[Clayart] clay for glazes

Edouard Bastarache edouardb at colba.net
Tue Oct 4 14:48:42 UTC 2022

Redart melts at cone 4

-----Message d'origine-----
De : Clayart [mailto:clayart-bounces at lists.clayartworld.com] De la part de John Post
Envoyé : 3 octobre 2022 18:19
À : Clayart international pottery discussion forum
Objet : Re: [Clayart] clay for glazes

Hi Mel,

Redart as a direct substitute for kaolin will lower how much alumina is in the glaze.
If you swap out the kaolin in the glaze for redart, you end up with 58% less alumina in the recipe.

Alumina and silica have a dramatic effect on glaze surfaces.
It's why Glazy.org maps out these values on the Stull Map which is in the upper left of each glaze recipe.

This is the Tony Hansen original 5x20 recipe.
Custer Feldspar 20
EP Kaolin 20
Ferro Frit 3134 20
Silica 20
Wollastonite 20

Here is a variation I made:
Post's 5x20 Redart with Ball Clay
Ball Clay 19.3
Custer Feldspar 18.3
Ferro Frit 3134 18.3
Redart 18.3
Wollastonite 18.3
Silica 7.5

By looking at the glaze ingredients you can see that the redart in the glaze actually ends up replacing the silica more than it replaces the ball clay.

This is because in order to match the alumina amounts in the original recipe, I had to add the ball clay back in. That ball clay brings silica with it, so that means I had to reduce the amount of raw silica in the glaze.

If anyone wants to look at these recipes on Glazy.org, here are the links to them.
Tony's 5x20 - https://glazy.org/recipes/266641 My redart variation of it - https://glazy.org/recipes/266737

What you will find is that they are both in the "Bright", glossy region of the Stull Map and have nearly identical unity formulas.

So for me, looking at the way the glaze calculation numbers work, I would think that redart is not going to be a great substitute for either kaolin or ball clay because you have to add one of those things back into the glaze to get the alumina value up to match the original recipe.

But one never knows until the new glaze idea is fired.

John Post

On Mon, Oct 3, 2022 at 8:56 AM mel jacobson <melpots at mail.com> wrote:

> For years we have seemed to always use EPK Kaolin for all our glaze 
> needs. It was the standard.
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