UH Blue

GLAZE OF THE WEEK 7

UH BLUE AND CARAMEL BROWN, TOO

  • Soft, cloudy blue glaze with brown on edges in reduction v Requires reduction, dull gray in oxidation

  • Works well on porcelain and dark clays

  • Can be used as an accent on some other glazes

  • Opaque and glossy finish

  • Quite stable, does not run very much

  • Nothing looks good on top of it, keep it simple

UH Blue is one of the workhorse class glazes at HPG which usually results in a soft, shiny blue, with faint beige speckles. It could be considered one of the “floating” glazes which has a depth of color blue with a lighter blue floating on the surface. In the background. Where the glaze becomes thinner on edges or raised areas, the color is like smooth, dark honey. In the vat, one would wonder where the blue comes from, because the color of the glaze itself is red-brown, full of iron oxide.

Even though there is no blue pigment in it such as cobalt, the blue color is a result of the oxidation/reduction state of the iron in combination with other minerals in the glaze formula. Both iron and rutile (iron plus titanium) are each about four percent in UH Blue. During the firing, the irons get into mixed oxidation states. Iron 2+ (Fe2O3) is oxidized iron and produced the “rust” or honey brown color. Iron 1+ (FeO) is reduced iron (oxygen content reduced) which tends to create the characteristic blue color for UH Blue called “chun blue”. If there is super oxidation which is represented by a combination of the two states (Fe3O4), colors sort of wash out to a nondescript gray-brown which we all hate. In fact, one of the tests for good reduction in firing is to see if UH Blue is blue.

This glaze works faithfully if it is treated well. The ingredients tend to settle, so it must be mixed very well each time it is used so it keeps the same consistency from one use to the next. The use of Bentonite at 2 or 3% prevents some settling, but more than that might prevent the blue color from developing. It is best applied when it is rather thick. In the vat it is usually the proper light cream thickness for one dip of the pot, but if it seems thin, the top of the pot should be dipped again, after the first coat is dry and the glaze remixed. The blue does not have a chance to come out if the glaze is too thin. Though it does not tend to run in firing, it is best not to glaze too far toward the foot of the pot. It is fussy in the sense that dust or dirt on the pot will prevent the proper effect.

UH Blue is brightest on an iron rich clay body, and it is very nice on Dave’s Porcelain or other white clay or as an accent on other glazes. It can be beautiful on Tenmoku or other iron-rich glazes where it breaks into a pearly blue comet streak. Shelle Avecilla gets fantastic results with UH Blue accent on McKenzie Celadon. Sometimes a bit of purple comes out, but don’t count on it! Other glaze accents on UH Blue can be ugly. Nothing seems to work because it already has several shades of colors in it. Oxblood makes it look like a bleeding wound. Best to keep the glaze application simple. Experiment with new combinations.

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Here is a “Whodunnit” mystery: Who was the person who first formulated this glaze? It has been around for years both at UH and at HPG to say the least and yet it is hard to find the person who deserves the credit for it.

Pat Harwood 5/22/2005//ed. Jan 2009 April 23, 2009 Nov 2010

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Naauao