Ian Currie on tue 19 dec 00
From: Heidi Haugen
To: CLAYART@LSV.CERAMICS.ORG
Date: Monday, December 18, 2000 4:46 PM
Subject: suface texture
>If two glazes have very similar si:al ratios (8.49-8.56)shouldn't their
>surface textures be similar?
G'day Heidi
The reason we often quote alumina/silica ratios as a guide to surface is
that there is an approximate correlation within certain limits. The problem
with generalizations in ceramics is that most (all?) are nothing more than
that, and there are exceptions. It was precisely to deal with this problem
(and others) that I developed the grid method outlined in my recent book:
"Revealing Glazes - Using the Grid Method" (see signature at bottom if you
would like to get a copy)
It's a little difficult to describe an alumina silica graph in words, but
perhaps I can explain the generalization as follows. Note that this applies
especially with high temperature glazes - the lower the temperature you go,
the more distorted the pattern gets.... so the following applies to
stoneware glazes, and more or less to midfire too.......
Usually glazes with alumina/silica ratios of around 1 to 8 (talking here
molecular parts - don't worry if you don't know what that means, but I think
you do..) ...these glazes will be shiny over a wide range of alumina and
silica values. This is because the 1 to 8 ratio usually puts the glaze
somewhere along the "eutectic trough". [On one of my grid tiles, this zone
extends diagonally from the bottom left corner C (low alumina/low silica) to
the top right corner B (high alumina/high silica) - and in many of these
tiles the diagonal band of shiny glazes from C to B is obvious. The tile
does need to be fired high enough for the pattern to appear... The diagram
below shows the layout of one of my grids with "X" meaning a glaze and "S"
meaning a "shiny" glaze]
A______B
X X X X S
X X X S X
X X S S X
X X S S X
X S S X X
X S S X X
S X X X X
C______D
Glazes with alumina/silica ratio of, say 1:4 will be to the left of the "S"
zone. Many of these are what we call high alumina glazes which are often
matt, or dry or in extreme cases quite underfired.
Glazes with alumina/silica ratio of, say 1:12 will be to the right of the
"S" zone. Many of these are what we call high silica glazes which are
sometimes opalescent, sometime white opaque, sometimes with a surface like
wet sugar, and underfired in the extreme cases.
The problem with these generalizations are that they break down if we
underfire, overfire, and in particular if we bring in materials that change
the eutectic landscape drastically.... And things that promote crystal
growth like TiO2 or Rutile will produce matt surfaces in glazes that would
be shiny otherwise. (And I think vice versa with B2O3) Note that the
generalizations I outlined above usually refer just to the base glaze.
Start adding other goodies in substantial amounts and all bets are off. And
all these "generalizations" I'm laying out now are themselves subject to
exceptions... For example I can show you an example of a matt underfired
(high flux) glaze turned shiny by the addition of 10% TiO2!
So how do we bring some order into this apparent chaos?
I use the grid. Rather than try to guess what a particular balance of
fluxes, alumina and silica will produce, I produce this grid where they vary
in easy steps, and the quality you were were expecting to find "here" will
often be found one or two steps away. So we don't need to rely on
generalizations or rules of thumb - we can see exactly what the materials we
have chosen will do IN SPECIFIC CASES over a wide range of variation. We
can see at a glance what the trade-offs are (for example "if I want the
glaze to melt earlier, will this cause it to craze?") - it is all laid out
on the grid tile in my hand. The method comes with techniques for
preparing, applying and firing multiple glazes easily, and also a page at my
website (or you can use the book) to easily calculate the glaze recipes.
If you think about it, this method will often compensate for variations in
materials. Especially if the material variation is alumina or silica, the
glaze you seek will simply be moved across the grid a step or so.
And it's useful for things other than surfact quality - it's an ideal method
for studying glaze melting ranges, crystal formation, opalescence and
opacity, glaze fit, colour variation etc. It's also ideal for seeing the
effect of varying clay body and firing.
And yes this is also inevitably a plug for my new book, but many have found
the method very effective, and I wrote the new book to outline the "recipe
version" of the method I've developed in the last few years. One of the
great advantages is that we now rarely need to venture away from the recipe
approach because of the way the grid lays it all out. Many of the sets of
glazes using the standard recipe grid are almost identical to what I would
be doing if I was using a Seger formula approach. And those that aren't are
simply skewed... but the glazes are all there, just moved around a bit.
If you are seeking stable glazes useful for internal utilitarian pots, you
can use the grid to find the glaze qualities you want, then get them
tested... but here one generalization seems to hold... if you maximize the
alumina and silica while keeping the glaze well fused, you will maximize the
stability of the glaze. On the grid, this means go as far as you can
towards the top right corner, corner B.
Hope this helps. Regards
Ian
P.S. If you would like to know more about my glaze method workshops, or if
you would like to purchase one of my books, go to:
http://ian.currie.to/
or just e-mail me. If you pay for the books by credit card at my web site,
they usually take about a week to arrive. (A bit longer at Christmas time!)
Incidentally, the value of Australian currency is currently way down, so the
book price to US, UK etc. is also down. All prices quoted at my website are
in Aussie dollars.
| |
|
