Bruce Girrell on fri 29 mar 02
The reason that I undertook this little project is that we were looking for
something to help us improve the surface quality of our pots while
burnishing. Of course, terra sig immediately comes to mind, but 1) the
process seems terribly wasteful to me, 2) commercially available terra sig
did not work well for us*, and 3) since we use a high kaolin (I'm trying to
avoid using the word "porcelain") clay body with its inherently larger
particle size, the yield from the terra sig process would be even worse.
I was looking for some way that I could improve the results from the normal
terra sig process or else replace that process with another. I chose the
latter. I felt that if I could mill clay to a sufficiently small particle
size then I would get what I needed without requiring many bags of clay to
I removed the clay that always adheres to the hopper lid of our pugmill, a
chunk about 8" square and about one inch thick. When I started this I was
not thinking in terms of a formal experiment, so I didn't weigh the clay,
but it was in the range of over a pound, but less than two pounds - in any
case, it was a lot less than a 50 lb bag.
I put about a quart of water into a blender, tore the clay into small chunks
and added them to the water with the blender set on high. I stopped when the
stuff in the blender reached a consistency that I would describe a thick or
heavy cream. I poured the clay into a rock tumbler loaded with two pounds of
glass marbles. Most marbles were about 1/2 inch dia, but some were about 1
inch dia. The combined slip and marbles almost completely filled the rock
To test the particle size, I sucked up 2 cc of the slip in a 3 cc syringe
and put it in a 25 ml graduate. I then added 20 cc of water (10:1
water:slip), covered the opening, and shook the graduate. After an hour of
settling there was a distinct water/clay interface at about the 6 ml mark
(mostly water, with 6 ml of slip).
When the ball mill was started its operation was noticeably quiet. This was
not surprising, considering the viscosity of the slip and how full the
container was. After a day of ball milling I did the settling test again
with almost the same results. After an hour of settling there was a distinct
water/clay interface, though this time it was closer to the 10 ml mark.
After three days of milling the sound from the rock tumbler was much louder.
I opened the container and found that the slip had dramatically decreased in
viscosity. This was opposite of what I had expected and I wrote to the list
about that. The responses that I got suggested that my clay had become
deflocculated from either the cations of the feldspar in the clay or else
from the breakdown of the glass in the marbles. Since terra sig is typically
deflocculated I did not consider this to be a problem and perhaps it was a
lucky bonus. This time the settling took longer. After one hour there was a
small water layer occupying only about the top two ml of the graduate. After
a day of settling, the water/clay interface was still indistinct. In fact,
there appeared to be a third layer - only about 1 ml - between the two.
I continued milling the mixture. After a week of milling I performed the
settling test again and even after a day of settling the water layer still
was cloudy. I could not determine where the water/clay interface was, though
when I poured off the liquid, there was a definite clay layer, perhaps 1 ml,
stuck to the bottom of the graduate.
At this point I tried some of the milled material on a pot. It burnished
beautifully (and, yes, I mean burnished, not polished - I used a smooth
rock). Because there was obviously still a lot of material that would settle
out, I continued milling for another week.
After two weeks of ball milling the settling test showed no settling after
an hour. After a day, it was still difficult to see if there was any water
layer. After three days of settling, I could discern a water layer, but it
was very cloudy. I poured the contents of the rock tumbler into a large
(about 14" square) Tupperware container, removed the marbles, covered the
container and let it sit for a day. After the day of settling I poured off
the liquid portion of the container. There was a layer perhaps 1/4" thick of
clay at the bottom of the container. This clay was very fine to the touch,
but I disposed of it anyway. I estimate that about 1/2 pound of the clay had
I obtained about 1 1/2 quarts of liquid material. The liquid that I poured
off simply does not settle. Apparently the particles are fine enough to be
held in suspension by Brownian motion. The results on the pots has been
great so far. A pot that would have taken me about three hours to burnish
took about one hour. I assume that I am actually doing a combination of
burnishing and polishing and that the polishing of the fine material is
covering the tiny gaps that normally would require additional burnishing to
smooth out. The surface of a properly prepared pot is like glass. I did one
pot that had been sponged to remove trimming/sanding marks, but had not been
smoothed after sponging and it did not turn out nearly as smooth. Proper
surface preparation is still essential.
We have bisqued one batch of pots that have received this treatment and will
do another batch this weekend. There has been no flaking at all (no flaking
during burnishing, either). There is a small reduction in shine but no more
than we see with our traditionally burnished pots.
Examination of the marbles shows that the surface of each marble is frosted,
supporting the theory that deflocculation of the slip occurred as a result
of breakdown of the borosilicate glass.
All in all, I have to count this as a major success.
1) I was able to convert a very small amount of our standard clay body into
a substance that, if it isn't terra sig for semantic reasons, at least for
all practical purposes acts like terra sig.
2) The process dramatically reduces clay waste at the expense of using a
small amount of electricity continuously for two weeks.
3) The resultant substance matches our clay body perfectly since it was
created from the clay body.
4) The product produces a high gloss surface with substantially less
5) It is possible to produce these results even using a high kaolin clay.
The amount of product that we got will probably last us at least a year or
more. However, when I produce the next batch I will be careful to record the
1) Quantities of materials used
2) Results of the settling tests on a more regular time basis
3) Specific gravity data
4) pH data
5) Quantity of final product and of waste product
Also, it should be possible to determine if deflocculation occurs as a
result of feldspar breakdown by charging the ball mill with porcelain balls
instead of marbles.
If any of you have managed to read this far and have suggestions for other
items that I should be aware of, should test, should quantify, etc. please
let me know.
Bruce "solving the mysteries of the universe one at a time" Girrell
*There were two problems:
First, the terra sig did not match the underlying clay color, which is very
white. As a result, brush marks would show in the fired pot. Not lumpy brush
marks, but brush marks as if you had dipped a brush in two different colors
of paint and used it.
The second problem was the consistency. It went on too thick. It may be
that, to increase product volume, the manufacturer took slightly larger clay
particles or maybe I just needed to thin the stuff somewhat. Anyway, it
didn't go on well for us.