[Clayart] Was Talc Substitution, Now Wollastonite #2 (a continuing of 2 posts)

Robert Harris robertgharris at gmail.com
Tue Oct 19 17:13:04 UTC 2021

David, I have never found any references that say that Kyanite converts to
mullite at such low temperatures. Almost all the ones I've found indicate
that conversion happens above 1300C (2372F). I have often wondered if the
Kyanite to Mullite conversion actually happens in our clays, or if it does,
whether it is somehow propogated by the other constituents of the clay. (I
imagine most of the data documenting conversion is done on fairly pure
kyanite samples). Do you remember where you got that number from?

This is an interesting paper
which also shows that complete conversion to mullite requires several hours
(below 1500C (2732f) - it occurs rapidly (30 mins) at 1600C (2912F).

Commercially Kyanite is converted to Mullite by heating for many hours at
1300C (2372F). So while it's possible that Kyanite will start converting at
Cone 10 temperatures, only a very very small percentage will be converted
under ordinary firing conditions. (Perhaps in a 5 day wood firing far more
will be converted, but that is a special case!)

The kyanite to mullite conversion is a very different process from
wollastonite melting. (Or any melting. It is irreversible conversion from
one crystalline form into another crystalline form). The majority of our
clay and glaze materials melt at temperatures far in excess of those we
fire to. This is the very essence of eutectics. I know very little of the
physical process of sintering, and then melting, but presumably at particle
interfaces eutectics come into play. So, of course one can get wollastonite
to melt at far lower temperatures than 1540C. I just made up a very molten
celadon that gets most of its calcium and magnesium from Wollastonite and
Talc. It is the interaction between wollastonite and feldspar that melts at
low temperatures. And both of those exist in clays too.


On Tue, 19 Oct 2021 at 05:54, David Woof <woofpots at hotmail.com> wrote:

> Hi Everyone,
> Let's agree to use previously mentioned Kyanite as a Ceramic material
> comparison.  And let's agree that those who swear by Kyanite will also
> admit the struggles with its limitations.
> Bear with me while I present a comparison favoring substitution with
> Wollastonite (CaSiO3) which does not cause these problems at the pyrometric
> cone temperatures we work at.
> In fact of comparison; cone 03/02 is threshold for dissociation of the
> combined mineral elements for Kyanite, vs cone 19 for Wollastonite.
> And as I mentioned in the accompanying previous post #1, the Flexural
> Modulus properties of Wollastonite are maintained in the clay body from wet
> plastic throwing state, thru the drying stages to the bone dry pre
> firing...and during the firing all the way into the cold out of the kiln
> fired ware state.
>  Wollastonite has been tested and used in Industrial Ceramics for many
> years now, and in many other Industrial manufacturing applications
> including plastics, paints, and roofing materials in part because of this
> flexural modulus property. (Fancy words for saying it can flex and resist
> breaking)
> Following; is a bit of the science behind what I have been passionately
> saying about and why I use it.
>  At temperatures above 1100 c/2012f  (cone 02)  Kyanite decomposes into
> Mullite and a Vitreous Silica in the reaction 3(Al2 O3*SiO2) > 3Al2
> O3*2SiO2+SiO2.     This dissociated transformation results in expansion of
> the vitreous silica which is the cause of the cristobalite we attempt to
> avoid in our stoneware when held at temps above cone 02 for long periods.
> Especially in slow ramp firing.
> **** Also, If we feel we must add fillers to a clay body there are other
> more innocuous materials than the raw ground silica/flint flours and fine
> silica grits as these also compound the free silica potential to cause
> cristobalite formation.
>  In contrast: Wollastonite begins its melt at 1540 c / 2804 f (cone 19)
> The then molten CaSiO3 continues to maintain a tetrahedral SiO4 local
> structure up to 2000c /3632 f which takes us off the cone charts of where
> we fire any of our clay bodies.   This extremely high eventual
> disassociation of the Calcium/Silica bond in Wollastonite is of no
> practical concern in our use as a valued glaze or clay body constituent.
>  And for the above reasons I have used Wollastonite in my Wood-Fired to
> cone 16 bodies, Gas fired cone 10,  Electric fire to cones 6 to 10 and a
> bullet proof Raku body at whatever temp as well.
>  And although Kyanite is generally recognized as a relatively effective
> but ("bare hands grinding") Raku Body, I have shunned Kyanite because of
> its inherent limitations in favor of the " stand up, smooth move" pleasure
> and protective confidence experienced with Wollastonite at all temps and
> firing protocols.
> Ok! Ok!  I've kicked this dead horse to death a second time! I know!!!!
> Well almost...
> The foregoing is well worth the second read and pondering for those who
> admit that they don't already know it all... or are no longer content with
> knowing "just enough" until they get in knee deep sh-- and yell "help me"
> on Clayart.
>  Misneach,
> David Woof,..................
> *******************************************************************************************
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