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

ronroy at ca.inter.net ronroy at ca.inter.net
Fri Aug 12 20:34:06 UTC 2022


Hi John,

Thanks for posting this.

One of the disappointing aspects of the clazy program is it's lack of  
expansion information. Has Matt made any changes in that regard?

RR


Quoting John Post <johnpost89a at gmail.com>:

> 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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Ron Roy
ronroy at ca.inter.net
Web page ronroy.net




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