Pottery by John on thu 15 jul 10
During a raku glazing and firing session the past weekend, I ran across an
unexpected problem. I made up a clear crackle glaze using the typical 80 t=
o
20 Gerstley borate to Custer feldspar ratio to glaze some pieces. For my
"best" piece, that would have some previously proven copper bearing,
low-fire commercial glazes for leaves and stems, I thought that having a
yellow crackle glaze would better suit the pot. I found a recipe
modification in Steve Branfman raku book, so, I added a tablespoon of Mason
6404 Vandium stain and about half that Tin Oxide (for brightening) to a wet
cup of the clear crackle glaze. It responded with a pleasing, just a shade
duller than lemon, yellow color for the wet glaze. I brushed on the
commercial glaze and my yellow crackle glaze, and after a suitable drying
period placed 4 pieces into my propane fired kiln. Three were glazed with
the clear crackle, and one with the other mix.
When the glazes all matured, the three clear crackle pieces turned out as
expected; nice crackle over a white clay. The other piece turned out with
coppery leaves and stems, but the crackle glaze was a sage green (which I
like) with no crackle at all. Anyone have thoughts as to why green instead
of yellow? and no crackle?
I researched the archives and found information about how bad vanadium is,
but nothing was there about unexpected color issues.
John Lowes
Sandy Springs, Georgia
http://wynhillpottery.weebly.com/
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