Does anyone have experience using pyrophyllite?

It's an aluminum silicate used to make clay bodies with an extremely low thermal expansion, so it's great for tiles or "flameware" stove-top pots.

I'm starting out with Ron Propst's "flameware" clay body and glazes. The glazes also need to have the same low thermal expansion in order to remain bonded to the clay.

Ron Propst's Flameware Formula
Spodumene (200 mesh) 30
Pyrophyllite (200 mesh) 10
Feldspar (200 mesh) 10
Ball clay (OM#4) 20
A.P. Green Fireclay 30
Western Bentonite 2
Macaloid 1

I used white firing Vee Gum T bentonite in place of the Western Bentonite and Macaloid. I have the moisture content about right and will start wedging the clay tomorrow. So far the clay body is very spongey like a marshmallow.

Unlike Ball Clay or Kaolin which shrinks as it fires, Pyrophyllite expands as it fires like Spodumene, Mullite, Kyanite, and Talc.

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Hi Norm:

I am new to this forum: my first post actually. Referred here by some friends; glaze research is on the top of my to do list. I have gone through many of the posts over the last week: interesting topics. I have been working on clay body formulations for the last four years; recently I added a 50lb bag of pyrophyllite to my inventory. Good for thermal expansion use, but does some interesting things to COE values. Working with crystalline glaze, I cannot do much to lower the COE of the glaze, leaving me with the only option of increasing the COE of the clay. Will see where this journey takes me. Glad to know I am not the only one who looks at odd ingredients closely.

Tom Anderson

Tom - Laguna Clay "New Zealand Frost" has an impressively high COE for a clay at 6.99 - I think it's essentially New Zealand China Clay with probably 10% Veegum and Nepheline Syenite to flux, hence all the sodium and high COE.

When I used "Zam Celadon" glaze with a COE of 7.86 on this body I saw a number of long linear cracks, suggesting the glaze wasn't the ideal match it appeared to be on paper.

One thing a maker of "flameware" (pottery to be used directly on a stove) told me is there is no substitute for sending a sample out to Ron Roy or someone else with a bench-kiln/dilometer to directly measure the COE of a body

The formulas used to estimate glaze COE are not always accurate, and sometimes not even close - and they're known not to be even slightly accurate when predicting the COE of a clay body.

Hi Norm:

Ty for the reference: I fired 50lbs of frost earlier this summer. The problem with firing crystalline glaze at cone 6 is it intensely dislikes sodium because Na has a cubic crystal structure that competes with the hexagonal crystal structure of zinc and silica. I had the pleasure of a three day meeting with Ron Roy earlier this year: aware of his dilometer. Been pricing them myself; rather expensive little toys. I am narrowing in on a custom clay body for tile: which I lovingly refer to as Coma Clay.


This is the only macro-crystalline glaze I've ever fired, as it behaves pretty much like a typical Cone 6 glaze. If you want more crystallization you simply refire it in a bisque. Interesting but not spectacular.

109.0%  Macrcrystalline Blue ^6
52.0%    Ferro Frit 3110
24.0%    Zinc Oxide
24.0%    Silica
2.0%      Lithium Carbonate
6.0%      Nickel Carbonate
1.0%      Red Iron Oxide

How costly is a dilatometer?  I've never asked. But they don't post prices for the Orton DIL 2010 STD, 2012 STD or others, which alone probably hints at the cost, similar to the original Segways. If you have to ask they price you probably can't afford it.

GM Norm:

Ron has two of them actually, and on occasion sends me links to used ones up for sale. If I had a smart phone I would know about them the same day, but I am not a big puter person outside of glaze forums. Have you looked at Imerys or Unimins technical grades of kaolin? Air floated, calcined, and some are magnet milled. Extremely low % of iron and magnesium: most below 0.30% of batch weight. In addition, you can order them in 2, 1.5, 1.0 , and 0.50um particle sizes. Have my formula limits for iron, magnesium, and titanium dialed in: almost done with my flux tests. So I will start doing some blends with particle size distributions shortly. Also need to finish my silica/ pyrophylilte combinations. .Thus far, if you exceed the 40% swap ratio, the clay begins to flake.


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