TECHNO TIP

UNDERSTANDING CERAMIC STAINS

Stains are fired blends of metal and ee-
Tamic oxides that have been reground into
afine powder. Stains containing otherwise
oxic oxides can be employed without
significant dangers. This is the first aspect
of something that stains have that colour-
ing oxides don’t: stability. A second as-
pectal stability is thut stains produce much
mere consistent and repeatable colour than
USING raw oxide colours.

Slains anc most popular at lower tempera-
turcs Where colours tend §o be brighter.
However mast stains can be used night up
to high tire. Premixed low fire glazes are
typically made by blending stains andeom-
mercial fit and other than following fir-
ing instruchons, users of these products
aive litthe thought to the
technical challenges that
Wert overcome to produce
them. This is a third key
advantage of stains: tbe
abalily to target a specific
colour. Many colours like
reds are difficult toachieve
and bevond the abilities of
end users,

While many stains are
Standard and their compo-
aliton is well known, oth-
ers are proprietary, Stain
companies don’ trelease the:
exact makeup of stains but
they do tell us the system.
For example, a green stain
might contain chrome, oo-
balt and silica and be la-
belled CrCoSi. Although
silica is nota coborant itsell, itis included
to create a stable silicate crystal structure
with the ather van,

Stain companics often supply multiple
products to produce a given colour using
different oxide systems (Le. chrome-tin
pink, manganese-alumina pink). Various
factors like the base glare chemistry, col-
cur shade, lemperature, and end use deter-
mine the system you should choose. Kunow-
ing how to calculate the oxide makeup of
a glaze or dealing with 4 stain company
thal provides good servioe are key factors
to being able to troubleshoot colour prob-
lems with stains,

February 2000

Tony Hansen
Thus stains do nat come with a unconkdi-
honal colour guarantee, The shade pro-
duced depends on many factors including
the host glaze chemistry, on/over!
undéerglaze use, glaze thickness, anvount
of opacifier, firing temperature, etc.
Actieving colours with stains is certainly
cusicr, but it is aot a no-brainer. Certain
systems are quite Hexible and produce
colour in many Kinds of glares (ie. cobalt
silicate). Other systems either require that
ceriun ovjides be present in thee ost glaze
in MIMmUmM wmounts or others not be
present at all. The symbiosis of host glaze
chemistry and stain, for example, can be
demonstrated with chrome-tn slains, They
Will net develop colour if zine is present or

pts: a8 ee

JULIA MAIKA majolicn pottery from ber exhibition Season to Taste
March (964 a4 the Gallery of BC Ceramics

if there is inadequate calcium, [t ts com:
mon to hear people say thal their pink stain
burned out, but generally the stain is being
used in an incompatible glaze base. An-
other interesting demonstration of these
factors is the colour chart of a typical stain
company. These charts show the stun used
Inone ar more fitted buse glazes that are
sclected to be compatible with as many
colours as posable. Certain samples will
also have whded opacilter and zinc, for
example.

Admitiedly, stains can produce homoage-
neous colour which can be less interesing
than the variegated and speckled colour

Potters Guild of British Columbia

elferts thatcan be acheeved Wilh maw metal
oxides hike iron, cobalt and copper, But
for manufacturing, tt cannot be ignored
thal stains are far more Comastent and
reliable to wee,

Potters love to paint stains over and under
plates to decorate wart. Majolica ware is
a good example. However, keep in mind
thal stains vary in thear willingness to
allow the underglaxe to envelope the cal-
our particles. Some tend to crystallize the
surface if used overgluve. Likewise, il
stains are used undergliute they vary in
their willingness to allow the overgplave to
penetrate through to form and interface
with the body. Stains don’ tsuspend well in
Waler to create a paintable material either.
Consequently 1015 necessary
to mix them with @ compat:
ible and paintable slave me-
dium for doing brushwork

Stains exist either in the con:
text of the huge industrial
ceramic industry of in the
hobby, pattery, and ceramics
markets. Large industnes ci-
ther have in-house technical
people or contract consull-
ants. Small users do mot have
this luxury. They should
know thal certain stain com-
panies, (i.c. Mason) have de-
Yelopedexcellentreputatons
for dealing with smaller yvol-
umes and providing support,

A fired glaze can leach heavy

metals Whether these metals

ae sourced from a stain or
from raw metal oxides. You should have
your glares tested for leaching if you are
making functional ware having stained
food surfaces. There are many facteurs that
determine if a glee is lewchable. For more
information sce the techno tip column in
the last newsletter or visit http:/!
digitalfire.com/education/ glaze!
leaching. htm

This fechag fip iv generously sponsored by
Plaincean Clay Gonited in Medicine Hat, If
[ou lurve a tectvtinnl quenion plecse septal i he
the Candle affice atenion the edinar