COLORANTS
The
term colorant is a broad based word for materials to permanently color ceramics.
These Include metallic oxides, carbonates, sulfates, and salts; raw stains;
calcined stains (commercial or homemade); and some nonmetallic. Colorants can be
used In clay, engobes, stains, overglaze, in the glaze, underglazes, decals,
washes, lusters, ceramic screen printing inks, porcelain enamels (metal
enameling), ceramic pencils, ceramic crayons, ceramic chalk, and crystal
formers. The metallics are: bismuth, cobalt, nickel, chrome, copper, Iron,
selenium, cadmium, uranium, manganese, zinc, and tin. The nonmetallics are:
cerium, erbium, neodymium, zirconium, and sulfur. They are used as they are, but
more often mixed with other colorants, mixed with and/or calcined with other
ceramic minerals. The major methods of coloring ceramics include the following
Colored
clay ‑
Colorants are mixed with the clay body.
Enoobe
‑ Colored fluxed
slip (engobe) is applied as a design or to give color and/or cover the clay
body. A glaze may be applied over.
Fuminc
‑ Metallic
carbonates, sulfates, and nitrates are painted, sprayed, dipped on green or
bisque ware for pit, saggar, or Raku firing. Like Egyptian paste the metallics
migrate to the surface to give flashes of color and luster effects.
On
the clay ‑
Colorants are applied by brush, spray, splatter, dip, pour, etc. to the surface
of the clay. An antique effect Is obtained by having the colorant applied then
washed off leaving the colorant in the recessed areas. A glaze may or may not be
applied over.
On
the glaze ‑
Colorants are mixed into base glaze for color. Some ceramic minerals that make
up the glaze have metallics (barnard clay, Albany slip, ash, earthenware clay,
etc.) that give glaze Its color (tenmoku, celadon).
Overglaze
‑ Colorants as
stains and washes are painted, airbrushed, splattered, stenciled, etc. to the
unfired or fired glaze. They fuse to the glaze once fired, but some will be matt
and may need a clear glaze applied over to give a sheen or gloss.
Overolaze
enamels ‑ Low
temperature glazes containing colorants are applied to a glazed surface and
fired at a low temperature to mature and fuse to the glaze.
Underglaze
‑ Colorants
(commercial stains or preparations) or washes are painted on as allover color or
designs. A transparent glazes Is applied over.
STAIN
A
stain is used to give color to and/or mixed with coay, engobe, or glaze or used
by Itself as a wash (over glazes, engobes, or clay body much like a watercolor),
underglaze color, and overglaze color. It is conducive to application techniques
of painting, spraying, screening, sponging, or used the same as the metallic
oxides. Stains like metallics, If applied too thickly, may crawl or act as a
flux or refractory..
A
stain is composed of a mixture of metallic(s), filler, flux, and alumina. This
mixture can ‑be used as Is (raw) or calcined. The purpose of calcination
the mixture of metallics and materials Is that the heat fuse the materials to
form a bond that is not affected by kiln temperature or atmosphere or the glaze
the same way that the metallics alone would be affected. Metallics like cobalt,
chrome, nickel, copper, Iron, etc. form colors in stains that by themselves
would not be possible. Thus the use of stains vastly Increases the color palette
and range. There are limitations and exceptions, particularly In the red,
yellow, and orange spectrum beyond a limited temperature and glaze composition.
Some stains are affected by the kiln's atmosphere and temperature.
There
is no set percentage of the various materials to make up a stain. The following
is averages of typical stains:
Materials
Percentages
Materials
Percentages
Metallic oxides, carbonates
4 to 85%
Alumina (china clay)
5 to 50°%
Flux (borax,
whiting, soda, etc.)
5 to 70°,6
Silica
0 to 50°ro
The
following typical stain formulas show the colorant, filler, stabilizer, and *calcining
temperature:
Blue Green Stain C/014*
Olive Green Stain C/014*
Leaf Green Stain C/014*
Silica
32
Cobalt oxide
32
Silica
35
Borax
20
Chrome oxide
24
Chrome oxide
20
Chrome oxide
18
Borax
14
Zinc oxide
15
Zinc oxide
15
Zinc oxide
14
Borax
18
Cobalt oxide
8
Silica
7
Cobalt oxide
9
China clay
7
Nickle oxide
6
Talc
3
100
China clay
3
100
100
Making and using one's own stains have the
advantages of saving money and obtaining a variety of different colors. The
disadvantages are time and extra equipment In making them. However, the variety,
unusual colors, and textures are well worth It. See Commercial Stains In
Appendix.
