Clay Coordinator on sat 6 oct 01
Oh Great Glazed Ones,
I have a question for you:
My understanding of cristobalite needs a little help in regard to
earthenware. Hamer/Hamer states that 2012 F is the temperature you need to
start converting quartz to cristobalite, and you need about 3-4 hours at
that temperature or higher.
Earthenware temperatures are around 1800 - 1950 F or so. Earthenware
glazes usually craze because they need so much flux (Na, K, B) to melt at
that low temperature. So cristobalite in earthenware is a good thing
because it helps with glaze fit. Since Ca and Mg are catalysts in the
production of cristobalite, you can get it from natural earthenware firings,
as many earthenware clays contain Ca naturally and/or have Talc (Mg)
additions.
Do you get cristobalite naturally in earthenware firings?
I have heard that in industry they add synthetic cristobalite or a
combination of tridymite and cristobalite, called disordered cristobalite,
in order to introduce cristobalite squeeze into tiles and other earthenware
produces, thus preventing the crazing of glazes.
Is that true?
Thanks for helping me to understand this.
Thanks,
John Britt
Penland Clay
Clay Coordinator on sun 7 oct 01
I have another question in regards to cristobalite formation.
Is it true that cristobalite is formed mostly from "free silica" rather that
silica introduced as filler in a clay body. Filler is generally introduced
as 200 mesh and it really needs to be very small to change to cristobalite.
So even though a 200 mesh size silica will contain some 325 mesh particles
it is not enough to cause a problem. The main cause of cristobalite is free
silica in the clay and the heating of the clay, which changes the metakaolin
to mullite and ejects 4 silica of molecular size. This is then easy to
convert to cristobalite. Is this true?
Thanks,
John Britt
Penland Clay
Ron Collins on sun 7 oct 01
This post made me think that I might not have done the right thing. I made
up some small cone 6 porcelain test batches, trying to convert my teaching
studio to all white, both earthenware and porcelain. I used all 305 mesh
silica, thinking that it would melt easier and make a smooth, sandable body.
It's in the bisque kiln right now......I didn't use larger flint...did I do
the wrong thing? Melinda Collins, Antigua, Guatemala
Ron Roy on mon 8 oct 01
Hi John,
I have done some dilatometry on this issue - Mimi Ostler has some info on
the subject as well - see page 165 in Out of the Earth Into the Fire.
It is not cristobalite that is generated but Enstatite - which has a high
expansion/contraction and helps to solve the low fire crazing problem.
Talc, if there is enough present, also reduces post fire moisture
absorption - another cause of crazing in underfired clay bodies.
I also have read somewhere that adding cristobalite is a way of handling
the problem - but it does nothing to prevent delayed crazing from moisture
absorption. Hard to find to. At San Cristoble - down Mexico way - they have
a mountain of it.
Have to be very careful to add the right amount each time - it really works
Old cone 10 kiln soft bricks will be full of it - do I have a deal for you.
Cristobalite is even worse than silica on the lungs though - be careful out
there.
RR
>Oh Great Glazed Ones,
>
>I have a question for you:
>
>My understanding of cristobalite needs a little help in regard to
>earthenware. Hamer/Hamer states that 2012 F is the temperature you need to
>start converting quartz to cristobalite, and you need about 3-4 hours at
>that temperature or higher.
>
>Earthenware temperatures are around 1800 - 1950 F or so. Earthenware
>glazes usually craze because they need so much flux (Na, K, B) to melt at
>that low temperature. So cristobalite in earthenware is a good thing
>because it helps with glaze fit. Since Ca and Mg are catalysts in the
>production of cristobalite, you can get it from natural earthenware firings,
>as many earthenware clays contain Ca naturally and/or have Talc (Mg)
>additions.
>
>Do you get cristobalite naturally in earthenware firings?
>
>I have heard that in industry they add synthetic cristobalite or a
>combination of tridymite and cristobalite, called disordered cristobalite,
>in order to introduce cristobalite squeeze into tiles and other earthenware
>produces, thus preventing the crazing of glazes.
>
>Is that true?
Ron Roy
RR# 4
15084 Little Lake Rd..
Brighton,
Ontario, Canada
KOK 1H0
Residence 613-475-9544
Studio 613-475-3715
Fax 613-475-3513
Ron Roy on thu 11 oct 01
Hi John,
There is an article in an issue of Studio Potter a while back - by Peter
Songhen that will say it all re silica and cristobalite - I don't have my
Studio Potter mags unpacked yet so I can't tell you which issue for sure -
I'm guessing end of 99. I did the dilatometry for him.
Yes - added silica is a contributer - if it is fine enough - when we tested
our 200M something like 94% went through a 400 mesh screen - the 400M
silica let through 96%. The explanation I got was - the grinding equipment
used has improved so much it is mostly sub micron.
All quartz is not the same - there are differences that do affect
cristobalite generation - the ground quartz does produce more cristobalite.
Peter proved it by sieving out specific sizes - the 100M, 200M silica did
not contribute to cristobalite generation - the unsieved of both 200M and
400M did.
I have many charts of raw clays - like OM4 and Bell Dark, fire clays
kaolins and they all have high cristobalite consentrations when fired to
cone 10R - that is because there is no spar added - if you want to avoid
troublesome amounts of cristobalite you need at least 10% spar and
preferably no very fine silica - you can add silica but make sure most of
it is 200M or bigger.
I have never found any cristobalite in any porcelain - they usualy have 20
to 25% silica - straight from the bag. The answer as to why would be -
there is also about 25% spar - the KNaO melts any cristobalite seeds before
they get going.
Anyway - the above is simlified to save me time - get a copy of his article
and you will have your answers.
RR
>I have another question in regards to cristobalite formation.
>
>Is it true that cristobalite is formed mostly from "free silica" rather that
>silica introduced as filler in a clay body. Filler is generally introduced
>as 200 mesh and it really needs to be very small to change to cristobalite.
>So even though a 200 mesh size silica will contain some 325 mesh particles
>it is not enough to cause a problem. The main cause of cristobalite is free
>silica in the clay and the heating of the clay, which changes the metakaolin
>to mullite and ejects 4 silica of molecular size. This is then easy to
>convert to cristobalite. Is this true?
Ron Roy
RR# 4
15084 Little Lake Rd..
Brighton,
Ontario, Canada
KOK 1H0
Residence 613-475-9544
Studio 613-475-3715
Fax 613-475-3513
Ron Roy on thu 11 oct 01
Melinda,
See my post to John on silica and cristobalite - never found any
cristobalite in any procelain - not a problem that is if you are using spar
to melt it. If you use MgO or CaO to melt you will though.
RR
>This post made me think that I might not have done the right thing. I made
>up some small cone 6 porcelain test batches, trying to convert my teaching
>studio to all white, both earthenware and porcelain. I used all 305 mesh
>silica, thinking that it would melt easier and make a smooth, sandable body.
>It's in the bisque kiln right now......I didn't use larger flint...did I do
>the wrong thing? Melinda Collins, Antigua, Guatemala
Ron Roy
RR# 4
15084 Little Lake Rd..
Brighton,
Ontario, Canada
KOK 1H0
Residence 613-475-9544
Studio 613-475-3715
Fax 613-475-3513
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