[Clayart] Blue haze on kiln shelves mystery

[Hank Murrow]

Dear Lisa;

You have me wondering if the Gillespie Borate is carrying cobalt out to the bottom surface of the tiles during the warmup. 
If so, a high melting wax on the bottom surface of the tile might prevent that migration, especially if it is applied before the 
glaze goes on the top.

Cheers, Hank in Eugene

> On Oct 31, 2022, at 1:26 PM, Dragonbelly Ceramics <lisa at dragonbellyceramics.com> wrote:
> 
> Hello, fellow clay folk -
> 
> I'm the glaze tech at a community studio in Ware, MA and have a
> mystery to solve. I've been working on variations of a base glaze to
> get a blue we like for the studio. I chose our studio white as a base
> (omitting the opacifier) because it's the most stable/happy glaze we
> have.
> 
> Over the course of the past 2 months, I've been testing various
> combinations of copper carb, cobalt carb, rutile, & RIO. I found a
> blue the studio all liked and did one additional test. While the test
> tiles look as I had expected, they left a blue haze on the kiln shelf
> in the shape of the square tile base. (See attached photos of kiln
> shelf and test tile).
> 
> Cone 6, oxidation, electric Skutt kiln, fired using preprogrammed slow
> glaze. Cones verified the firing at cone 5-6 (depending on the shelf).
> 
> White Base:
> 
> minispar 200    46.1
> Silica                 19.7
> Gillespie Borate 13.5
> Whiting               8.2
> Dolomite             5.9
> Zinc Oxide           3.9
> Bentonite             2
> 
> 
> Problematic color tests:
> 
> WB-10:
> +cobalt carb 2
> +copper carb 2
> +rutile            1.5
> 
> WB-13
> same as WB-10, +zircopax 7
> 
> 
> Variables I've already accounted for:
> 1. clay body
> I've used the same batch of test tiles, made months ago with bmix and laguna 90
> 
> 2. White base chemistry - at the start of this process, I mixed a
> large batch of the base and have been using small batches of it to
> create colors to test. The initial tests (using similar oxides in
> slightly different concentrations) did not cause any problems.
> 
> 3. Specific gravity of the tests
> All measured at or close to 1.38, using a digital scale and a 10-0 ml
> graduated cylinder.
> 
> 4. application on the test tiles: all tiles dipped to a count of 3 by
> one person (me)
> 
> 5. Kiln placement - same haze was seen with tiles places on multiple shelves.
> 
> Does anyone have any idea what's causing the blue haze??
> 
> Best,
> Lisa Janice Cohen
> 
> (Attached photos of one of the test tiles and of the shelf)
> -- 
>        http://www.dragonbellyceramics.com
>           where imagination meets function
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