William Lucius on mon 28 oct 02
No fair! I go away for a trip to Italy and you start discussions about
things that need my input. And by the time I get back you have moved on to
other subjects. But here is some information about B/W pottery firing
anyway, just in case anyone is still interested.
Basically there are two routes to black-on-white earthenware pottery such as
that made by the prehistoric and early historic pueblo potters. Both
require the use of boiled sticky plant sugars (bee weed being my personal
favorite) in the paint. In some areas where potters had access to smectite
(not kaolin or kaolinite!) body or slip clays capable of absorbing carbon,
the organic (vegetal) paint was painted directly on the vessel. In other
areas the organic paint was mixed with iron oxide of variable purity and
painted on the vessel (I have refired thousands of black-on-white pottery
sherds and have yet to record the use of manganese dioxide in the paint,
whereas manganese is the oxide of choice for the production of black-on-red
pottery).
Regardless of the paint type, the firing procedure is the same. After a
period of preheating around the firing pit, the pots are place on sandstone
slabs that serve as kiln furniture and covered with a relatively open
cribbing or tent of wood fuel which when ignited results in the vessels
reaching orange heat quite rapidly (the entire firing may be completed
within 45 minutes). During this time the atmosphere in the kiln changes
from strongly oxidizing to neutral. After the peak temperature has been
achieved (+ or - 850 degrees C) and much of the fuel has turned to ash or
coals, the entire pit is covered with a layer of dry soil, effectively
stopping the firing. The plant sugars in the surface of the paint have
turned to ash, and the iron oxide or organic paint underneath the ash are
strongly reduced, resulting in a black paint color. If available, light to
white firing body clays are used, which due to the neutral firing atmosphere
maintain a light gray to white color. If iron rich clays are used, a white
slip is applied prior to painting and firing. While still porous, the
vessel tends to be well fired and will ring when struck due to incipient
vitrification of the clay platelets.
If you want first hand experience in this type of firing, I would echo Tim's
invitation to attend the firing at Leupp. I hope to be there with my own
pots, and I am ever willing to talk about pottery, especially from the
perspective of ceramic technology.
William A. Lucius, Board President
Institute for Archaeological Ceramic Research
845 Hartford Drive
Boulder, CO 80305
iacr@msn.com
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