TECHNO TIP: VARIEGATING GLAZES

While industry avoids so-called ‘reactive
glazes’ (for consistency problems) potters
avoid the ‘porcelain sink’ look and con-
stantly seek ways to get glazes with sur-
face variations in colour
and texture. Here are some
ideas on how to give drab
glazes an exciting vari-
egated or mottled surface.

Highlighting: Employ
‘highlights’ of the same
colour but in a different
shade. For example, vary-
ing the thickness of a trans-
lucent (partly opacified)
coloured glaze will vary the
intensity of colour (espe-
cially where it thins on the
edges of sharp contours).
Variation can be achieved
by pouring, double dipping,
brushing, waxing, amd in-
cising techniques,

Physical means: Create variegation by
splatter-spraying or sponge-stippling asec-
ond layer of glaze with a contrasting col-
our of shade (or the same colour in a more
matte or glossier version). A tngger or
pump operated garden sprayer works well
for this. Use a fluid glaze over a more
stable matte (where spots tend to “feather”
into the underlying layer) of vice-versa
(where they tend to sit on top). While
brightly coloured varicgated glazes look
grew, subtle variations can be very effec-
tive. An example is a glossy blue speckle
or pattern on a matte blue background,

Surface Crystal Growth: Very fluid
glazes are a must in forming crystals on the
surface during cooling (low alumina, high
flux). TiO2 materials like titanium dioxide
and rutile seed crystal networks encourage
their growth. A thin rutile wash applied to
a glaze surface can act a5 a crystal growth
catalyst. High calcium and boron encour-
ages the formation of calcium-borate crys-
tals, high zine plaxes also crystallize when
they are Muid, The addition of up to 4% tin
in such glazes can magnify the effect.
Slow cooling greatly enhances crystal
growth. Small amounts of lithium (e.g.
1%) can have a remarkable variegating
effect on rutile glazes, especially when
colourants like iron are present. Incustry

by Tony Hansen, Plainsman Clays Ltd

calls crystal growth on glaze surfaces
“devitnification’.

Specking Agent: You can add acolounng
oxide that contains
particulate matter that
speckles the glaze surface,
Manganese granular,
ilmenite, and granular
rutile are examples but
these materials are heavy
and tend to settle in glazes
that are too fluid. Ondi-
nary iron oxide and cobalt
oxide often produce small
specks in unmilled glazes,

Multi-layering: Double
layering of differem glazes
produces variegation well
when the lower layer is
more fluid and the upper,
stiffer glaze tends to break
into islands revealing rivu-
lets of the lower one. Be aware of the
problems associated with double layer glaz-
ing (cracking and crawling during drying
because cither layer shrinks too much, is

applied too thick, or onto wet ware). Use
glazes with lower or less plastec clay con-
tent for multi-layered work and check out
www.ravenscragslip.com,

Phase Differences: The glass matrix in a
fired glaze can separate dunng melting
forming globules of different glass chem-
istry. These reflect light differently and
thus variegate. “Techies” look for chemis-
tries [hil encourage phase separation,

Combinations; Use combinations of the
above to variegate surfaces even more.
The popular Floating Blue recipe is a good
example. [ts colour varies with thickness
s© it highlights irregularities in the sur-
face, Phase separation in the translucent
matrix makes the colour ‘swirl’ in patterns
of blue. Titanium crystals in the matrix
make it sparkle. The growth of calcium-
borate crystals on the surface appears to
Noat over a deep bluc background,

Knowing the ‘mechanism’ of the variega-
tion in your glazes is important to enhanc-
ing and controlling them, and it will get the
admiration of people who see your work.

CALL FOR SUBMISSIONS
GRANVILLE ISLAND PUBLIC MARKET
ANNUAL CRAFT ADJUDICATION

Craftspeople interested in selling their work at Granville Island
Public Market should submit four samples on
Sunday, February 18 9:00-17:00
1218 Cartwright Street on Granville Island, Vancouver, BC

Samples are judged in categories, depending upon items received.

Crafts NOT considered are clothing, crafts containing non-CSA approved
electncal components, mass produced or manufactured items, kits or goods
made from kits, imported goods and scented goods unless scent is enclosed.

There will be a $15 cash registration fee per category entered.
Applicants will be setting up their own display.

For display criteria or other information please call
Market Coordinator 604.666.6477 option #4

Potters Guild of British Columbia

February 2001