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super-refined terra sig

updated tue 30 apr 96


Vince Pitelka on mon 15 apr 96

Russel, Marilyn, and others interested in terra sig -

I got a number of inquiries about my reference to "super-refined terra sig
which gives a high luster in a single coat with a single polishing." Here's
the scoop. I expect that my approach is very similar to that of others who go
for a highly-refined terra sig. Too many of the recipes out there do not
separate out the large particles adequately, and the product is not a true
terra sig. I started researching this when I began doing my "ancient clay"
classes and workshops at U-Mass about ten years ago. First experiments were
from the standard recipes in books and CM, and the results were unsatisfactory.
I wanted what I had seen on ancient Greek and Roman pots. Finally I discovered
the work of a German ceramic chemist named Schumann, who researched terra sigs
and discovered the real secrets, which had been lost for about 1600 years.
Schumann's info guided me to the current system I use. When there was a
discussion of terra sigs recently on Clayart I seem to recall seeing similar
information, so others may have gotten it from Parmalee as I did.

I usually use redart or ball clay as a starting point. I start with a small
amount of hot water, and into it I dissolve .25% (1/4 of 1%) soda ash AND .25%
sodium silicate (it seems to work better than either one by itself), based on
the dry weight of the clay I am going to make into terra sig. I then add the
clay, and add water, blending with a jiffy-mixer, until the mixture is very
thin, checking with the hydrometer until the reading is 1.2 for the redart slip
or 1.15 for the ball clay slip. I place the bucket with this mixture up on a
table (for reasons which will become evident) and leave it undisturbed for
exactly 20 hours. After the 20 hours I siphon off the uppermost, thinnest
liquid with a winemakers siphon - this tool is ideal for the task, because it
has a length of flexible clear plastic hose connected to a stiff clear plastic
length about 20" long, with a small "cap" on the end so that the tip of the
siphon sucks from above rather than below. This makes it much easier to tell
when you begin to get to thicker material, which is absolutely critical.

The 20-hour settling time may seem arbitrary, but it's not. There are several
forces at work in this deflocculated mix. The deflocculant introduces same
electrical charges, which causes the particles to repel one another and stay in
suspension longer. Also, there is the ever-present atomic vibration which
causes particles in liquids to naturally disperse. Working against these
forces is gravity, causing particles to settle out. At 20 hours, gravity has
caused all the heavier particles have settled out, while the finest particles
are still in suspension, due to atomic vibration and deflocculation.

I start the siphon with the tip just barely immersed in the settled mix. DO
place it up on a table BEFORE the 20-hour settling period. I slowly feed the
tip of the siphon down into the mix as the thin liquid is siphoned off. I keep
the tip so close to the surface that it periodically will suck a small amount
of air. If it sucks too much the siphoning action will stop, but having it
suck a little bit of air is critical, because it gives a good indication of how
thick the liquid is. As soon as you get to thicker liquid the siphon will
begin to suck much more air. As soon as this hapens, STOP SIPHONING. Resist
the temptation to keep siphoning, because the product will be inferior. I have
never tried to do anything with what remains in the bucket, which is MOST of
what I started out with.

You can siphon into any container, and the siphoned liquid will of course be
far thinner than the original specific gravity, and will be unuseable, so the
question then is how to concentrate it. I am the proud owner of a 36"-diameter
restaurant wok, which I bought at a flea market for one buck. Using this as a
slump-mold, I made 24"-diameter terracotta evaporating dishes with a raised 2"
edge. I place one of these on a chair and siphon the terra sig directly into
it. The water soaks into the terracotta (any bisque-fired claybody would work)
and evaporates from the back and from the rim. It takes about a week for the
terra sig to get back to a useable specific gravity (dependent on temperature,
humidity, and air movement). Don't cover the evaporating dish. Anything that
settles into it out of the air won't do it any harm. Don't worry if it seems
to be solidifying around the edges. When it has thickened considerably, scrape
the solidified stuff loose with a clean rubber scraper, and agitate the mix
with a whisk. If necessary, work the lumps against the bottom of the dish with
a very clean sponge to bring all the terra sig back into suspension. Decant
some into a tall container and check the s.g. If it is still thin let it
evaporate some more. If it is thicker put it in an appropriate container and
add water.

