It is a very variable glaze, but all of its variations are great - provided it gets hot enough. It also does an interesting toadskin thing when it is not quite hot enough which most people probably would not consider a successful surface but it has some nice qualities, especially on sculptural work.
It is a glaze that likes to be hot, though: maybe 03? Hot enough that thin or delicate earthenware bodies warp in the kiln.
Because Portland Pottery is right in the middle of an urban neighborhood, we try to use a minimal-smoke approach - which means combustibles on the ground, trashcans over top after a few seconds of free burning. We use pine or cedar pet bedding as our combustible - it's cheap & easily accessible, but contains some pitch which will create black smoke marks on the finished piece that need to be scrubbed off to see the colors. Different combustible material will give you different glaze results, to a certain extent - it's all about how fast they burn and how loose they are to let oxygen through. My very favorite combustible is dry leaves: they burn quick and clean, and create very little rough texture - but good luck getting enough, in June.
It's tricky to get the FAB into the pile of combustibles while it is still molten enough to get the wonderful copper results but not so molten that is gets a crusty sawdust texture. You can see a bit of the texture on the third photo.
Here's the recipe. This ia for a 4000 gm batch, so WILL NOT translate as percentages - you'll have to do the math on that. (Hint: multiply everything by 2.5.)
Frit 3110 2800
Gerstley Borate 200
Flint (325) 200
Soda Ash 400
Copper Carb 140
More oxygen in post-fire gives more turquoise, less gives more copper. Join e for the next raku workshop at Porland Pottery on September 16th, from 12-4 pm, or mix it up yourself & have some fun!
To register for the workshop, call 207-772-4334. Bring 5 - 8 bisqued pieces, made of porcelain, groggy stoneware, or a body made specifically for raku. Wear cotton clothing & closed-toed shoes.