[Clayart] Blue haze on kiln shelves mystery

[Lis]

Cobalt, copper and chrome all volatilize in the firing and the vapours 
will land on the kiln shelves. Saggars would contain the vapours.

Lis

On 2022-11-01 12:11 p.m., W Seidl wrote:
> Perhaps a stupid question...  Have you tried firing those tiles in a 
> saggar?  That might eliminate/mitigate any environmental issues going 
> on in the kiln during firing.  Still happening(?), then look to the 
> glaze. The "sealed" environment inside a saggar, blah blah...
>
> Just a thought. Might not be a cure, but might get you closer to the 
> cause.
>
> Best,
>
> Wayne in Maine
>
>
>
>>> 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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