[Clayart] Blue haze on kiln shelves mystery

Dragonbelly Ceramics lisa at dragonbellyceramics.com
Mon Oct 31 20:26:32 UTC 2022

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

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:

+cobalt carb 2
+copper carb 2
+rutile            1.5

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??

Lisa Janice Cohen

(Attached photos of one of the test tiles and of the shelf)
           where imagination meets function
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