Don Goodrich on fri 15 sep 06
Yikes, Scott!
If you want your sister to have many more birthdays,
refrain from making that mug as you've suggested.
First, using an underfired raku clay is asking for trouble.
If a clay body is rated from low-fire up to cone 10,
someone is lying to you. If it really matures at ^10,
it's underfired at cone 6 and will likely remain porous.
If the glaze does craze, this will allow moisture into the body.
Heating coffee in the microwave may well cause the mug to
fail somewhat violently. (John H. has advised us against
careless use of the word "explosion", so I'm avoiding it.)
It would be best to use a stoneware or porcelain body that's
formulated for cone 6, if that's your temperature of choice.
Second, the glaze recipe you gave doesn't appear to be
stable. I've never used Wayne's Blue, but its
Alumina:Silica ratio is 1:6.53. This is likely to
allow some leaching of cobalt or copper.
Why 6 percent of either? 3% of cobalt will give a really
dark blue, and you could probably cut back on the copper
as well. If you must have colorants on the inside of a
drinking vessel, perhaps it would be better to find a clear
base glaze with an Al:Si ratio closer to 1:10, and add
colorants to that. Test to find out if it crazes before
using it on a mug.
Hope that birthday isn't tomorrow...
Good luck,
Don Goodrich
Scott Hunnicutt on fri 15 sep 06
hey all,
I am making a coffee mug to give to my younger sister for her birthday and
I was thinking of glazing the inside with wayne's blue. I am working with
raku clay which is rated from cone low fire(06, I think) to high fire cone
10. I have tested the glaze on loafer's glory and it crazed. Has anyone
fired wayne's blue to cone 6 on raku clay before? and did anyone have
crazing on the piece? if you did have crazing did you alter the original
formula?
here's the original recipe: kona f-4 44%,gerstley borate 26%,ball clay
15%,silica 10%, and dolomite 5%.
I added 6% cobalt to make a deep blue or 6% copper carbonate for a dark
green.
thanks for any answers,
Scott hunnicutt
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