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

David Woof woofpots at hotmail.com
Tue Oct 19 04:49:34 UTC 2021

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.


David Woof,..................

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