[Clayart] Blue haze on kiln shelves mystery

[William Schran]

Couple things to consider: Boron containing compounds can cause glaze
spitting. Whiting as it decomposes can out gas losing CO2.

William Schranwschran at twc.com703-505-1617

	-----------------------------------------From: "Dragonbelly Ceramics"

	* 
To: clayart at lists.clayartworld.com
Cc: 
Sent: Monday October 31 2022 9:31:31PM
Subject: [Clayart] Blue haze on kiln shelves mystery

 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 [1]
 where imagination meets function
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Links:
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[1] http://www.dragonbellyceramics.com

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