Joss Research: Ceramics: Glazes: Red Temmoku

“Red Temmoku”

(11 October, 2004, with a followup)

Here is a Red Temmoku bowl, fired to cone 10r. The bowl is about
6 across. It is made of Loafer’s Glory, from
Highwater Clays;
the glaze consists of Brick Clay from western Wisconsin;
Wood Ash (Oak, unwashed); and Red Iron Oxide (84% purity).
I strongly suspect that the yellow teadusty or
“corn-pollen” sprinkles, which show up
particularly well on the interior, result from the ~2.6%
MgO content of the Brick Clay; I get essentially the
same effect in other iron-rich glazes if I add enough
Mg. (Oddly, although all or nearly all of the ancient
Chinese high-iron Jianware glazes contain noticeable
MgO, very few modern Western “Temmoku”
glazes seem to use magnesium. Go figure.)





(Click any of the small photos to get one that is
1280×960 px. If you want something even larger, you can
change the “.10c.” in the filename of the
large image to “.22c.” for the full-size
originals, 2272×1704; just be aware that those files are
about 1.5 MB each, and may take a while to download if
you are on a slow connection.)

This bowl was donated to a benefit auction in Minneapolis.

Note: The word “Tenmoku” properly
refers to Jian teaware styles of the Song dynasty, not
to glazes. I am using the more modern romanization of
the word, “Temmoku”, to refer to glazes. The
spelling difference is regrettably slight, but it seems
to me that the distinction is important — I have
seen a claim, from someone who failed to understand this
issue, to the effect that there can’t possibly be
any White Tenmoku pottery. This turns out not to be the
case; I have held a White Tenmoku bowl in my hands. It
was grayish, to be sure, but definitely unlike the usual
dark or “Hare’s Fur” or teadust or
oilspot glazes you see on most Jian teaware.





A 21st Century Reprise

(2010.0712)

The history of this glaze is complicated, and (for me,
at least) fraught. When I first started making my own
glazes, I read Robert Tichane’s book about Ash
Glazes and decided that I wanted something like the
typical Jianware glaze, but dark cherry/mahogany red
rather than the yellowish color he described. This is
because Jian teawares are made of a dark stoneware-type
clay, and a yellowish glaze looks really dark on them;
I am using porcelain, and a yellowish glaze is not
anywhere near as interesting, at least to me.

My first attempt at one of these was successful, but as
soon as I tried it again, I failed. This taught me two
important lessons: you have to sift the clay and the
ashes before you weigh them out, because otherwise you
don’t know the actual proportions of the glaze. It
also helps to keep a searchable record. Soon after that
I started using glaze calculation programs.

Some years later, when I made the bowl in the photos
above, I got the red color back again. I fired a few
things, and then I decided to add more glaze to the
bucket. The result was not red. I have been searching
for the key to this for several years now, and every
time I think I have it, I get another brown or opaque
glaze test out of the kiln. This is discouraging and
tiresome, but I have certainly narrowed down the
parameters to some extent. I keep going for two reasons:
first, the fact that I really want this color. Second,
the fact that every once in a while I get a hint of it
on a piece or a test tile. The issue seems to be that
it is tweaky, and that the key parameters (whatever
the turn out to be) must be within a fairly narrow
range of values.

I initially thought that the particular type of Red
Iron Oxide was key, but I have since seen some red
on pieces that used ordinary synthetic high-purity
RIO. (…But see below for more about this.)

I initially thought that a hot reduction firing was key,
but I have some test tiles with red on them that were
fired at cone 9 in the electric kiln, and I have seen
some red on at least one piece that I fired to cone 9o.
(…The key in that sentence is “some red”;
see below for more.)

Recently, I have been digging back through glaze recipes
and my notes on them, and I have discovered an odd
thing: the percentage of brick clay that I use in the
glaze has crept up stealthily over the years. As far as
I can tell, the original was only about 40% clay,
whereas recent tests have been above 70%. This would
seem to be a key factor, except that the glaze on the
bowl above seems to have been made with about 70% clay.

I continue to be puzzled, and I continue to test various
conjectures. Recent tests have been more like a Kaki
(“persimmon” — rich crystalline brown,
rather like the inside of a ‘Hyakume’ or
‘Chocolate’ persimmon), quite opaque, so my
next step will probably be to revert to a 50-50 mix of
clay and ash, which should be pale green and
transparent, and start adding iron oxide to see what
happens to the color.

Wood Ash being the highly variable material that it is,
however, this means not one series of tests but rather
at least 4. I have ash from a wood-fired pizza place,
from Dr. Becca Levin’s fireplace, from a charcoal
grill, and from a Black Locust tree that Edwin Gould
burned. (I also have smaller quantities of ash from
several sources including Fred Paget, and if I
can’t get enough variation from the four I list
above, I can always try some of the others. The problem
is that if I want to mix a lot of the glaze, I need a
fair amount of ash; I am fervently hoping that the ash
is not the key here.)

There is also the obvious possibility that there are
several keys, all of which have to be positioned
accurately. It might help if I had analyses of all of my
materials, but that is rather expensive, so at least for
now I am proceeding on a grossly empirical basis.
Eventually, however, I may have to grit my teeth and
start saving for a battery of XRF runs. (I do have XRF
analyses of the brick clay, performed by Mary Simmons
some years ago, and they are extremely helpful; but
without analyses of the ashes I am still to a large
extent floating around like an untethered balloon.)

I will post more about this as it happens.





Further Update, Mid-2012

I am now beginning to suspect that the red I see in
pieces that come out of cone 9 firings in the electric
kiln may not be precisely the same red I see in pieces
that are fired in reduction. My most recent test results
suggest that high temperature is important — at
least one glaze that comes out red at cone 11+ seems to
be brown at cone 10. (My test kiln fires so quickly,
btw, that cone 10 is about 1305-1307° C, and cone 11
is more like 1320° C or a bit higher.)

I am also drawn to the conclusion that the RIO I use is
important; with ordinary high-purity synthetic RIO I
seem to get a nice rich brown, rather than red.
That’s not 100% certain yet, and I need to do a
little more testing; but either way I think I’m
going to have to get the 84-86% pure material analyzed.

Most of my recent tests, btw, have been with mixtures
that are close to 60% brick clay, 30% wood ash
(unwashed), and 10% RIO. Different ashes are giving me
different degrees of redness, and I will probably have
to get some of those materials analyzed as well.






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Phone: +1 240 604 4495.



Last modified: Sat Jul 14 01:29:50 EDT 2012


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I am a Researcher of the Joss Research Institute. I work primarily on lasers and ceramics, with occasional excursions into other areas.

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