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help! my copper has gas...

updated sun 24 jan 10


Melissa Schooley on fri 22 jan 10

I've been testing several glazes looking for a nice, icey blue,
translucent cone 6.=3D20
I've been using copper carb and cobalt carb, in amounts of 1% and 0.1%

No matter WHAT base glaze I use, I'm getting some serious bubbles in my
glaze. Any color that I mix up that does NOT contain the copper, is
smooth and glassy. As soon as I add the copper, I get thousands and
thousands of teeny-tiny bubbles all throughout the glaze.=3D20

I understand that copper carb can give off lots of gas, so I tried
switching to copper oxide and cobalt oxide but that didn't help either.
What the heck is going on?

Here's the base recipe that resulted in the LEAST amount of bubbles (but
still WAAAAY to many to yeild a clean, translucent glaze):

Whiting: 15.8
Frit 3134: 23.2
EPK: 21.1
Silica: 34.9
Custer feldspar: 2
Nepheline Syenite: 2.9

It was fired to the following schedule:

Ra 1: 200'/hr to 212', hold 20 mins.
Ra 2: 375'/hr to 2000',=3D20
Ra 3: 212'/hr to 2188', hold 20 mins
Ra 4: 999'/hr to 1832',
Ra 5: 100'/hr to 1382'

As I stated, other colorants don't cause any problems. The glaze comes
out beautiful, bright and clean. No bubbles what-so-ever. Is the copper
raising the melting point of the glaze? I thought copper was a flux and
might lower the melting point but the glaze doesn't look overfired - I'm
not getting large blisters or anything like that. Is it volatizing? I'm
nowhere near cone 8 which is where I thought copper did that. Could it
be my bisque? - But why then, don't the other colors bubble?

I understand that whiting can give off lots of gases, and tried a base
with wollastonite in it rather than whiting but it actually looked

Any help would be GREATLY appreciated!

Melissa Schooley
Raging Bowl Pottery

Handmade Porcelain Celebrating the Art of Fine Craft

ivor & olive lewis on sat 23 jan 10

Dear Melissa Schooley,

If you check the chemistry text books you may find the black Copper oxide i=
unstable and decomposes to oxygen and red Copper oxide at your firing

Best regards,

Ivor Lewis,
South Australia