Stephani Stephenson on sun 15 apr 01
For the person who wanted to hear about single firing:
I have been single firing for 2 years and here are some observations:
I will add about 3 % bentonite to a glaze to help keep the glaze in
suspension in the the glaze slurry/tub.
A glaze that settles will cause problems in single firing as much or
more so than with bisque glazing, because you can't go rinse off a piece
, re-dry it and start over. So the glaze in the tub has to be consistent
throughout, top to bottom. Bentonite really helps, as do other
suspension agents.
An added benefit of Bentonite is that it increases the shrinkage of the
glaze by a bit, which usually improves the glaze fit, as it allows the
glaze to shrink at a compatible rate with the clay.
If your glaze already has a good percentage of clay in it, likely you
will not need to add any bentonite, as the clay itself will act as a
suspension agent and will increase shrinkage of glaze. A starting point
is to observe how your own glaze acts in the slurry, in the
dry-pre-fired stage and when fired.
It seems to me that bentonite improves the flow of the glaze as it is
applied to the pot. In non technical terms it makes it 'silkier 'and
also
seems to improve the physical durability of the dried unfired glaze
surface. It also helps the glaze adhere to the clay body.
I have never had a problem with glazes fitting over each other, but I
tend to use a family of glazes . But the problem of glazes peeling or
flaking off from each other is no different than when using glazes on
bisque ware. Using a base glaze with variations works well in this case.
Many of the glazes we use in single firing have a clay component of
10-18% anyway.
Because we low fire , the rest of the glaze usually contains a good
percentage of frit and other ingredients which are not plastic .
So adaptations to the glaze formula usually have to do with adjusting
the glaze for good suspension in the slurry stage.
Typically, as with regular glazing, you will check your fired product
for melt, fit, crazing, or shivering, and then decide whether to further
adapt that glaze or move on to another.
I would suggest that you try your glazes as is, before worrying too much
about how to adjust them for single firing. you will likely find that
most glazes work quite well as is. I really have not had much problem
using glazes designed for bisque on raw ware.
One reason for this, from my own observation, may be that I start off
with a thicker glaze slurry, i.e. less water is added to the glaze
slurry.
Not drastically thicker, but somewhat thicker.
Because less water is in the batch, less water is absorbed into the raw
ware (good for minimizing the weakening effect of water on raw ware.)
and less water evaporates from the glaze coat as it dries, lessening the
shrinkage of the glaze at this stage of the game. So you have few
problems with the glaze flaking off. This at least gets the glaze off to
a good start as it enters the firing. ( we glaze when ware is bone
dry)
It seems to me that the ingredients, (clay, with or without bentonite)
that improve glaze behavior during these stages also help improve
fit during the firing itself.
A quick overview of different products involved is as follows:
Macaloid : a suspension agent, similar to a a 'clean, white' type of
bentonite. It is a approx. $7.93 per lb. in U.S.
Vee gum-T: a macaloid type bentonite, also used as a surface hardener.
It is approx. $7.01/ lb., U.S.
Vee gum-ceramic: a blend of Veegum and CMC. don't have a current price
CMC (carboxyl methyl cellulose) : an organic cellulose gum which
functions as a thickener, binder and suspending agent, $8.04 per lb,
U.S.
Bentonite :(325 mesh recommended for glazes) is $1.26 lb/ U.S.
(prices are for one pound quantities)
I sometimes use CMC as a binder for coloring stains and oxides, but I
have stuck with bentonite as a glaze additive because it has worked,
and I am familiar with its behavior in the slurry, unfired and fired
stages. Also it is a very affordable material! It would be good to hear
what percentages of the other products people use.
Before I started single firing I thought it was a mysterious finicky
process which would only work if everything was perfect within a very
narrow range! Probably because during my education I never saw or
practiced single firing, except in pit , saggar firing, wood or salt
fire : situations involving un glazed ware (or ware with Terra sig,
etc. )..I met one person who single fired but I think he liked to
maintain the mystique. But really , there is no more or mystique to it
then regular glazing. (perhaps I should also say, no less mystique as
well!)
Try it and first observe how your standard glazes work.
I should mention that we single fire tile . We also single fire basins,
fountain spitters, sculpture and large architectural trim and the like.
The larger pieces we fire are thicker than the typical thin walled
thrown pots, so the technique may well vary with those who single fire
thinner ware.
I still bisque fire certain forms that are thinner walled or for one
reason or another, difficult to raw glaze. Also I still bisque pieces
that I want to stain
and rub off, for textural or aesthetic reasons.
Walked down to the beach at sunrise this Easter morning. What a
beautiful day.
best to all
Stephani Stephenson
Alchemie Studio
Leucadia CA
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