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terra sig in wood & salt fire, was re: terra sig + raku = pop-offs??

updated wed 4 aug 04


Geralyn W. Barry on tue 3 aug 04

I have experimented with both burnishing and using terra sig on pots in
both woodfirings (cone 11+) and cone 10 gas reduction salt firings (with
both small and heavy amounts of salt - NaCl). I have noticed that just
having a smooth surface encourages interesting effects in atmospheric
firings, and you do end up with a "smoother" pot in most cases, even though
it does not have the same shiny quality of such a surface fired to a lower

In fact, one all-time favorite pot (of my own pots) is a very plain 4-sided
handbuilt "plant pot" I made from Georgie's (an Oregon-based clay company)
"Trail Mix". It is rated by them as a cone 4-7 clay, but handles
temperatures up to cone 10-11 just fine (supposedly because of some
fireclays in the body). I like to work with it, but I think it can be
really ugly and rough and gray in a woodfiring, so I sometimes apply terra
sig made from OM4 ball clay by either spraying (on this particular pot) or
brushing onto bone-dry greenware. I then just lightly hand-burnish (with my
fingers and palm).

This particular pot was fired for 3 days in an anagama kiln. The pot was
positioned with one of the 4 "corners" facing into the path of the flame at
a slight angle. One side is all dull and ash-covered with small-scale
crinkles but still smooth (giving the appearance of mottled gray-green
lichen growing on rock). The other side that faced the flame is similar,
but the back edge of that side shows traces of a buttery-smooth shiny red
where the surface was more protected from the ash by one of the "bulges" in
the center of each of the 4 sides. Those two front sides (facing the flame)
are nice, but the other two sides are my favorites - with more of that
smooth, rich red than on the front, but still with some of the same ash
effects on the "bulges". The red varies on the pot from looking like smooth
red-brown leather to actually quite shiny, depending on the side. I'm
pretty sure that red is from the iron in the OM4. And there is also a
wonderful pattern of blushing and ash inside - don't think I put anything
inside but maybe a very light application of shino... maybe nothing. I'd
have to check my notes on that. If I find a photo of this pot, I'll put it

In a cone 10-11 residual (or lower amount of NaCl) salt firing, burnishing
gives a very smooth surface that I think enhances the blushing that can
occur with the right claybody and a small amount of salt. Much of my work
is burnished "dirty" porcelain (with enough iron content to blush nicely).

With burnishing, you still get the color reponse of your underlying clay
body if you leave it bare in an atmospheric firing. If I have a clay body
that I like working with but don't like how it reacts in a wood or salt
firing (usually a rougher clay with more iron - a clay that I think can
look kind of dead in those firings), then I will apply slips or terra sig
and see what that does to the surface. Those treatments can encourage more
variation in the response of the surface to the atmosphere in the kiln.

After this Bourry box-wood/salt kiln I'm working on is built, I hope to get
back to expermenting more with surfaces using terra sig and slips. I think
there have been a couple of articles (at least one) on using terra sig and
other slips in the woodfire - maybe one in Ceramics Technical several years
ago - can't lay my hands on it at the moment.

Geralyn Wood Barry in western Oregon
almost ready to start building "the kiln" - maybe this weekend!

At 12:30 AM 8/3/04, you wrote:
>actually, i'm very curious about reaction of ash
>on smoothed surface. i'm also ignorant how terra-sig
>or burnished surface reacts to cone 11 "yakishime"
>firing. it won't stay shiny but does it remain smooth
>and how fire color behaves differently, if it does,
>from rough shigaraki-like clay surface? as a matter
>of fact, i wanted to experiment in anagama firing
>this week but alas, i lost my only one terra-sig/
>burnished test piece during shipment.