CALCINED STAIN can be
individually made and will require a crucible, ball mill or mortar and pestle,
100 mesh screen, and kiln. Crucible are purchased or made. To make a crucible
use fine Iron free grog (10 to 20 percent) wedged Into porcelain. Crucible forms
are made by wheel or pinch formed. Before making a large batch of any stain, a
testing should be done. The procedure for making stains are:
A.
Materials weighed, mixed, and 100 mesh screened
B. Some formulas recommend ball milling the materials wet or dry for 1
to 3 hours. Small amounts are done in a mortar and pestle for 1/4 to 1/2 hour.
C.
Wet materials are screened and washed twice then set out to dry. Dry
materials are screened.
D. Dry mixture Is placed In a porcelain
crucible and fired in a kiln to the recommended temperature, usually C/2 to
C/11. In amounts of 5 pounds or more the melt will need to soak at the
peak temperature to make sure the entire mixture is
heated evenly.
E. The heating the
stain mix results in the mixture being hard like a glass rock (clinker), soft
rock like sandstone, firm like dry clay, or powder consistencies. The soft will
only need screening while the harder compound will break up with light hammering
but the very hard need heavy blows to break the clinker into small pieces. These
small pieces are ball milled for several hours to further break down the
materials to finer consistency. Commercial companies dump the hot clinker Into
water whereby the thermal shock shatters the clinker to speed up the hammering
process.
F. The broken pieces
are placed in a ball mill and wet ground for 4 or more hours until it will pass
through a 100 or finer mesh screen. Some materials will need to have the water
changed
several times during the grinding to wash out the
soluble salt. Small amounts can be ground by hand in a mortar and pestle.
G. The wet stain Is screened (100 mesh or finer)
and air dried. Commercial stains are screened 120 mesh and finer.
The benefits of
making up raw stains are
saving time, having a variety of colorants, and being able to develop new or
different colors. The time saving of mixing up a large batch of raw stain so
that it is available when smaller amounts are need to make a glaze, engobe,
wash, and such. Making stains, washes, ceramic pencils, etc. requires time and
preparation, however it is worth It. If a particular glaze Is used often and the
glaze formula calls for several different metallics, making this colorant
mix Into a large raw stain batch will
save having to constantly weigh these out. When this glaze Is prepared, only
the stain needs to be weighed Instead of several different metallics.
RAW STAINS are not calcined and often the same
formula as calcined stains. The procedure Is the same (above A through C) as
calcined stains. Even some commercial stains are not calcined, In particular
browns and blacks. It saves time, ball milling, and kiln firing. Depending upon
the firing temperature, Ingredients, and the glaze it is used in, the coloring
power and color range of some
yellow, red, and orange stains may be limited to low temperatures, while others
will have the full temperature and glaze type range. Raw stains are easy to
make, cheaper than commercial stains, and are an Interesting process.
Unfortunately, the colors possible with raw stains are limited as compared to
calcined stains.
COMMERCIAL STAINS Coral, pink, and certain yellow,
orange, red, and purples can only be obtained by the use of stains. It Is very
difficult to make these In one's studio without precise endeavor, extensive
equipment, and extended ceramic supplies. Heating the materials In a crucible,
pouring the hot clinker Into water, grinding the fragments, ball milling,
washing, and screening Is time consuming. Several companies make stains and
provide a variety of colors. See Appendix for listing of stains.
The following table lists the possible colors,
metallic oxide, and additive(s) used to obtain both raw and calcined stains.
FIGURE 9 ‑ STAIN COMPOSITION FOR METALLICS,
AND ADDITIVES
Color Metallics
Additives
Yellows Uranium
Calcium
and oranges
Antimony Lead, tin, calcium
Vanadium Zrcopax,
tin
Red Iron Alumina,
zinc, calcium
Rutile Tin,
alumina, zinc.
