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a gb free ^06 white crackle glaze

updated thu 25 may 00


Bruce Girrell on wed 24 may 00

This past weekend was glaze mixing day at the Girrell household. We've been
using a white crackle glaze that was OK, but just didn't make our hearts go

I looked back through my pile of printouts of glaze recipes. Lo and behold
there was one that didn't call for gerstley borate. I mixed up a couple kg
of it and another glaze that did use GB and fired them together on a scrap
bowl and some test tiles.

The recipes:

Ferro Frit 3134 100
Silica 6
EPK 10
Tin oxide 10
Pinch of bentonite (I used about 2 Tablespoons)

Gerstley borate 65
Tennessee ball clay 5
Nepheline syenite 15
Flint 5
Tin oxide 10

Other possibly important stuff:
The clay body for the test pieces was Great Lakes Clay raku body (I forget
the exact name, but it's the one with only fine grog). All test tiles and
the bowl were fired together and all placed in the same reduction bin.

The results:
Both recipes produced a nice crackle on the bowl, but the GB free recipe had
a larger mesh size, i.e., the cracks were further apart. The glaze that we
had been using produced a crackle that had too fine of a crackle, so this
was a good thing. The GB recipe gave a brighter white and I attribute that
to the thickness of the glaze coat. Frit based glazes just don't seem to
adhere as thickly.

The results on the test tiles were not as telling. The tiles had a lot of
texture, which in my experience tends to reduce crackle, and they had been
washed prior to glaze application, so the glaze was not going on thickly.
The ones that did show crackle were mostly the GB free recipe with several

Melting of the GB-free glaze was nice and uniform without any bubbling.
Despite the high frit content, the bucket properties are pretty good.
Apparently, the EPK and bentonite provide enough suspension for the frit so
that it doesn't sink and harden. The GB recipe showed the usual bubbling
during the firing. The GB-based glaze goes on the pot nicer with a nice
thick uniform coat. The frit glaze goes on uniformly, but tends to be

All in all, the GB free recipe seems to be a winner. I added a little more
opacifier to boost the whiteness and if we take the time to get it on
thicker, I'm sure it will do well. More tests tonight.

Credit where credit is due dept:
The recipes were from a large collection put together by Dewitt Gimblet. See for the whole list. The GB based
recipe is called Basic White Crackle and is attributed to Steve Branfman.
The GB-free recipe is named Tak and was collected from mel jacobson, who
credited it with this note: "is a kurt wild came from a student
named takahara. it is nice...we have all used it."

All for now.

Bruce and Lynne Girrell
in northern Michigan getting excited about going to Europe together.
Sometimes that day job has its benefits.