[Clayart] Clayart Digest, Vol 105, Issue 13

[David Finkelnburg]

Jeremy,
      Historically, glazes were suspended with kaolin (China clay) or ball
clay.
      Bentonite works as a glaze suspender because of its relatively large
amount of surface area compared to kaolin.
      As you understand, roughly 10% kaolin or ball clay will suspend most
glazes, as will 2% bentonite, though 2% may be more than you need.
      Since your goal, as I recall, is a crackle glaze, you need to examine
the Coefficient of Thermal Expansion (CTE) as calculated by your software.
For glazes that craze, the greater the CTE the more crazing (crackle) you
will have.
            Regards,
                 Dave Finkelnburg

On Fri, Aug 23, 2024 at 8:25 AM Jeremy Keala Ceramics via Clayart <
clayart at lists.clayartforum.com> wrote:

> Hello,
>
> Thanks for the feedback. I've been thinking about your answers and how to
> re-formulate my question so it's more precise :)
>
> To give a bit more context to my question:
> I started out looking at examples of Kuan glaze recipes, and as David said,
> most are some combination of Feldspar, silica and calcium with bentonite as
> a suspension agent. But then, reading through Nigel Wood's book I happened
> upon a table of "Ge" glaze recipes, which appear similar to Kuan glazes in
> their results. The two recipes are:
>
> recipe 1
> Potash feldspar : 63.5
> China clay : 6.5
> quartz : 22
> whiting : 6.5
> iron oxide : 0.8
>
> Recipe 2
> Potash feldspar : 28.3
> China clay : 26
> Quartz : 27
> whiting : 14.5
> dolomite : 2.5
> iron oxide : 0.75
>
> Seeing the kaolin present in these two recipes, I thought it might be
> possible to not use bentonite. But as I'm not very good at using glaze
> calculators for the moment, I was wondering about any suggestions of how to
> go about replacing bentonite.
>
> As an example, I tried using ball clay to modify this recipe from John
> Britt's book :
>
> original recipe
> custer feldspar : 79
> whiting : 9.5
> silica : 9.5
> bone ash : 2
> bentonite : 2
>
> modified with ball clay
> custer feldspar: 63.8
> ball clay : 10
> silica : 13.1
> whiting : 11.1
> bone ash : 2
>
> It shows up pretty similar on a glazy calculator, but I'm not very
> knowledgeable about these calculations, so I was wondering if I was missing
> some important detail in trying to replace bentonite with ball clay? Also,
> given that original kuan glazes didn't use bentonite, I'm wondering how
> this glaze settling problem was managed "back then"?
>
> So I get that it's not generally easy to replace bentonite in a recipe, but
> at the same time, I don't really understand why I can't replace it with
> ball clay, like in this example that I tried. Hope my question is a bit
> clearer. I just didn't want to write such a long email the first time
> around!
>
> Thanks for your suggestions, they have been very precious throughout all my
> glaze testing.
> Jeremy
>
>
>
>
>
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