copper Red with magnesium part 2
Here are three local reduction copper glazes.
Previously I'd shown the effect of higher viscosity resulting from high
alumina and low silica on local reduction copper glazes.
Then I'd shown the effect of increasing the MgO in alexanderBowl_Z15_0.
Now I show the effect of increasing MgO and Copper in another of these
glazes. MgO is increased via a corresponding decrease in CaO.
As with alexanderBowl_Z15_0, the
the surface of the glaze has separated into a viscous layer with a fluid
component bubbling through. The viscous component has become
150 deg F an hour to 250 deg F
400 deg F an hour to 1800 deg F
300 deg F an hour to 2050 deg F
120 deg F an hour to 2310 deg F with a hold of 20 minutes at 2310 deg F
300 deg F an hour to 1750 deg F then a half hour hold at 1750 deg F
300 deg F an hour to 1700 deg F then a Three hour hold at 1700 deg F
25 deg F an hour to 1650 deg F then a one hour hold at 1650 deg F
The glaze with added .5% copper carbonate, 2% tin oxide, and 1% silicon
carbide.
K2O .13
Al2O3 .48
SiO2 2.64
molecular percent Silica 64%
The glaze with added 1% copper carbonate, 4% tin oxide, and 2% silicon
carbide.
K2O .13
Al2O3 .49
SiO2 2.67
molecular percent Silica 64%
The glaze with added 1% copper carbonate, 4% tin oxide, and 2% silicon
carbide.
K2O .17
Al2O3 .49
SiO2 2.63
molecular percent Silica 64%
alexanderBowl_Z23_0, the higher MgO version of alexanderBowl_Z15_1, shows
minimal red and has a coarser texture.
alexanderBowl_Z23_1, the higher MgO and higher alkali metal version of
alexanderBowl_Z15_1, is nearly indistinguishable
The green markings seen in the higher MgO versions of alexanderBowl_Z15_1 is
the green of an oxidation copper glaze.
However this shift in color is similar to that seen in part 1, in the glaze
which had only increased MgO.
Alternatively, perhaps this is the result of non-reduction of the copper in
the glaze, rather than later re-oxidation.
more
prominent, with the red nearly disappearing. A deep grass green is visible in
the clear fluid part of the glaze.
Glaze alexanderBowl_Z15_1
derived glaze alexanderBowl_Z23_0 with higher MgO and higher copper
carbonate
derived glaze alexanderBowl_Z23_1 with higher MgO, higher Alkali metals
and higher copper carbonate
bowls are ~4 inches in diameter
oxidation firing to cone 10 in an electric kiln
Firing profiles
Up Fire profile
Down Fire Profile
Clay body is a grolleg porcelain from Tacoma Clay Art Center.
glaze compositions
Empirical Formula glaze alexanderBowl_Z15_1 :
Na2O .05
CaO .63
MgO .19
Empirical Formula glaze alexanderBowl_Z23_0 :
Na2O .04
CaO .55
MgO .28
Empirical Formula glaze alexanderBowl_Z23_1 :
Na2O .05
CaO .49
MgO .29
Remarks
The prominence of the glossy state has
increased greatly and, where thick, the glaze is bubbly. A large mass of grass
green has appeared. This is seen in the second set of pictures.
from alexanderBowl_Z23_0,
which is higher only in MgO. This is seen in the last set of pictures.
This might be an artifact of the
excess copper in these two glazes.
It does seem that the increased MgO
affected the color of the copper.
Did the MgO interfere with the
reduction of the copper?