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colors, pitfire, unglazed, terra sig, & clayart overload

updated mon 31 mar 97


Susan Benson on tue 11 mar 97

Dear Amazing Clayarters,

I want to thank you for the best part of my morning routine---YOU! I am
suffering from information/inspiration overload, and loving every minute of
it, but now I need to ask several questions.
The threads on pit firing and terra sig. have really made me want to try
them out... (probably simultaneously, but not necessarily) I live less than
1/2 mile from a public beach with good sized fire rings: Concrete, approx. 3
1/2 ft. diameter, 18 in. deep. Is this a suitable size/container for a pit
firing? City ordinances allow fires in the fire pits up til 10pm, when the
beach officially closes.
I am still very new to clay--less than 1 year, and so these next
questions may be very basic, if they are, please be patient. Since pit
firing is a low temperature process (relatively), I know that the clay will
never come close to vitrification, but would a claybody formulated to vitrify
at ^5 become any harder /more durable than a ^10 claybody when fired this
way? Or would terra cotta be a better way to go?
Also, Aardvark Pottery Supply in Santa Ana CA has a ^5 claybody called
Cassius Baltic, which is ebony black, and very smooth, looks gorgeous in the
examples I've seen, both unglazed and w/clear. I'd love to try this one w/
terra sig., prepared in the way Vince described, but I have also been
following the manganese thread, and this body contains it. Would wearing
gloves and being super careful to wet-clean every surface before the stuff
could dry and become airborn be good enough precautions? The kiln has an
excellent venting system. I have put off buying the clay, because this
thread has me pretty concerned. Please let me know your thoughts, I learn
so much from you every day.
Thank you!
near Oceanside, CA where it's getting warmer, & the flowers and my
sinuses are both blooming. Spring hath Sprung.

Teresa Murphy( on wed 12 mar 97

Hello to all!
I've been in the clayart circuit for a few days now and am thoroughly
enjoying it.
This question is for Susan who mentioned a ^5 claybody called cassius baltic
Ardvaark Pottery Supply in Santa Ana, CA. Could you share the mailing
address and/or phone number of this place? I am interested in attaining some
of this particular clay. Thanks in advance!

Vince Pitelka on wed 12 mar 97

Susan -
The best bodies for bonfiring/pitfiring/sawdust-smoking are porous and open.
Tribal potters often use as much as 50% tempering materials (sand, grog,
crushed seashells, volcanic ash, etc.) in their clay to open it up. A
terracotta sculpture body will probably work fine. Fine-grain, gritless
claybodies are the least likely to survive, but if one manages the fire
carefully they can survive.

In most situations, vitrification is counter to thermal-shock-resistance.
You do not want a bonfiring body to be vitrified, for the reasons explained
above. My favorite body for bonfiring is also my favorite raku body and is
also an excellent high-fire sculpture body - equal parts Cedar Heights
goldart clay, ball clay, fire clay, and fine grog. As indicated by an
earlier post today, don't risk bonfiring greenware. We prebisque all the
work to ^018, and I transfer them directly from warm kiln to bonfire setup.
We then do the bonfire in less than an hour.

It is not clear whether you want to make terra sig from the basalt clay, or
use terra sig on it. If you want to make terra sig from the clay you may
run into some complications. Normally, when adding oxides or mason stains
to prepared terra sig, it is best to ball mill the mixture to reduce the
colorants to a finer particle size, otherwise you are just introducing
coarse particles which to some degree negates all the trouble you went to to
make a proper terra sig in the first place. In this case, it is doubtful
that the colorants in the basalt clay have been ball-milled, so if you use
my proceedure for making terra sig, most of the colorants will remain in the
sediment when you decant off the terra sig.

I have heard many people comment favorably on the Cassius basalt clay, and I
expect it would be fun to experiment with it (aside from the necessary
precautions for manganese toxicity). But keep in mind that many
commercially-made basalt clays are intended primarily to give interesting
color in oxidation firing, and if they are reduced they can flux so badly as
to bloat and slump (because of very high iron content).

Regarding the manganese toxicity, in normal use, raw terra sig does not
normally come in contact with your skin that much, so I see no problem with
manganese as a colorant in the sig. The primary problems with manganese
toxicity are in wedging/handling clays containing manganese, and in the
glaze lab.
- Vince

Vince Pitelka -
Phone - home 615/597-5376, work 615/597-6801
Appalachian Center for Crafts
1560 Craft Center Drive, Smithville TN 37166