Ray Aldridge on wed 23 jun 99
At 09:16 AM 6/21/99 EDT, you wrote:
>----------------------------Original message----------------------------
>Do not, under any circumstances, do the dumb thing I did, i.e. mix a 5
>gallon bucket (20 lbs of glaze materials) based on a l00 gm test. The
>beautiful fleshy pink w/gold stripes glaze I got at l00 gms turned into a
>glassy pool on my kiln shelf and was a total loss of glaze materials, time,
>and hopes. I learned that lesson but good!
>
This is why, even on the simple and reliable glazes I use, and even when
making up a long-in-use standard glaze, I test a new bucket before I use it
on pots. If it works like it's supposed to, I screen the dregs of the old
bucket into the new bucket and carry on.
If you do get a disastrous bucket, the reasons can be several. It might be
that it just doesn't behave the same as it did on the test tile. But this
can be addressed by using test tiles that are as much like your actual pots
as possible. In my case, I use stencils and slip with sgraffito. My tiles
are thrown as an open based ring and cut into segments that have an
L-shaped cross section. I apply a small stencil, dip in the slip, and
strike a few lines through the slip. The only caveat with this approach is
that if you're working with a fluid glaze, it may move more on a tall form
than on a relatively short test tile, and forms which tend to collect a
roll of fluid glaze, like wide bowls, may also not be adequately addressed
by a simple upright test tile.
Thickness counts for a lot in glaze results. Not only should you test at
the specific gravity you'll use for your ware, you should make your test
tiles the approximate thickness of the ware you'll be glazing, because a
thick cross section will suck up a lot more glaze than a thin, easily
saturated bisque. Also, the test tiles should be fired to the same
work-heat and schedule as the ware-- this is why I recently acquired a
small electric kiln for testing-- one that had a firing profile similar to
my larger kiln. I'd always used one of those cheapie on-or-off tiny
testing kilns to check my glaze recipes, which worked okay as far as
crudely testing glazes for excessive fluidity and so forth- catching big
errors in measurement. But it didn't work well when trying to work out
major changes in glaze technology, because glazes would still have to be
put through the big kiln before I'd know whether or not they'd really be
usable.
Another reason for a disastrous bucket might be that simple error in
measuring out the ingredients. You can sometimes discover what went wrong,
by a combination of observation and testing. If the glaze is too fluid,
perhaps you added a double portion of fluxing oxides. Or perhaps you
neglected to add the clay portion. (Missing plastic clay will show up in a
glaze slop that keeps settling the instant you stop stirring, for example.)
Or it may not be that simple-- you might have dipped the silica component
out of the feldspar sack. If you think you might know what went wrong, you
can test modifications, comparing them to the original test that you liked.
In any case, even if you can't figure out exactly where you went wrong, you
may be able to rescue a glaze bucket by modification. If the color of the
glaze is fairly inoffensive, you might be able to convert it to an
acceptable liner glaze just by adding a little of this or that. The glaze
mentioned in the original post might just need a little kaolin to keep it
on the ware. It might be worth testing a small sample to see if it could
be rescued without too much trouble.
I do a fair amount of this sort of thing. For example, I recently made up
a half bucket of a blue green glaze that relied on stains for color. After
I tested it on a full-sized pot, I decided the color was just a little too
loud for my taste, so I made up another half bucket of glaze without the
colorants and added the two together. I got a very pretty pale celadonish
color that I like a lot on porcelain.
Of course, sometimes you just have to accept the loss of the time and
materials. If you're firing fairly high, the materials generally aren't
very expensive.
Ray
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