Low Alumina Nickel Colors
firing to cone 10 in oxidation
slow downfire at 50 deg F an hour in the interval 1850 deg F to 1700 deg F
slow downfire at 25 deg F an hour in the interval 1700 deg F to 1650 deg F
One hour hold at 1650 deg F
All of the glazes shown here have relatively low alumina and silica, and might be described as base heavy glazes.
Additionally these glazes are all low in both calcia and magnesia.
In the seger formula of these glazes, Alumina varies from .23 to .35, and silica from 1.4 to 2.4.
We draw the line between low and moderately low alumina at .3, low and moderately low silica at 2.
Thus a glaze is described as having low alumina if its alumina is < .3, similarly as having low silica if its silica is < 2.
The descent is long, yet these glazes are all derived from the glaze number 404 in Emannuel Cooper's book "The Potters Book of Glaze Recipes".
Blue glazes
glaze cooper_404_2MZn, porcelain bowl ~ 3 inch diameter
This glaze has moderately low Alumina and moderately low silica with bases which are predominately Lithia and Baria.
This is a moderately saturated teal blue, glaze is well matured though pinholes if thick.
glaze cooper_404PAlBMAlk, porcelain bowl ~ 3 inch diameter
This glaze has low alumina and low silica, its bases are predominately Zinc and Baria, with no lithia. Besides having lower silica than above glaze it also has a lower silica alumina ratio.
purple / mauve
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glazes cooper_404_2Li, cooper_404_3Sr, cooper_404_9_0
These three glazes are similar in having both moderately low alumina, and moderately low silica.
The bases of cooper_404_2Li are dominated by Baria and Lithia.
The bases of cooper_404_3Sr are dominated by Baria with the reamainder split between strontia and lithia.
The bases of cooper_404_9_0 are dominate by zinc, followed by baria, then Lithia.
It is likely that the more subdued color is the effect of higher alumina, and the concommitent increase in silica.
Cranberry Colored Glazes
glaze cooper_404_PALB, porcelain bowl ~ 3 inch diameter
This glaze has low Alumina, low Silica and low Alkali metals.
The bases are split between Baria And Zinc with some Boron
glaze cooper_404PAlB_Ball, porcelain bowl ~ 3 inch diameter
This is the above glaze with Ball Clay substituted for EPK Kaolin
The bases are virtually the same, Alumina is a bit lower, silica a bit higher, giving a higher Silica:Alumina ratio.
The evidence is that one can get intense and varied colors with Nickel in c/10 glazes which are low in alumina and silica.