Ron Roy on fri 10 oct 03
Hi Taylor,
Another good question! I am impressed again - really!
If the glaze is still pyroplastic (soft) when the quartz in the clay body
goes through it's inversion at 573C - on the way down - then the glaze
simply adjusts in size.
Quite common in glazes that are high in boron because boron stays soft to
quite a low temperature - around 500C I think.
If the glaze is solid (frozen) at 573C then it cannot adjust to the
decrease in body size and there is tension between clay and glaze. In this
case the glaze gets compressed as the clay gets smaller - because of the
quartz inversion - the glaze never has free quartz - so it does not get
suddenly smaller at 573C - so it is compressed - which means in fact it
remains bigger than the clay - which is necessary for it to not craze.
So adding free quartz to a body is a possible cure for crazing.
So how come adding quartz to a glaze is a cure for crazing? Because melted
quartz (amorphous silica) has a low expansion - if the glazes has a lower
expansion(expansion on heating Contraction during cooling) it is less
likely to contract more than the clay during cooling so it will be less
likely to craze.
Something to understand though. All clays and glaze have a "overall"
expansion/contraction - exactly reversible on heating and cooling - for
ever.
If the overall contraction of the glaze (after the glazes has solidified)
is more than the contraction of the clay - the glaze will craze.
I hope I am not going to far - but if you really want to understand this
there is a bit more.
Lets say the glaze is frozen at 800C - and the glaze crazes before the
quartz inversion - will adding more quartz to the body help? To put it
another way - will increasing the contraction of the body at 573C help to
cure crazing that has already happened at say 600C?
Go for it Taylor - be of good courage!
RR
>I want to understand this statement; I think it is something
>important for me to understand. Knowing when a particular
>glaze goes super cooled (that's just for Ivor, I'm not trying
>to sound smart. Honest) is helpful especially with regards
>to using compression because if the glaze is still molten
>when the reversion takes place said compression is lost? Did
>I get that right?
>
>Is this an issue with a large number of glazes? I'm assuming
>that it would be an issue at only a limited range of heatwork.
>Is my brain still melted on this?
>
>I do appreciate your and John's work on these matters.
>
>Taylor, in Waco.
>
>-----Original Message-----
>From: Ron Roy [mailto:ronroy@TOTAL.NET]
>Sent: Thursday, October 09, 2003 12:29 AM
>To: CLAYART@LSV.CERAMICS.ORG
>Subject: Re: Ron
>
>... Knowing when your glazes actually go
>from pyro plastic to solids seems to me to be a very helpful bit of
>information if you are trying to determin when to slow cool for
>instance.
>It certainly is crucial if you are trying to use the quartz inversion to
>keep glazes in compression and avoid crazing.
>
>...
>
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Ron Roy
RR#4
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Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
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