Dave Gayman on thu 21 feb 02
Is there a rule of thumb for turning the surface of commercial lowfire
glazes from glossy to matte? These would NOT be for food or liquid
contact. I want opacity, so that's not an issue.
Short of an easy answer all ready to hand to me, what are your thoughts on
the following methodology:
Since I can't delete fluxes already in the glaze, dry out a small amount
(probably pint / half liter) of a commercial glaze to obtain dry weight.
1. Do a line-test with additions (say 5% by weight) of kaolin. +5%, +10%,
+15% (would I need any more?)
2. Do a line-test with additions (say 2% by weight) of flint/silica. +2%,
+4%, +6%, +8%, +10% (or did I get this backwards -- that is, would I get
more flatting from kaolin, less from flint?)
Anyone have suggestions for any other flatting materials?
Dave
Stephani Stephenson on fri 22 feb 02
Dave wrote
Is there a rule of thumb for turning the surface of commercial lowfire
glazes from glossy to matte? These would NOT be for food or liquid
contact. I want opacity, so that's not an issue.
......
Anyone have suggestions for any other flatting materials?
Hi Dave
One question : are you looking for opacity only, or primarily? matt
glazes are by nature opaque but opaque glazes may not necessarily be
matte.
What kind of opaque and/or matt surface?
for opacity , zirconium (usually zircopax) can be added. Other
opacifiers are Tin Oxide and Titanium dioxide.
Whiting, zinc , magnesium carbonate, dolomite, titanium, barium
(toxic), and clay may also steer your glaze to a matte glaze, in
addition to the silica and/or alumina.
Matts are not so simple though.
Like opacity, one way a matt effect occurs is when a material does not
dissolve or melt and remains suspended in the glaze, blocking the
transmission of light, if you will..
Alumina matts tend to have a dry look, like an underfired glaze, where
it looks like the melting point of the glaze has not been reached. (You
can also add alumina in the form of china or ball clay.)
Another way a matt glaze works is that certain materials are added in
enough of a saturation/concentration that the excess material
precipitates out in the melt. (That is inexact language, but this is a
short answer! someone else chime in here!) Silica matts can work in
this way.
Also, materials which are dissolved in the glaze melt can form
crystals upon cooling. These crystals interfere with the transmission of
light. The light shining through the glaze, hitting the pot surface and
bouncing back is 'interfered with' by these crystals. Of course not all
crystalline glazes are matt glazes, but sometimes crystallization
produces some of the more complex looking matts. Slow cooling promotes
the formation of crystals (they have a longer time to form at high temps
in the kiln).
silkier matts , not as dry as alumina matts can be obtained with the
additives containing calcium and magnesium.
Since you are just adding these things to a commercial glaze , you won't
really understand completely what is going on, as you don't know the
other ingredients in the glaze or the exact %.
also, whenever you add complex materials especially there are other
'side effects', Some of these materials will alter the viscosity, color,
etc. of the glaze. sometimes materials will act in ways you least
expect. For example you can increase silica, thinking it will raise the
melting temp of your glaze only to find you get an even glossier glaze.
This is due to eutectics wherein materials which usually do not melt in
the low temp range, combine in proportions where they do melt at lower
temps....not a good explanation, but moral of the story is sometimes
the unexpected happen!
try some line blends. you won't know the real % of your additions, so
just be consistent.
I'd say if you are really interested in seeing what causes matting in
glazes, start with your own basic glaze from scratch. use a standard lo
fire clear, there are many . then begin to adapt it.
There are commercial matt lowfire glazes out there, though I am not too
familiar with them.
You can also start from the other end, with a very matte underglaze,
something like Amaco velvet undergalzes and begin adding flux, in the
form of frit , till you get just the right combo of matt and melt.
Hope this helps
Stephani Stephenson
steph@alchemiestudio.com
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