Ron Roy on wed 14 oct 98
Hi Autumn,
Surface tension: First of all has nothing to do with expansion. Liquids
have a surface tension - like water - the spiders can stay on top if there
is enough tension. Each glaze will have a different amount - if high the
application imperfections will tend not to smooth out. If it's too low the
glazes get runny. Majolica painters like it high so the brush work stays
put and control the application to minimize the runs and drips - or scrape
em off.
Viscosity is similar to surface tension in it's influence on a glaze, If
high the glaze will not smooth out well - if too low the glaze tends to
run.
Expansion means expansion on heating and contraction on cooling. Every
oxide in a glaze or clay has it's own expansion factor - glazes expand and
contract on heating and cooling according to the proportion of different
oxides (a simple explanation) - when a tile has a glaze on only the top
surface and the glaze contracts - on cooling - less than the clay the tile
will "cup" - bend backwards - it's actually a way of testing glazes to see
if the glazes winds up too big. In real life you want some of this - glaze
winds up to big for the clay - so your glazes will not craze - but too much
will result in cracking pots or pieces of glaze flaking off - shivering.
In the opposite situation - glaze winds up too small for the clay - there
is no effect because the glaze crazes and the tension is relieved.
Crawling is usually either a raw glaze problem and the glaze cracks up as
it dries on the pot or during sintering in the kiln - Magnesium carbonate
is the ideal material to promote crawling because it not only makes the
glaze crack up during sintering but also promotes high surface tension
which prevents the glaze from getting back together as it melts. Crawling
can also be caused by simple mechanical adhesion problems - like when a
second glaze is applied over an already dry first glaze that is powdery.
These problems are usually solved by changing the materials in the recipe,
timing application differently and/or adjusting early stages of firing.
Simple answers to complex problems but understanding leads to solutions.
Your statement "Or using a glaze with more expansion?" is exactly right if
the problem is cupping (glaze winds up bigger than the clay) but there
would be no turned up edges - just a gentle even rounding of the whole
tile. A more understandable way of saying what you meant, at least for
those not familiar with ceramic science, would be "using a glaze (or
changing the one you have) with more contraction on cooling - that way you
would reduce the bending or maybe even eliminate it. Don't go too far
though - you don't want the glaze to craze either.
>----------------------------Original message----------------------------
>I am wondering if the warping of the tiles might be from the glaze having a
>"tight" fit and contracting? This wouldn't affect a pot that had glaze
>inside and outside. Perhaps if the tiles were thicker this wouldn't
>happen? Or using a glaze with more expansion?
>
>Which brings me to asking about the effects of surface tension of a glaze.
>Could too high a s.t. cause dunting or would that be from too low an
>expansion? I can't quite imagine what surface tension in a glaze means
>especially if it's not quite the same as viscosity, which I gather it
>isn't. How do these characteristics (surface tension, viscosity, low
>expansion) play a role in crawling glazes?
>Autumn Downey
Ron Roy
93 Pegasus Trail
Scarborough, Ontario
Canada M1G 3N8
Tel: 416-439-2621
Fax: 416-438-7849
Web page: http://digitalfire.com/education/people/ronroy.htm
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