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Rock Recipes for Knapping
the knapping community there are several ways to heat treat stone. The following
is a brief description 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.
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!
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,
Roaster Oven Cooking
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 vermiculite
"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
for 6 hrs then 50 per hr to 500° for 6 hours.
for 8 hrs then 50 per hr to 525° for 5 hours.
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
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.
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.
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.
right at 500°.Some of the more transparent is better off at 450°-480°.
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.
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.
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-specie 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.
the following materials, dry out at 200° for 8 hours, then ramp up 50° per
Kay County 500-600
Edward's Flat 400-550
Gray Boone 650
Mexican Agate 500
Petrified Wood 300-480
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
Brazilian Agate 500-600
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VIRGIL G. RICHARDS