Cooking

The roaster oven is a good middle ground when it comes to cooking. It is less expensive than a computerized kiln (much!) yet gives more control than a fire pit in your back yard. obviously less expensive than a kiln, but also provides a bit of control. The negative is that some roasters don't reach higher temps. The inner liner should be removed to allow higher temperatures. Many use a sand or vermeculite "bath". This is done by place spalls or slabs upright standing upright, not touching one another. The heat on roasters varies as well, with hotter temps being generated in the corners and bottom
of the pan.

Rock Recipes

Kay County Flint (Kay County, OK)

250° for 6 hrs then 50 per hr to 500° for 6 hours.

Mook Jasper (Mook Station, Australia)

250° for 8 hrs then 50 per hr to 525° for 5 hours.

Kaolin (Union Co. Ill.)

250°F for 2 hrs then 50°F/Hr to 350°F for higher grades, up to 600°F for coarser grades. Hold at peak 6 hrs

Knife River Flint (ND)

I have had success with flakes and THIN spalls with 25°F/Hr to 350°F, when 350°F is reached, bring down temp. 25°F/Hr. Lower grades cook better than high grade.

Brazilian Agates

A maximum of 450°F should do fine for the Moss agate. 50°F per hour/holding at 450° for an hour or two. Don't overdue it! Try going to 400° as a test first.

Montana Agate

I tried cooking it first in the nodule. Many fine cracks occurred. I cook it in slabs at 500°F. Works like obsidian and very glassy.

India Agate (bloodstone)

Treat right at 500°.Some of the more transparent is better off at 450°-480°.

Flint Ridge Chalcadony

The best grades can be worked raw. The light blue varieties are a bit tougher and require 200° for 2 hours then bring up the temp 100 degrees an hour till it reaches 480 degrees. Hold for two hours, shut down and allow it to cool. Tougher varieties can be taken up to 500° for 36 to 48 hrs.

Flint River Flint

Heat at 250 degrees for 2-3 hours.Increase heat 50 degrees per hour until top temp of 375 -450°. Soak at top temp for 8 hours. Turn the device off after the soak time and let cool.

Sunset Jasper (Richardson Ranch, Oregon)

Heat-treat at 400-425° (it gets brittle at 450+°). Don't go over 500. I have found that it's not necessary to soak it, just bring it up slow and shut the kiln down. The yellow stone will be orange to dark red, but this beauty is only skin deep. Some sunset has whites to purplish colors in it.There is a sub-spiece of sunset that is yellow and appears to be coarser in grain, though it heat-treats fine. The main difference is that the color will penetrate all the way through. 400 = yellow, 425 = orange, 450-475 = dark red, 500 = KABOOM.

Other Materials

On the following materials, dry out at 200° for 8 hours,then ramp up 50° per hour
temp indicated. Hold for 8 hours. Ramp back down at 30-50° per hour.
  • Burlington 600-650
  • Mozarkite 550
  • Kay Cnty 500-600
  • Ft. Hood 400-550
  • Edward's Plat 400-550
  • Coral 425-650
  • Novaculate 775-900
  • Tequovas 425
  • Alibates 425-500
  • Montana Agate 550
  • Chalcedony 300-350
  • Gray Boone 650
  • Peoria 550-650
  • Mexcan Agate 500
  • Jaspers 350
  • Petrified Wood 300-480
  • Georgetown 425-450
  • Grimes Grave 400-475
  • Bulls Eye 450-plus
  • Coastal Plains 400-450
  • Flint Ridge 500-600
  • Sunset Jasper 450-475
  • Castroville/Uvalde cobbles 425 whole, 500 slabs
  • Pedernales 400-450
  • Brazilian Agate 500-600
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