Mike Bailey on fri 6 aug 99
In message , Evan Dresel writes
Dear Evan,
Your question interested me so I ran your glaze and the additions
through my computer programme (made by David Hewitt). I thought you
explained the test very clearly so it was easy to follow!
However, I'm at a loss to know why the test didn't work - as you say, it
should have reduced the crazing. So, a few random thoughts and
questions.
Are you definitely seeing crazing? (You probably are but one needs to
ask!)
Did the glaze go stiffer, more satin or matt? I think it should have
done as its moved so far from the eutectic.
Did you definitely use china clay and flint for the additions (not a
mis-labelled bag)?
If the answer to these is 'yes' then I suppose it may be explained by
the fact that the progression is less fused and therefore the COE is not
comparing like with like. That is, the comparative use of the
calculation depends on the oxides being in a similar state of fusion.
Less fused materials behave differently to glassier one. There was an
interesting posting by Michael Banks about this a few weeks ago - re.
clay and glaze COE give different results.
This is only a suggestion and I'm reminded of a story we were told -
warning us against theorising. It goes like this.
This learned philosopher / scientist (someone like Faraday or Newton)
wants to make a point to his fellows in the Royal Institute, and at the
end of their monthly meeting sets them this problem.
You know, he says, that when you fill a bowl of water to the brim and
put a stone in it, the amount of water that is displaced is equal in
volume of the stone. Eureka they cry and all nod in agreement.
How is it then, he asks, that if you fill the bowl of water to the brim
that, unlike a stone, you can carefully put a small goldfish into the
bowl and no water will be displaced!? They all agreed to give this some
thought.
At the next meeting many explanations were put forward. Most agreed that
it was to do with the ability of the fishes skin to reduce surface
tension, but lots of other suggestions were put forward too and great
discussions ensued.
Meanwhile our scientist filled a bowl with water and, as they watched,
he added a small fish......
Anyway, I feel a bit like those philosophers - offering an explanation
without testing! but I'll be very interested to hear what anyone else
has to say about the crazing.
Cheers,
Mike Bailey.
>----------------------------Original message----------------------------
>Maybe the glaze wise-ones out there can help me understand the results
>of my tests to correct crazing in a glaze. The original glaze is:
>
>Patty's Crystal Clear (from ClayArt)
>Cone 6 oxidation
>
>Nepheline syenite 30
>Gerstley borate 21
>Whiting 8
>Kaolin 10
>Flint 31
>
>Add: Bentonite 2
>
>GlazeChem gives the expansion as 67.4 x 10e-7 per degree C, but my
>original test tile crazed. So I tried to fix this by raising the Si and
>Al while keeping the alumina:silica ratio the same. To do this I mixed
>59 flint with 41 EPK which has close to the same ratio and essentially
>nothing else. Then I mixed a 200g batch of the base glaze, dipped a
>tile; added the flint-EPK mixture in 3% increments (6 g) up to 15 %
>dipping a tile at each addition. The expansion thus should range down
>to 63.6 x 10e-7 per degree C. For comparison my other glazes seem to
>fit with an expanion of around 69 x 10e-7 per degree C.
>
>Ok, the original recipie tile showed nearly no cracks after 2
>freeze-boil tests but the others did with the worst cracking at about 12
>%. The first tile may have been dipped a bit thinner but why would the
>crazing get worse with decreased expansion?
>The cracks in the tiles don't look quite the same as in other badly
>crazing glazes I have seen. They don't run as straight and don't always
>seem to meet at 90 degree angles which seems more typical to me (the 90
>degree "orthogonal cracking" is seen in mud cracks and tundra frost
>polygons and has something to do with how the cracks form but I can't
>remember exactly what from my geology student days). My glaze tests
>have a moderate number of bubbles (I'm really on a quest for a bubble
>free glaze) and that may have something to do with the appearance.
>
>Any ideas of what is going on here? Is this some symptom of too low
>expansion and I was just making things worse?
>
>-- Evan in W. Richland WA who hopes he doesn't fry his brain
>volunteering at the Art Sale this afternoon
>
>pedresel@3-cities.com
>
--
Mike Bailey
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