Colors in glazes are effected by the clay, slips,
stains, or underglazes below them. Most ceramic colors, however, are a
result of metallic oxides being dispersed in the fabric of the glaze
itself. Under varying circumstances, these colorants can give very
different results. The Three Main Factors Effecting Glaze ColorThere are a number of variables that can effect a glaze's color. These mainly fall within a group of three major factors. - The
composition of the glaze. This not only includes the colorants in the
glaze, but also other glaze materials that interact and effect that
colorant or combination of colorants.
- The temperature to which
the glaze is fired. Some colorants are volatile and will dissipate into
the kiln atmosphere if fired too high. Others give different colors at
different temperatures.
- The kiln's atmosphere during firing and, in some cases, during cooling.
Chromium OxideChrome oxide can yield a
variety of colors: red, yellow, pink, brown, and especially green.
Chrome is volatile at cone 6 and above and may jump from pot to pot
causing streaks and smoky effects. - Chrome-red: needs lead glaze fired at cone 08 or below. Extremely toxic; not for functional ware.
- Chrome-yellow: needs lead-soda glaze fired at cone 08 or below, or will begin turning green. Extremely toxic; not for functional ware.
- Chrome and zinc yield brown.
- Chrome
plus tin yields pink, grayed pink, and warm browns. Color depends on
proportions of these oxides in glaze and in relation to each other.
- Small
amounts of chrome plus cobalt can yield teals at cone 9 and higher when
fired in reduction. Magnesia glazes aid in producing nice colors.
Cobalt Oxide and Cobalt CarbonateCobalt
is an extremely powerful colorant that almost always produces an
intense blue. Cobalt carbonate tends to be used more by potters because
it has a finer particle size and is less intense. - In glazes with a high magnesia content, very small amounts of cobalt can give a range from pink through blue violet.
- Magnesia
and cobalt in glazes fired at cone 9 or higher can yield blue mottled
with red, pink, and purple. Very hard to control and duplicate due to
the narrow temperature and atmospheric range.
- Cobalt and rutile can produce in mottled and streaked effects.
- Cobalt with manganese and iron will yield an intense black.
Copper Oxide and Copper CarbonateCopper is a strong flux
which can make a glaze more glossy. At cone 8 and above, copper is
volatile and can jump from pot to pot. Copper generally gives green in
oxidation and red in reduction. Copper oxide is more intense than copper
carbonate, as it contains more copper by weight. - In alkaline glazes, copper will produce turquoise.
- Copper yields a lovely range of greens in lead glazes. Copper increases lead's solubility. Toxic; not for functional ware.
- Copper in barium high-fired glazes produce intense blue and blue-green in both oxidation and reduction. Toxic; not for functional ware.
- Copper in low-fire raku glazes can yield metallic copper. Over time, however, the glaze will oxidize to green.
Iron Oxides in ClayNot
many potters would challenge iron's place as the most important of the
ceramic colorants. Iron's natural presence in most clay bodies produces
clay colors ranging from light gray to deepest brown. Under clear
glazes, iron-containing clay bodies can show a very similar range of
colors. Iron-containing clay bodies that have been fired but are not mature, such as bisqueware,
often are a salmon or yellowish pink color. If a pot is glazed with a
lower-temperature glaze and fired below the clay body's maturity
temperature, a salmon, ocher or reddish brown color will show through. Iron Oxide TypesMost iron used in glazes is introduced as red iron oxide (ferric oxide, Fe2O3).
Yellow iron oxide is another form of ferric oxide; although its raw
color is different, it is chemically identical to and acts the same as
red iron oxide. Black iron oxide (ferrous oxide, Fe3O4)
is courser and generally not used. Crocus martis is an impure iron
oxide which can be used to produce speckled, rough, or spotty effects. Iron Oxide in GlazesGenerally speaking, iron produces warm colors ranging from light tan and straw to deep, rich browns. - High-fire glazes containing bone ash and iron can yield persimmon reds and oranges.
- Iron and tin in high-fire glazes result in a mottled cream color, breaking to red-brown in thin areas.
- Iron fluxes in reduction atmospheres. It is less active and can sometimes even act as a refractory in oxidation atmospheres.
- Iron in high-fire reduction can yield lovely, delicate iron-blue and celadon green.
- High-fire,
high-iron content glazes fired in reduction will yield glossy dark
brown or brownish black. In thin areas, the iron may reoxidize during
cooling. Reoxidization will result in those areas turning red or gaining
red highlights.
Manganese DioxideManganese
is usually introduced into glazes as manganese carbonate. Black
manganese dioxide is more often used in slips and clay bodies, where its
coarseness yields spots and splotches. Manganese, when compared to
cobalt or copper, is a fairly weak colorant. It is toxic; handle with
caution, using all safety precautions. - In high-alkaline glazes, manganese yields rich blue-purple or plum.
- At cone 6 and above, manganese produces brown.
- In lead glazes, manganses yields soft purple tinged with brown. Extremely toxic; not for functional ware.
Nickel OxideNickel oxide, when used by
itself, gives notoriously unpredictable results. It can be used to
produce quiet grays and browns, but nickel is almost always used to
modify and tone-down the colors produced by other colorants. RutileRutile
is an impure titanium ore containing some iron and other materials. It
is a very interesting colorant which is generally tan in oxidation and
gray in reduction. Rutile encourages crystal growth in mid-range and
high-fire glazes. It is well known for creating lovely streaky and
mottled effects. - In boron-containing glazes rutile produces pronounced streaks or spots, especially in glazes containing other colorants.
- In fluid glazes, rutile encourages opalescent blues.
- Rutile increases opacity.
Other ColorantsOther colorants that are used less often include: - Antimony: used for yellow in low-fire glazes.
- Cadmium and Selenium: very similar, producing bright reds. Both burn out extremely easily. Toxic; not for functional ware.
- Gold: gives a range of pink, red and purple.
- Ilmenite: as a colorant, very similar to black iron oxide.
- Iron
Chromate: produces shades of gray, brown and black. Iron chromate plus
tin may produce a pink or reddish brown; if applied with a brush, can
yield black hazed or haloed by pink. Toxic; handle with care.
- Platinum: gives gray.
- Silver and Bismuth: used in luster overglazes.
- Uranium Oxide: gives red, coral and yellow colors. Note: even fired into a glaze, uranium remains radioactive. Toxic; handle with care.
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