Rick Mckinney on wed 27 may 98
Clayarters,
A question for the glaze gurus out there. In different glazes at different
times I have noticed that CuCo3 sometimes creates a gray to black darkness.
For instance in the Pinnell Sr Matt often listed on this listserv (a
turquoise matt) if it applied thinly, then the glaze is semi-opaque and very
dark gray to black. But if it applied thickly, then it is a nice turquoise
matt, though it might still have some dark streaks or areas. This can occur
with Soda Green glazes that derive their greenness from CuCO3 as well. What
I don't understand is where this black comes from.
The recipe for the Pinnell Sr Matt mentioned above is as follows:
Lith. Carb 1.0
Sr CO3 20
Neph. Syenite 60
M23 10
Silica 9
TiO2 5
CuCO3 5
Thanks for any help or thoughts on this matter. -=Rick=-
Rick McKinney
1321 Montclaire Ct.
Appleton, WI 54915-2803
Tel: 414-830-2226
Email: mcki0023@dataex.com
Ron Roy on thu 28 may 98
Hi Rick,
Hamer says 5% so fills the glaze that some copper cannot be held in
solution and comes to the surface. Somethimes it can be rubbed off. So the
black is the pure copper oxide on the surface of the glaze - easily
disolved by acid foods and will discolour. There have been reports on
ClayArt of this happening with some oribe glazes. If you don't want it -
reduce the amount of copper in the glaze till it stops and then test with
acid foods like vinegare - at room temp for 24 hours to see if the copper
is reducung the durability of the glaze.
What kind of glazes do you want to work on during the course at Canador by
the way?
RR
>----------------------------Original message----------------------------
>Clayarters,
>
>A question for the glaze gurus out there. In different glazes at different
>times I have noticed that CuCo3 sometimes creates a gray to black darkness.
>For instance in the Pinnell Sr Matt often listed on this listserv (a
>turquoise matt) if it applied thinly, then the glaze is semi-opaque and very
>dark gray to black. But if it applied thickly, then it is a nice turquoise
>matt, though it might still have some dark streaks or areas. This can occur
>with Soda Green glazes that derive their greenness from CuCO3 as well. What
>I don't understand is where this black comes from.
> The recipe for the Pinnell Sr Matt mentioned above is as follows:
> Lith. Carb 1.0
> Sr CO3 20
> Neph. Syenite 60
> M23 10
> Silica 9
>
> TiO2 5
> CuCO3 5
>
>Thanks for any help or thoughts on this matter. -=Rick=-
>
>
>
>Rick McKinney
>1321 Montclaire Ct.
>Appleton, WI 54915-2803
>
>Tel: 414-830-2226
>Email: mcki0023@dataex.com
Ron Roy
93 Pegasus Trail
Scarborough, Ontario
Canada M1G 3N8
Tel: 416-439-2621
Fax: 416-438-7849
Web page: http://digitalfire.com/education/people/ronroy.htm
Tom Buck on sun 31 may 98
Rick: The chemistry of copper in glazes is something like this:
Below 200 C, copper carbonate basic stays as such (green powder).
Above 200 C, the carbonate decomposes and the stable copper compound is
now Copper II Oxide, or CuO, which will show as black under some
conditions.
Above 1026 C, the black oxide loses oxygen and becomes Copper I
Oxide or Cu2O, which shows as red. And it is stable to 1800 C (above that
reached by all pottery kilns).
In a reducing atmosphere, CuO and Cu2O will, in steps, be
converted to Cu metal (hence, the bright penny colour of some raku
glazes).
In an oxidizing atmosphere, there will be a balance between CuO
and Cu2O such that some copper atoms will fit into the glass network
polymer and some will "dissolve" in the molten glass. In such a case, the
resultant glaze (assuming a clear base) will transmit light in the green
part of the spectrum, ie, the glaze will be green.
The molten glass can only hold so much dissolved copper oxide, so
any excess will be dispersed rather than dissolved, yielding perhaps a
colloidal dispersion or "solution" that could on cooling see CuO formed
again and if clumped the black will be evident.
So, if you are seeing black on your pot, chances are good that
your glaze contains a lot of copper carbonate.
Tom Buck ) tel: 905-389-2339 & snailmail: 373 East
43rd St. Hamilton ON L8T 3E1 Canada (westend Lake Ontario, province of
Ontario, Canada).
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