I know that I wrote about critical theory last semester, but that class is now OVER and I can move on to something more fun: finding the perfect blue celadon.  On the surface, the two concepts have little in common.  That's a valid conclusion, because they really do have little in common.  A Venn diagram would look like this:   O   O       See?  Barely in the same room.

I have several reasons for taking this ceramics class.  First, the critical theory class was so intense, it nearly pushed me 'round the twist.  Second, ceramics is near and dear to my heart, and it's time to get back into the lab and produce ugly pots.  I've not-so-patiently been waiting for this class, Glaze Calculations, to be offered again.  It's usually offered every other January and should have come into rotation last winter, but the man who teaches it was on sabbatical last year.

I'm also taking the class as a research project.  You see, I write about science and teach my students to do the same.  I'm keeping careful track of my test results so that I can write about them and help others to understand the wonders of the elusive blue celadon.

It's such a cool project!  Celadons are some of the world's oldest and (in theory) simplest glazes.  Usually the color palette spans mossy greens and soft browns, because the sole colorant is iron.   These glazes are fired at cone 10 in reduction, so the iron colors are fairly predictable.

But when the stars align, magic happens and a light robin's-egg blue emerges from the kiln.  My project this semester is to examine how and why that happens.  I'm also trying to determine what happens if you mess with the amounts of potassium and sodium in the glaze by varying the types of feldspar used.  Fun, huh?

Family Test #1

The first step was to find a good base glaze.  I did the family test (i.e., testing an entire family of glazes) for Ian Currie's .7 Limestone set, as found on pp. 16 and 55 of his excellent book, Revealing Glazes.  The recipe is based on Bernard Leach's base recipe:

          Bernard Leach's Cone 8 Limestone Glaze
          40% Feldspar
          30% Silica
          20% Whiting
          10% Kaolin

Here are Currie's four corner recipes for this particular glaze:

          Currie's .7 Limestone Glaze Test (300 g each)

          Glaze A
          126 g  Feldspar
          54 g    Whiting
          120 g  Kaolin (EPK)
          0 g      Silica

          Glaze B
          73.5 g  Feldspar
          31.5 g  Whiting
          75 g     Kaolin (EPK)
          120 g   Silica

          Glaze C
          210 g   Feldspar
          90 g     Whiting
          0 g       Kaolin
          0 g       Silica

          Glaze D
          105 g   Feldspar
          45 g     Whiting
          0 g       Kaolin
          0 g       Silica

I mixed up 300 g dry (500 g wet) of each corner glaze.  Volumetric blending (using the chart found on p. 102) yielded 35 test recipes that varied the amount of alumina against the amount of silica.   I tested on a grid tile made of Dan's Equal Parts clay:

          Dan Murphy's Equal Parts White Stoneware
          25   #6 Tile Clay
          25   Custer Feldspar
          25   Hawthorne Clay
          25   Flint  (200-mesh silica)

This clay body contains 0.5% iron naturally, so it is not a surprise that there was a fair amount of blue-gray in the test, even though I had added no colorants.  I'm going to have to shift to a porcelain that doesn't have so much iron.

[pic of test tile]

I really liked the way tiles 11 and 17 look, so those will be my next tests.  Tile 11 is a nice matte glaze and has this recipe:

          Clear Matte 11
          51.33   Custer Feldspar
          22.0     Whiting
          26.67   Kaolin
          0          Silica

That's right: no added Silica.  Apparently the Custer and the kaolin are enough.  I'm looking forward to seeing my tests on this one.  We're firing again on Monday, so I only have to wait a few days.

Here's the recipe for Tile 17:

          Clear Gloss 17
          49.44   Custer Feldspar
          21.19   Whiting
          18.13   Kaolin
          11.25   Silica

Looks normal, doesn't it?  That's because it falls in the expected ratios for this kind of glaze.  The fun will come when I start swapping feldspars.  I'm hoping to push the potassium amounts until it becomes a chun.