They actually are two completely different glaze bases. The crackle pattern in both is caused by the same basic situation however. The crackle patterns in the two of them are subtly different also, although in the photos that is hard to see.
There is a property called the
Coefficient of Reversible Thermal Expansion......usually abbreviated as C.O.E. in ceramics. When the glaze layer shrinks more than the underlying clay body as the kiln cools (has a higher COE), the glass which is then in tension, fractures. That is what crazing actually is.
I use molecular calculation to formulate the glaze chemistry after firing to put that COE expansion number in the "9" range (I won't go into the units or the "rasied to powers" here) when the normal glaze fit for my clay body is about 7. If I get it up to 9, I always will get that kind of crackle.
Then the grey-green glaze also has reduced red iron oxide in it, whereas the "purple" one does not. The "purple" one actually came out of the high fire glaze kiln as a milky white glaze. Exactly the same as this one:
Then gold chloride with fluxes was painted onto the white piece in a very, very thin layer. The properties of fired gold when it is present in a molecularly thin application cause the light to refract and be seen as purple instead of the expected gold color. It also stains in the craze lines,.......and is a fired in color... not like tea staining.
Hope that explains it. Too much science... takes away the marvel and the magic
. And thanks for the very kind adjcetives!!!!
best,
................john
