Bye, Bye, Bisque

by Kip O'Krongly

When I first began working with earthenware back in 2008, I started out as simply as possible: red clay, white slip and a clear glaze.  While I was delighted by the potential of earthenware, it took over two years of research, with much guidance from a very generous clay friend (thank you, Scott!), to hit on a clay body, slip and glaze that all chemically complimented each other. 

Early on in these earthenware experiments, I worked in a largely traditional low fire format: bisque firing hotter to burn out organics, followed by a cooler glaze firing.  Given the nature of my surface process, after the bisque everything needed only a quick dunk in the clear glaze and then would head right back into the kiln. Frustrated by the additional time and energy wasted by a second firing, it finally clicked: Why not lean on the research out there and to try to eliminate the bisque altogether? 

Leather hard goat platter ready for green glazing

Leather hard goat platter ready for green glazing

The initial tests were promising, but pinholing and blistering posed some serious surface issues.  To combat this, I turned to altering my firing schedule as a solution. When single firing, all the organic materials that typically burn off in the bisque have to also escape through the glaze.  This can often lead to glaze trouble, so a slower firing schedule and proper venting are crucial.  When I fire, keeping the peeps open until the firing cone drops and having a few holds along the way has greatly reduced my surface imperfections.

If you are interested in single firing, the first step is to test how your glaze shrinks as it dries.  Make test tiles and apply glaze at both leather hard and bone dry to see how the glaze responds (if it doesn’t crack and fall off your leather hard tile as it dries, you're set to skip the spray gun).  My glaze has over 25% clay, so I brush onto leather hard pieces with no problems (as seen in the goat platter above).  If your glaze doesn't respond well to leather hard application, don't fret!  You can still green glaze bone dry work by spraying.  Once you're all glazed up, it's time for firing.  (And a fun aside - since your pieces haven't done their final shrinking, they can touch!) 

Here is the firing schedule I've been using to single fire to cone 04:

Segments: 4
Ramp 1: 200°, Temp 1: 180°, Hold 1: Vary, depending on dampness of work
Ramp 2: 80°, Temp 2: 250°, Hold 2: 30 minutes
Ramp 3: 150°, Temp 3: 1562°, Hold 3: 45 minutes
Ramp 4: 108°, Temp 4: 1890°, Hold 4: 35 minutes (or until the cones fall!)
*I have found that every kiln varies in terms of what the digital temperature will read and what the cones will say.  Even though I know my kiln, I still put in witness cones and watch them fall to ensure I hit the right temp.

If you still find you have trouble with pin holing, blistering, or bubbling using the above schedule, you can follow the great advice of Tony Hansen from Digital Fire and add in an additional hold 100 degrees below peak to clear out micro bubbles, and/or a controlled cool (108 degrees an hour to 1500) to help with any blisters. It's amazing what you can solve with some firing tweaks, and a real joy to have finished pieces after one cycle through the kiln!  

Happy single firing!

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For a pdf version of the single firing schedule above, click here!