I originally started using the resulting product at a specific gravity of 1.2
or even higher, but ran into trouble with it peeling and chipping. Now I thin
it to 1.13 to 1.18 for both the redart and the ball clay, depending on the
results I want. When I said I use one coat that was a bit misleading. I apply
to bone dry clay, often sanded. I use a wide soft brush, and I simply brush on
repeated strokes until I get an opaque buildup (still extremely thin). As soon
as the surface liquid has soaked in, I polish with a soft piece of flannel. I
usually get a glassy shine in one polish. It's magical. The amount of terra
sig I brush on depends on how opaque I want the coat to be, and how much I want
it to smooth out the texture of the clay. It is possible to get a very high
shine with an almost transparent coat, because the shine results from the clay
platelates laying flat on the surface, and in this refined terra sig the
particles are so fine that a distribution of them over the surface will give a
good shine and yet still allow the clay beneath to show through. A good terra
sig may be the world's most perfect substance.

Expect to use a LOT OF CLAY to get a good terra sig, but the results will be
worth it. To get a gallon of redart terra sig takes about 75 pounds of redart
clay. Ball clay gives a higher yield, since it is finer to begin with. As I
mentioned above, I have never tried to do anything with the deflocculated
residue left from the settling process. It would be good for making thick
slip. If you add it to a claybody you would be deflocculating the clay, which
will reduce plasticity.

I have fired all my terra sigs to a maximum of ^02. I have applied very thin
coats to bisqueware and fired them with adequate results, but never as good a
shine or as durable a surface as when applied to bone dry. Terra sig applied
to leather hard tends to loose it's shine when it dries. At ^04 the redart sig
gives a bright brick-red-orange color, and the ball clay gives a PURE white.
The redart sig, when properly made, is denser, and in a blackware bonfire gives
beautiful brown-to-black colors. The ball clay sig in the blackware firing
gives intense jet-black.

I have never fired sigs to stoneware temperatures. If you get around to it
before I do, let me know what happens. Of course, a redart sig would turn to a
glaze, but more refractory clays might give great results. Ball clay may be
the best bet. Kaolins and fire clays give extremely low yield in terra sig,
because of the coarseness of the particle size. Stoneware clays give better
results, depending on the fraction of fine particles. Experiment away.

I always like to work with pure clay terra sigs, because they give the best
shine. It is of course natural that others will want more color, but unless
you can ball-mill the mixture the shine will be reduced. At U-Mass we
experimented with both oxides and mason stains and got good results by
ball-milling the thickened evaporated terra sig and colorants for a day or so.

Good luck, email me any questions, and please let me know of your results,
either on Clayart or via email.
- Vince
Vince Pitelka -
Appalachian Center for Crafts - Tennessee Technological University
Smithville, TN on tue 16 apr 96

Vince and all you sig users out there-
I too have played around with the terrasig issue a bit and since most
of the work I was doing at the time was high temp it only made sense top
(to) fire it as hot as I could. Results... both ball clay sigs and EPK
sigs all turned to a lusterless white slip at c/10. (Vince, I doubt you
remember me syphoning off some of your ball clay for this test... but hey
it was just sitting there.)
Never did try it at c/6 though. I also wonder about making the sig more
refractory so that less sintering will occur at higher temps. Anyone else
out there tried this at all?

Alex Solla

Louis Howard Katz on wed 17 apr 96

I think that you will find that the shine of sigs will disappear well
before cone 10 with any clay. This does not mean that sigs have no uses
in medium and high temp kilns. Ultrafine clays do coo things in solid
fuel and vapor kilns. Very watery sigs are nice to dip pots in to
accentuate texture. Wax resist decoratioon with sigs at cone ten in
residual soda can be very striking.

*Louis Katz *
*Texas A&M University Corpus Christi *
*6300 Ocean Drive, Art Department *
*Corpus Christi, Tx 78412 *
*Phone (512) 994-5987 *