Making the switch from gas to electric..

[Lisa Nouvelle]

God morning friends. I am making the switch from cone nine gas firing to cone six electric. I use one glaze for most of my work and that is Leach White which I have reformulated to work beautifully in the lower temperature. In the gas environment I used iron oxide under the glaze to create great shading/shadowing effect along with the speckling and iron spots with the clay I was a happy camper. The oxide showed up any textures nicely as well. I also used the glaze heavier for a great shiny opaque white.

In the cone six firing I have no problem with the shiny white but I have not been able to recreate any interest at all with the oxide wash or iron spots or even letting the clay color show through. The clay with lots of spots in it looks like a cheap effect instead of an exciting or interesting development.

Any suggestions? Thank you so much in advance for any input you have.

[Norm Stuart]

Congratulations! You've shown the look created by the slow-cooling dense firebrick used in a ^10 kiln can be easily replicated with a computerized kiln at ^6, programmed to slow-cool between 1800F and 1500F.

Red iron oxide does not degrade into black iron oxide extensively until ^7, so if you want a black iron oxide look at ^5/^6 you do do need to start with black iron oxide. Alternately if you want a red iron oxide color, using synthetically produced red iron oxide with near 100% purity is the easiest path.

Cone 10 red iron glazes are complicated by the fact that almost all of the red iron oxide will have become black iron oxide, so the kiln temperature has to be held for several hours on the way down at a temperature favorable to oxidizing iron. Cone 6 with a slow-cool is a far less complicated process.

After looking at the Warm Jade tiles below, you may find that your best solution at ^6 is to add magnetite or black iron to the clay body itself.

This is an oil spot glaze, the under-layer tinted with black iron oxide, the top layer tinted with yellow mason stain.

Warm Jade is a glaze with no added iron. The only iron present occurs as impurities in the components of the glaze ingredients like ball clay 1%, rutile 10%, gerstley borate 0.4%, and any iron in the white burning clay.

Digitalfire suggests some of these iron impurities are black iron oxide while others are red iron oxide.

http://digitalfire.com/4sight/material/ball_clay_80.html

http://digitalfire.com/4sight/material/rutile_1204.html

http://digitalfire.com/4sight/material/gerstley_borate_806.html

Warm Jade
44.2%    Nepheline Syenite
25.2%    Gerstley Borate
18.9%    Silica
11.7%    Ball Clay OM4
  5.0%    Tin Oxide
  4.0%    Rutile
  3.0%    Copper Carbonate

This tile is Warm Jade slow-cooled on a white burning clay with some iron impurities.

This second Warm Jade tile is also slow-cooled on a white burning clay with NO iron impurities.

None of the black spots you desire.

I've not made White Bird glaze with black iron oxide in place of red iron oxide. It would be interesting to see the result.

You'll achieve the look you want shortly.

[Lisa Nouvelle]

Next set of results in. I modified the White Bird glaze recipe to 12% Zircopax instead of 6% each of Tin Oxide and Zircopax. I will attach the photo, the glaze has a thousand tiny pin holes(?) in it. The color is great, much more white although less shiny as well. It broke nicely over some darker clay and was more brown than orange as I used black iron oxide this time.

Any ideas on the tiny bubbles?? Thanks to all!

Lisa

[Norm Stuart]

You want a well-melted crystalline glaze surface which scatters light. You've made the glaze more viscous when melted, so the glaze doesn't have time to lay down after off-gassing. This stiffness also inhibits the flow of iron to the surface of the glaze.

David Pier describes a matte glaze in 2010 in "Ceramics Arts Daily":

http://ceramicartsdaily.org/ceramic-glaze-recipes/glaze-chemistry-c...

Glossy glazes are very smooth, smooth on the scale of the wavelength of visible light (390nm–750nm, where a nm is ~1/400,000,000 of an inch). Thus any bumps, pits, or undulations on the glaze surface are smaller than approximately 390nm or 1.5/100,000 inch, so as far as the light is concerned the surface is perfectly smooth. If the light’s wavelength is larger than the bump, then you won’t be able to see the bump. Conversely, the protruding crystals in matte glazes are larger than this and therefore scatter the light.

Oxides employed to create matte glazes are MgO, CaO, SrO, BaO, Al2O3, TiO2, ZnO, and MnO.

Digitalfire says this about Tin Oxide compared with Zirconium Silicate:

http://digitalfire.com/4sight/material/tin_oxide_1642.html

http://digitalfire.com/4sight/material/zircon_1722.html

http://digitalfire.com/4sight/material/lithium_carbonate_975.html

Zircon will stiffen the glaze melt more than tin. Zircon is a more yellow white while Tin is more blueish.

Glazes lacking melt fluidity can experience problems with crawling, blistering and pinholing since they are lacking in the ability to heal disruptions. Adjustment of the glaze chemistry may be needed (increasing the flux content while maintaining the SiO2:Al2O3, for example).

Sometimes people use a partially melted refractory glaze and call it "matte". But this leads to problems such as leeching and poor glaze durability - and won't allow the iron eruptions you're looking for.

As one possible starting place, compare your revised White Bird glaze with these:

matte glazes, which rely on Titanium Dioxide / Rutile for crystallization (with a slow-cool) and Lithium as a flux (lithium carbonate or lithium fluoride or spodumene). Only the Dry Yellow Pumpkin, which relies on spodumene and a much higher percentage of titanium, exhibits some glaze stiffness similar to your revised White Bird.

http://cone6pots.ning.com/photo/lemon-frost-vase
http://www.flickr.com/photos/glazes/1085517444/
http://ceramicartsdaily.org/daily/a-high-fire-reduction-potter-deve...
http://www.flickr.com/photos/glazes/1086357083/
http://www.potters.org/subject54205.htm/

Another possible starting point are these two recipes for a matte "Waxy White". One recipe uses Ferro Frit 3134, the other with gerstley borate. These matte glazes rely primarily on calcium and magnesium crystals to form the matte surface, another method from the "Ceramic Arts Daily" article. Neither glaze contains titanium or lithium -- but adding titanium dioxide would certainly make these glazes more matte.

http://glazestuff.wordpress.com/2012/04/02/waxy-white-6-ox-diane-be...
http://www.flickr.com/photos/glazes/1085533046/

Also keep in mind that the original White Bird recipe will be all white if applied too thick (as the iron can't rise through the thick glaze layer), all brown if applied too thin, and just right it shows iron breaking through the white in a manner you're looking for.

[Norm Stuart]

Annabelle Aylmer's glaze on her tea pot,

http://cone6pots.ning.com/photo/annabelle-aylmer-02

. . . looks like it might be the sort of ^6 white glaze over iron you're looking for.