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Last week, I unloaded a kiln full of only glaze tests which I did on the new test tiles we made with the extruder.  I have to say that I am very pleased with the performance of the tiles themselves.  The glaze tests are another matter.  Let’s just say that out of a few hundred combinations and recipes, I think I’ll keep or expand on maybe five.  I keep telling myself that you never know what a recipe is going to look like until you fire it in your kiln under your conditions.  That’s my justification for appearing to waste so much time and energy.

In addition to 12 line blends, I also took all of my existing glazes and tested them over and under each other.  That’s 8 x 8 = 64 tiles right there.  In general, my glazes do not play well together in terms of appearance.  However, this was very educational, and now I know which ones combine to do something interesting, if not attractive.  I can also safely say that none of the combinations seem to crawl off one another or anything else catastrophic.

I did manage to generate some interesting purples, a new silica white, a medium waterfall green, a floating green-on-brown, and a floating blue-on-black (à la Midnight Peacock, but simpler because it’s one glaze not two, woo hoo!!!).

I also learned the following tidbits which might help you in some of your testing.  Remember, everything is dependent on the chemistry of the base glaze and your firing schedule.

(If you’re reading this article, you’ll probably be interested in my Glossary of Glaze Colorants as well.)

1.  Although lithium (from lithium carbonate, spodumene, or petalite for example) contributes to a lovely glaze finish in some base glazes, it can also kill certain color responses.  For example, at our temperature, I can get some great iron greens in certain feldspathic base glazes.  However, adding an appreciable amount of lithia-containing mineral to these base glazes make them turn an unimpressive brown.

2. Maybe this is obvious:  Two base glazes which have the same oxide analysis but use different minerals to get there do not necessarily look the same when fired.  For example, you can get calcia from whiting or from wollastonite (and others).  If you substitute wollastonite, you obviously have to decrease your flint content, to keep the silica the same, but once everything is adjusted and the molar amounts of all the different oxides are similar (within 0.1) you aren’t guaranteed that one base glaze will fire like another.  Fascinating, isn’t it?  From a purely chemical standpoint, one might assume that oxide source wasn’t nearly as important as final oxide content.  However, it seems that source does matter.  So, choose wisely.

3. Calcium does indeed have a bleaching effect on iron.  I don’t know if you would see this in a (high) iron-containing glaze, but in a clear glaze, increasing the calcium content will bleach the tan appearance coming from the iron in the clay body.  This is particularly noticeable on our white stoneware.  We use Phoenix from Highwater Clay which is pretty darn white in oxidation at cone 10.  However, there is a noticeable difference when clear glazes that differ only in calcium content are used on this body.

4. Black iron oxide (FeO) and red iron oxide (Fe2O3) perform differently in the same base glaze.  Now, I’m not talking about intensity of color, since by weight, red iron oxide is not as “strong” as black as there are more iron atoms in a given weight of FeO than Fe2O3.  At cone 10, Fe2O3 breaks down to FeO and thus, if correcting for molecular weights, one should be able to obtain the same color from FeO as Fe2O3 if you adjust the percentage correctly.  However, red iron oxide, in my experience, consistently generates a warmer brown than does black.  Browns made from black iron oxide tend toward a grey-green cooler hue versus those made with red which are warmer, more like burnt sienna.

5. Very small amounts of chrome in the appropriate base glaze are awesome and can do some miraculous things.  In the wrong base glaze it turns out cat-sick green.

6. Magnesium causes copper to tend toward a warmer and softer green whereas lithium generates yellower greens.

7. The titanium dioxide in rutile can have a lovely effect on basic cobalt blue, however, you have to be careful of the pinholing that is associated with adding TiO2 to a glaze.  I have worked on ameliorating this phenomenon by extending what I refer to as the “open” time of a glaze.  By that I mean, the amount of time a glaze is molten in your kiln, before a skin has formed on the glaze surface through which bubbles have to pass contributing to pinholing.  Logically, you can extend the open time of the glaze by maintaining its maturing temperature for longer either through heat or chemistry.  If you’re trying to extend the open time via different fluxes, you may need to take into account the temperature at which compounds such as oxygen or carbon dioxide are evolved from the molten glass.  Please note that “open time” is my own special word, meaning if you use it in front of someone other than me, they probably won’t know what you’re talking about!

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