Hanne Bjorklund on tue 13 feb 01
I too was striving for a large white Raku crackle, and found a website that
solved the problem.
Try burnishing your pots before subjecting them to the traumatic journey
through a Raku firing.
The theory is that with an extremely smooth surface, there is nothing for
the glaze to ' hold on to' when it shrinks, and a large crackle develops.
I tried this on spherical pots, thrown in a very fine (200 mesh) white
clay, and it worked!
I let them air cool for a few minutes to develop the crackle, and then
placed them in a container full of shredded paper for the smoking and left
them for half an hour,-- maybe more. Lovely BIG crackle.
Don't dunk them in or spray them with cold water. If you do, the crackle
shatters into tiny bits and you end up with a mottled grey pot.
So now I am looing for a new mountain to climb.
This was the recipe I used:
WHITE CRACKLE SHAPIRO
Gerstley Borate 63%
Barium Carb 6%
terryh on wed 14 feb 01
my default crackle white glaze is "colemanite 80, neph-sy 20, plus opacifier
10". i change the Colemanite/Neph-Sy ratio a little to change the crackle
size. for instance i use "colemanite 75 (70), neph-sy 25 (30)" for a little
larger crackles. actually, i use both together to exploit different
crackle-size distributions. the melting temperature changes as the ratio
changes, so i cannot change the portion too much if i want to use both
together. consequently, neither i can change the size significantly.