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Lesson Plans

Re: glaze question

[ Thread ][ Subject ][ Author ][ Date ]
Robert Alexander Fromme (rfromme)
Sun, 10 Oct 1999 21:19:45 -0500

Date: Sun, 10 Oct 1999 21:14:22 -0500

>At 03:45 PM 10/10/99 PDT, you wrote:
>When a student applies too much glaze is obviously melts to the bottom. I
>will then refire the piece upside down so the glaze melts towards the top.
>Depending on the shape, if some excess glaze was removed, etc. it will even
>>out without all the filing.

With each firing, the flux in the glaze eats deeper into the clay body of
the form. The additonal duration and the added cooling and heating also
effects the amount of quartz and christobalite which develops in the piece.

Here are some things to think about.

There are three parts to any glaze, the acid, the base and the neutral (The
RO2, the RO-R2O, the R2O3 ).. The acid is the glass former and in most
cases that is silica (or in some cases at low temperatures it is boron).
The base is the flux which makes the glaxe melt. The neutral is alumina and
it keeps the gaze melt viscous, kind of like honey rather then water. It
flows less and it resistss recrystalization upon cooling. The amount of
melting is effected by th relationship and particular type of the fluxes to
the glassformer and the neutral. Example: At lower temperatures the
relationship of base to neutral to acid might be something like 1 to .2 to
2 , whiile at higher temperature it might be 1 to .4 to 4.

Your potential problems are:

1. Firing the kiln to a temperature above the maturation temperature which
was intended for the glaze.
2. Soaking the kiln for too long of a time at the maturation temperature
(temperature and duration both effect the melt)
3. Using a clay body which has extra fluxing inclusions which may be melting
out into the glaze and encouraging additional melt.
4. Mixing two glazes together (or putting one type over another) which may
create an Eutectic, the lower melting point created by specific mixtures of
chemicals. (Example: sodium silicate meltes at a lower point then silica or
5. Using a glaze which was poorly formulated in the first place or where
there is a lack of quality control at the mixing site. (as the mines
continue to work, the chemical composition of the earths change and so clay
and glaze companies need to keep up a constant quality control operation or
their origianl formulas will change as the chemicals have changed at the
mine even though the mixing formula for the glaze has remained the same.
6. The weather, the way you stack a kiln, the relationship of the ware to
the elements or the firebox and bag wall all have an effect upon the uneven
firing and the creation of hotspots.

Years ago they used to have some of my online glaze lessons at the ceramics
site at SDSU and I was told that they were mirrored at the clay web. I
don't know if they are still there but you may find them helpful iif you can
find them. Back in the years when I was trying to survive as a potter, I
taught clay and glaze formulation courses at the Southwest Craft Center in
San Antonio. About ten years ago, I tried to teach a few interested
students online while teaching the course. The pages on the web were copies
of those original email lessons.

Hope this helps.