Joseph Coniglio on wed 28 apr 04
I have something I have to say about craze, it traps bacteria and food
odours in my stoneware oven pots and discolors the surface like an
old pot!
---And I'm sorry to say while many attempt to achieve craze and
crackle. If I could say at this moment in time very ANTI craze. My pots
gotta cook food in them. I can't have it!!
While there is plenty of gripe about unsafe minerals like lead etc in
what should be food safe glazes, I'm pretty bothered I haven't been
able to find a fool proof TOTALLY non craze, non crackle glaze at ^10
reduction.
I need a Red Plum, a Dark Blue and a Green. Satins are fine!!
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A nice shiny? non craze glaze in any color at ^10 is a dream!!
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Info:
I check for fit, I don't over dip. Don't dig runny. I like the unpredicable
nature of reduction, and cool the kiln down real SLOW for thermal
shock.
(also) What glaze component is a big no-no...ie a big contributor to
craze if any main one could stand as a culprit?
Any ^10 reduction non craze recipes anyone??
Thanks Joe
Dave Finkelnburg on thu 29 apr 04
Dear Joe,
You asked, >Any ^10 reduction non craze recipes anyone??
How can I break this to you gently, Joe? You know I am a huge fan of
your interest in classic big Greek terra cotta pots. And I figure anyone
who makes functional pots is clearly of superior intelligence. So, I hate
to be the one to say this to you...but there just is no magic glaze recipe
that doesn't craze...at any temperature, or atmosphere.
As you know well, crazing occurs when the glaze shrinks more than the
cooling pot. This stretches the glaze, puts it in "tension," and it starts
cracking. To avoid that, what you need is a recipe for a glaze that almost
exactly fits your clay body.
As for materials, sodium and potassium are relatively high expansion
fluxes, so you can't use too much of them if you need to lower the
coefficient of expansion of your glaze. Lithium and magnesium are
relatively low expansion fluxes, but both profoundly influence the response
of some colorant oxides, so you need to use them with care. For example,
too much MgO in a glaze with cobalt will come out purple rather than blue!
As Jim Tabor wrote, silica lowers glaze expansion. It also makes glazes
glossy. And if you use too much, a glaze won't melt. :-(
Before you even look for glaze recipes you need to understand how much
your clay body shrinks when it cools. This is called its coefficient of
expansion (COE). For porcelain this is about 6 x 10-6 inches/inch/degree F.
For stoneware the number is about 7 more or less, same units. So when
someone says 70, they mean 70 x 10-7 in./in./deg. F. The numbers are
different if you use degrees C (as the intelligent world does), but the idea
is identical.
There are two ways you can measure COE. You can pay to have a
dilatometer study done. Or, you can follow the approach of Jim Robinson in
his Studio Potter magazine article, "Body Building for Potters." He made 10
glazes, with COE's from about 5 to 8.4, then fired 10 test pieces, each
glazed with one of the glazes. The glazes with the high COE's craze, but
somewhere down the line crazing stops. That's about where the glaze fits
that particular clay body.
Armed with COE information for your clay body, then you can start
examining glaze recipes, looking for ones that may fit. It will help, as
Jim Tabor wrote, to use glaze calculation software to estimate the COE for
any particular recipe.
You do need to appreciate that this is not an exact science. We are
working with raw materials for which we typically do not have precise
analyses. Firing atmosphere and temperature influence the COE of the clay
we use. What's more, our clay body COE and glaze COE both change a little
as raw materials vary from batch to batch!
You can, and will if you persist, find glazes that fit your clay.
However, just don't expect a bull's-eye the first time.
Good potting!
Dave Finkelnburg in Idaho, who, in the interest of keeping this
post from being a 3,000-word magazine article has criminally oversimplified
pretty much everything above...sorry...
Jim Tabor on thu 29 apr 04
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>Hi Joe-
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>....(also) What glaze component is a big no-no...ie a big contributor to
>craze if any main one could stand as a culprit?
>
>Any ^10 reduction non craze recipes anyone??
>
>Thanks Joe
>
Two approaches come to mind to minimize or eliminate craze problems.
Your body and glaze need to fit better and getting someone else's glaze
that works well for them may not work for your clay.
First approach would be to glaze green with slip glaze developed from
your body and flux it. A screened slip from the body to get out coarse
materials can be used for a simple blend with a flux or better with a
combination of fluxes. Consider using a frit for this test with a 1000ml
of slip (A) and 1000ml flux (B) at glaze consistency. See how parts
ratios of 1) 3A/7B, 2) 2A/3B, 3) 1A/1B, 4) 3A/2B, and 5) 7A/3B look when
fired to ^10R. Apply (dip thick and thin) to dry tiles to fire
vertically or better on small bowls covered inside and out. Two to five
tests of the each mix would give you a better indication in a kiln with
uneven areas of temp and atmosphere. If you can reduce everything from a
workable test into a recipe to 100% than plug it into a glaze
calculation program to find out the expansion that works best for you to
be craze free and keep future glazes at or around that number.
Second approach if you have to glaze bisque begins with plugging in your
best (least crazed) glaze into a calculation software program and
looking at the expansion it has. It probably needs to be lowered
somewhat. Silica additions will lower that number and you can increase
it to combat crazing. High expansion fluxes can also be lowered to do
the same thing but your melt may be adverse to the desired surface or
color. Amounts of calcium, magnesium or zinc are some fluxes that may
influence your colors but nail down expansion first. When you get that
magic number for a glaze that fits your clay than new glazes are easy to
develop around it. Additions of 5+ ultrox or zircopax will also reduce
crazing tendencies but will also influence color and opacity.
The expansion # I have students work to in new glazes is 70 for my body
and they create glazes that fit with a +or- of around 3. We use Hammer's
limits of ^! temp stability and also play outside the box. Calculations
are made using John H.'s GlazeMaster (around $50- cheap) and it is easy
to use even with teen angels. I've used many blend systems in my decades
of glaze development, as in "old school" (first approach), but favor
calculation for new glazes. Bringing buckets of scrap glaze to new life
and making ash glazes are best left to old school approaches. I hope
this helps.
Jim Tabor
In "Dulsa",OK and looking forward to relocating back to TEXAS
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