Pottery In Australia Vol 38 No 3 September 1999

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interesting though such crystals are in themselves,
has in most cases, contributed as little to the
aesthetic significance of the piece as it has to the
function.' Subtlety and crystalline glazed pots could
be seen as contradictory. Herein lies the challenge.
Technically the development of crystalline glazes
depends on many variables, some of which
include:
colour and type of clay body: White firing,
dense porcelaineous type bodies are more
suited to crystalline glazes as darker bodies
adversely effect the glaze colour.
• The type of surface: Crystals develop differently
on vertical and flat surfaces. Forms may need to
be modified to suit to the glaze.
• Fluid glazes: For crystal growth, the glazes need
to be fluid with a viscosity which must
encourage the growth of crystals. There is
minimal alumina in these glazes and fluxes
which promote crystal growth include zinc,
odium, potassium, barium, magnesium and
lithium. A fluid glaze is formed in which the
oxides move freely. When cooling tarts, motion
within the glaze slows down, and period bonds
form as crystallites. These act as nuclei around
which larger crystals may form. The slower the
cooling rate the larger the crystals may be. Some
oxide work better than others. Appropriate
setting procedures are required for pots before
all of your shelves are ruined with glazes that
have run off the ware. A base-width collar and a
saucer to catch glaze runoff can be useful.
• Glaze application and thickness: Glazes can be
brushed, dipped or sprayed, but spraying offers a more
sub~:e way of varying the glaze thickness, especially
towards the base of the pot.
• Temperature and the length of the firing - I fire slowly
up to full cone 10 in a neutral atmosphere, (60°C /hour
from 1100°C to cone 10), and soak for around 10
minutes to even out temperature differences within the
kiln, then turn off the gas burners which are relit at
1090°C. This temperature is then maintained for 3-4
hours before it is shut off and closed up. The total
firing time is around 15-16 hours.
The temperature at which the kiln is held determines
the shape of the crystals such that at 1140°C. needle
GLAZES FOR CRYSTAL DEVELOPMENT-
CONE9-10:
1. Frit4110 46 Addition:
Silica 21 Cobalt carbonate 0.5
Zinc Oxide(Dense) 27 Copper carbonate 2
Kaolin 1
Titanium 4.5
2. Pot Felspar 28 Addition:
Barium Carb. 5 Cobalt carbonate 0.5 to 1%
Dolomite 4 Iron Oxide 2-4%
Zinc Oxide 20
Titanium 3-7%
Lithium Carb.
7
Kaolin 3
Silica 33
3. Frit 4110 47 Addition:
Zinc oxide 25 Cobalt carbonate 0.25
Silica 20 Manganese dioxide 1
Titanium 6.5
Kaolin
Alumina hydrate 0.5
Other possible colourants:
Cu C03 1-3% light green
FeO 2-8% red. 1 orange
MnO 0.5-4% beige, irn1y
like crystals form; at 1100°C, the c1ystals are battle-axe
shaped and at 1050°C the crystals are round. I find the
glazes benefit from a longer slower firing.
• Colourant: The main colouring agents are red iron
oxide, copper oxide and carbonate, cobalt carbonate,
nickel oxide, manganese oxide, rutile, illmenite and
vanadium. Often two or more can be used in
combination. oo
Peter Wilson who is a potter and lecturer at Charles Sturt University
in Bathurst, SW.
READI:'\G
Rhodes, D. Clay and Glazes for the Potter
Hopper, R. Ceramic Spectrum
Creber, D. Crystalline Glazes
Thanks to Peter Deck and Ted Secombe
38/3 SEPTEMBER 1999 + POTTERY IN AUSTRALIA 25