[Clayart] Was Talc Substitution now Wollastonite #1 of 2 posts

David Woof woofpots at hotmail.com
Tue Oct 19 04:48:28 UTC 2021

Hi Everyone,
Regarding the direct substitutions as given by Glaze Calculation programs.
It must be understood that each of the minerals used in Ceramics have important individual characteristics and properties, which also carry consequences, either positive or negative, when one attempts to substitute without informed consideration.

One such well known is that CaCo3, Calcium Carbonate (whiting) in certain amounts in concert with other certain mineral combinations will cause pin holes in the fired glaze.
Substituting Wollastonite CaSiO3,..... for the CaCo3 and adjusting the other glaze ingredients for a new balanced formula will (in most cases) eliminate the pin holes.
But the resulting glaze would probably not display the same characteristics as the original,..although if successful, one would have created a New Glaze!!!"

Another example I will cite is of the mineralogy of Wollastonite which could be in its own right a valuable material in many applications but is often underrated and misunderstood by potters.
But not by Industry which always uses the most cost effective and results efficient materials.

Wollastonite offers much more to a clay body or glaze than simply as a convenient source of, or substitution for, calcium and silica (or as a minor fluxing agent) as Glaze Calculation Programs would have one believe!!!

In its mineral state Wollastonite occurs as bladed crystal masses and the separated crystals often show an acicular particle shape.
Its acicular structural crystalline character provides a "dimensional stability" and what is termed a 'flexural modulus" to materials it is added to.

(Read: Of significant importance to Potters and Ceramic Industry is that at our firing temperatures, 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 up and then down into the cold out of the kiln fired ware state.

It is not difficult to translate this to beneficial reasons for application in our glazes and clay bodies* before * during* and post firing*

In simple description , in its crystalline structure Wollastonite works as twined and interlaced "little rollers" **rather** than as needles jumbled together to tack and pin the clay particles together as in the "fabled and inordinately popularized" KYANITE.
Kyanite does this well for a Raku body in the forming stage (as well as its potential to tear skin!!!)
Kyanite also contributes problematic free silica in fired stoneware bodies!!!

Slowing down and thinking, learning, prevents much wondering, failures and frustration... let's not just seek answers for a pressing hurry up immediate need or glaze fix.

Study, learn, appreciate the intrinsic qualities, then make wise effective use of our materials.


David Woof..................................................................................................................
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