tammy brown on tue 3 aug 04
Hi all...
Can anyone tell me why I got these results?
I dipped a large sculpture in a clear matte glaze and fired this buff stoneware clay body to cone05 at a medium speed. For this sculpture the results are acceptable but I got shiny spots and dull spots. I am thinking that has something to do with the speed of friing, the speed of cooling or maybe even the thickness of application. It is a Mayco commercially prepared glaze that I diluted 10%.
Can anyone offer suggestions as to why this occurred?
Thank you
Tammy in Columbus
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Snail Scott on tue 3 aug 04
At 05:25 AM 8/3/04 -0700, you wrote:
>Hi all...
>Can anyone tell me why I got these results?
>I dipped a large sculpture in a clear matte glaze and fired this buff
stoneware clay body to cone05 at a medium speed. For this sculpture the
results are acceptable but I got shiny spots and dull spots.
How even is the thickness of the piece? And how large is
'large'? And how fast is 'medium speed'?
It may be that the shiny spots on your matte glaze
were too thin, since I sometimes find that thickness can
improve matteness in matte glazes. More likely, though,
is that the matte spots are the thin areas. Possibly the
whole piece didn't spend enough time in the firing to
really develop the matte surface. So, in the areas where
the glaze was thin, the glaze may have partly soaked into
the porous underfired stoneware. This is common with
brushed glazes, when it's hard to tell how many coats
you've applied. Since you dipped, the discrepancy may be
due instead to variations in the thickness of the clay,
since thicker areas will bemore absorbent. Or, the glaze
may have puddled a bit while still wet, creating thick
and thinspots that way. (The 10% thinning should be fine,
assuming it was a standard 'brushing-consistency' glaze
to start with.)
Can you tell which areas have the thicker glaze coat, if
there is any difference?
Also, some matte glazes aren't really 'true mattes', i.e.
microcrystalline, but are simply underfired recipes that
would be glossy at higher temperatures. If this is one
such, perhaps the shiny areas are thinner areas that
reached a higher temperature in the firing. This seems
unlikely, as I generally find that an equal glaze layer
will mature just about the same unless the thickness
discrepancy of the clay is really huge.
Also, what do you mean by 'shiny' and 'dull'? Most true
matte glazes are not matte like flat housepaint, but
merely matte compared with glossier glazes. They still
have some sheen, unless they're truly underfired. So,
are the dull places dull like a bare slip (or flat
paint), or dull like a properly fired matte glaze (sort
of satiny)? And are the shiny parts shiny like a gloss
glaze (like glass) or like a matured matte glaze? Have
you used this glaze on this clay before, to have a
basis for comparison; i.e. do any of the areas look
like 'normal'?
-Snail Scott
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