[Clayart] Rhodes 32, Stull Maps and Biaxial Grids (long)

[Kathi Koester]

Dear John,

Thank you for your older teaching posts: I happily recited
your rhymes ("slipping is dipping...") and skunk projects when I taught my
daughter elementary clay at home. Your writings are appreciated. I wish you
and your health the very best!

Sincerely,
Mrs. Kathi Koester
MN

On Fri, Aug 12, 2022 at 2:52 PM John Post <johnpost89a at gmail.com> wrote:

> I took a break from clayart for a few years.
>
> During that time I had a back surgery.
>
> I knew that the recovery would take a while so I signed up for Matt Katz’s
> glaze class through his Ceramic Materials Workshop.
>
> If you happen to take the class it will forever change the way you think
> about glazes and your approach to solving glaze problems.
>
> In the Ceramics Materials Workshop online class much discussion is centered
> around something called the Stull Map.
>
> In 1912 RT Stull mixed a set of glazes that all had the same flux ratios.
> The ratio he used was .3 R2O to .7 RO. What he varied was the amount of
> alumina and silica in each glaze. He fired these glazes to cone 11 and
> plotted the glaze characteristics on what is now known as the Stull Map.
>
> The .3 to .7 ratio for fluxes produces the most durable glazes. You can
> vary that ratio a little bit either way, but if you go too far away from
> it, your glazes will become less durable. I do not wish to type a super
> long explanation about this, so if this interests you, sign up for Matt’s
> classes. (This ratio has its roots in some of Seger’s work.)
>
> The Stull chart can be seen on the website https://glazy.org when you look
> at individual glaze recipes there.
>
> I read the discussion in July’s Clayart archives regarding Rhodes 32.
> I noticed there was discussion about Rhodes 32 regarding what factors
> contributed to its matte quality.
>
> Here are a few things I picked up in Matt’s class about matte glazes...
>
> For a matte glaze to be a true matte, it needs to fall on the left side of
> the Stull Map.  Its silica to alumina ratio needs to be between 3:1-5:1.
> The
> silica to alumina ratio for Rhodes 32 is 4.72 so it meets that requirement.
>
> To test if Rhodes 32 is a true matte, you could increase the temperature
> you fire it to. A true matte will always run. Since this is already a cone
> 10 glaze, that probably is not the best option unless you have a hot spot
> in your kiln.
>
> Another test would be to add silica to the glaze.
> True mattes always turn glossy if you add silica to them.
>
> I might also add that the R2O to RO ratio for this glaze is .19 to .81. It
> is a little bit outside of the ideal .3 to .7 ratio for the most durable
> glazes.
>
> If I was trying to fix Rhodes 32 to be more durable I might try several
> things. First I would move this glaze around on the Stull map by changing
> its alumina and silica amounts. I would create a 4 corner biaxial blend
> exploring how changing the amounts of alumina and silica in the glaze
> affected its resistance to cutlery marking. I would use volumetric blending
> and create a 5 x 5 grid with 4 corner glazes.
>
> If this did not yield a result that worked, I would alter the existing flux
> ratio by adding more R2O flux to bring it into the .3 to .7 ratio. After
> doing that, I would run another biaxial grid exploring alumina and silica
> ratios with the new .3 to .7 ratio.
>
> And if that still did not get the glaze where I wanted it to be, I would
> add one mole of boron - either with frit 3134 or Gerstley Borate. A cone 10
> glaze can take a mole of boron without it turning into an over-fluxed
> disaster.
>
> I have been a huge Ian Currie grid fan for a long time. A Currie grid is a
> biaxial grid.  Ian used to say that using his grid method was like “Fishing
> for glazes with a net instead of a pole.”   Glazy has a feature to create
> custom biaxial grids and every time I use it to create a grid I think of
> Ian. The biaxial grid feature on Glazy basically plots a grid on the Stull
> Map with the click of a button.
>
> When Ian was presenting his workshops in the US, one of the complaints he
> received was that his grid method produced too many glazes that could not
> be used on functional ware. Using the custom biaxial grid feature on Glazy
> allows one to dial in an area on the Stull Map for exploration. That area
> can focus on the functional glaze regions of the Stull Map or one can cast
> a wider net like the original Currie grids. Basically Glazy allows you to
> use biaxial grids to explore only functional glazes if you understand how
> to locate the functional region on the Stull Map.
>
> Biaxial grids allow you to see trends. I recently created a grid that
> exhibited all of the characteristics of true matte glazes that I
> discussed. The
> grid I created had runny matte glazes on the left hand side of the Stull
> Map that turned glossy as they moved to the right on the map as that is the
> side where more silica is being added. Matte glazes on this grid that were
> over-fired turned runny.
>
> I will post a few photos of my recent biaxial grid on my blog along with a
> little more explanation and some pics of the Stull Map. If you are
> interested. You can check it out here http://www.johnpost.us
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