May Luk on mon 27 feb 06
Hello;
I have a query regarding a glaze stemmed from the
thread 'Clear Bead Glaze'
My post didn't show and here it is again. Ivor has
sent me some clarifications privately and I took the
liberty to post it at the bottom.
Regards
May
London, UK
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Dear Ivor and friends;
I am ignorant in shino and salt glaze. For my own
further education in glaze, can I also try to list my
notes on reticulation?
The breaking up of a glaze surface caused by the glaze
shrinking
From the clay body. This clay body is usually coarse.
Smooth clay surface Is easier for the glaze to form a
smooth homogeneous layer. High sodium oxide (high
alkaline) contributes to high expansion, thus high
contraction to the body. Beading glaze has high
surface tension and viscous when molten. Alumina oxide
and feldspar would help giving high surface tension
during fusion.
I am investigating a broken up glaze that is the
reverse of the
Beading glaze, coverage wise. The glaze forms a
network and the negative 'bald spots' (exposed clay
body) is in perfect circles in various sizes. It's
almost like a sheet of cookie dough after you cut out
all the cookies. Do you know what that glaze is
called?
Best Wishes
May
London, UK
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Dear May,
Good to hear from you. Happy New Year.
Interesting thoughts.
I think you have something in saying that there is
some breakdown or fracturing of some Glaze slops when
they dry, be that on dry unfired clay or on bisque
ware. Whether or not the texture of the clay or bisque
surface aggravates the effect would have to be tested.
Another thing to check on is the strength of the
adhesion between the two
materials.
Once there are fissures in the dried glaze a
prediction can be made about the progress of the glaze
as a kiln heats up. Fine corners and edges of thin
tablets of glaze will tend to be the first locations
to melt. They are exposed to the heat before the clay
body. As they melt they will shrink, each towards it
own centre. Adhesion to the clay may not happen until
thin fragments are curling away from the clay. They
may even become balls before they adhere to the clay.
Coefficient of Thermal Linear change does not come
into play until the glaze has cooled below what is
known as the Glass Transition Point, where it is solid
and no longer fluid. In a beaded glaze the glaze
coating is discontinuous so shrinkage applies to glaze
beads but not to the spaces between them.
The glaze you are investigation sounds interesting.
Sounds like a
holey cheese, Emmental of Gruyere. One possible cause
could be the
emission of gas from the clay after the glaze has
melted. Or you may have a glaze recipe that is giving
off a lot of gas. Since you have a continuous glaze
matrix then Coefficient of Thermal Linear Change could
affect it on cooling and you may get cracks between
the voids.
Tw'ill be interesting to see what the rest of the crew
think.
All the best,
Ivor
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