Reds Chrome
Tin, calcium, silica
Selenium/cadmium Tin, calcium, alumina
Uranium Soda,
tin, alumina
Gold Tin,
alumina
Pinks Chrome
T1n, calcium, lead, silica, zircopax
Manganese Alumina
Purples Chrome
Tin, boron
and Ulacs Manganese
Alumina, tin
Cobalt Tin,
magnesia
Blues Cobalt
Alumina, zinc
Cobalt/chrome Alumina,
tin
Nickel Alumina
Copper/cobalt Tin,
alumina
Grays Nickel/cobalVchrome Silica, calcium
Browns Iron
Zinc, alumina
Chrome Zinc,
tin, iron
Manganese Zinc,
alumina, nickel
Blacks Iron/manganese/cobalVcopper
Alumina, silica
Iron/manganese/copper Alumina, silica
(cobalVchrome free)
Iron/cobalVmanganese/chrome
Alumina, silica
(copper free)
Greens Chrome/copper/nickel Alumina, silica
Rare earth elements
(cerium oxide, neodymium oxide, praseodymium oxide, and yttrium oxide), produce
some soft pastel colors as well as experimenting both for a stain and for
unusual glaze effect such as crystal and crystalline glazes. See Appendix for
more details on metallics.
METALLIC OXIDES AND OPACIFIERS
Metallics are the
source of color, and in large amounts will opacity In a glaze. Often the
metallics are oxides or carbonates but other forms are used Including carbides,
chlorides, nitrates, phosphates, silicates, sulfides, and sulfates.
Color Is the
absorption, reflection, and refraction of various
light wavelengths that are detected by the eye and its associated nerves. Light reflected to the eye stimulates
the various color cones of the retina, creating the basis for the brain's
perception of color. Pigments and glaze colorants are both perceived in this
matter, but there the similarity ends. Pigments, as in paints, produce easily
predicted colors when mixed, (Le. yellow and blue make green). This Is not
necessarily so In glazes, for there Is the chemistry of the glaze involved. In
ceramics, unlike pigment, the color and color change are caused by chemistry.
Examples are the abundance or scarcity of oxygen and metallic molecules and
crystal and chemical structure. When blending of blue and yellow in ceramics,
the result may be anything but green. This Is a chemical blending. The following
Is a guide for the various colorants and opaciflers.
FIGURE 10 ‑ COLORANT AND RESULTING COLORS AS
AFFECTED BY MAJOR FLUXES IN A GLAZE
Metallic
Dominant Firing Atmosphere
Resulting color(s)
Colorant Flux
Oxidation (OX) or Reduction (RD)
CHROME
Alkaline OX
‑ Yellow‑green, chrome.
RD ‑ Pale green to
browns‑green to almost yellow. Very refractory.
Lead
OX ‑ C/010 to CI08
(high lead and low alumina) give brilliant orange to
orange‑reds; soda and
lead give bright yellow using 1 percent chrome, and
with 4 to 5 percent In gives
pink; temperature CI04 give chrome greens.
RD ‑ Not reduced.
Boron
& Magnesia
‑
Dull greens ail temperatures and atmospheres.
High
Zinc
‑
Brown all
temperatures and atmospheres.
High
Barium OX ‑ In lower
temperature, brilliant yellow‑greens to light green.
RD ‑ Brown to pale
green.
COBALT
Alkaline
‑
Powder blue, Intense blue to blue‑black at all temperatures and
atmosphere
Lead
‑
Typical cobatt blues but less Intense than alkaline based at all
temperatures
and atmospheres.
Boron
‑
Rich blues of typical
cobalt at all temperatures and atmospheres.
High
Magnesia OX ‑
Purple.
RD ‑ Pink, violet, to
red‑blue.
High
Zinc
‑
Strong
green‑blues in all temperatures and atmospheres.
High
Barium ‑
Gray‑blue in
small amounts to Intense blue in all temperatures and atmo
spheres.
COPPER
Alkaline OX
‑Intense blue to blue‑green, In low alumina a bright blue. (Usually
runny)
RD ‑ Gives blood reds.
Lead
OX ‑ Rich grass green
yellow.‑green; with tin light grass green with deep rusty
red when thin.
RD ‑ Not reduced.
Boron
OX ‑ Brilliant blue‑green and turquoise; with rutlle results
in blue‑green
streaks; In reduction blood reds, purples to whitish reds In
all tempera
tures and atmospheres.
High
Magnesia OX ‑
Purples.
RD ‑ Liver to pale
pink‑reds.
High
Zinc OX ‑ Develops strong
greens.
RD ‑ Hinders copper
reds In reduction.
High
Barium OX ‑intense blue to
blue‑green.
RD ‑ Tomato reds; a mix
of barium calcium, and soda gives blood reds.
IRON
Alkaline OX
‑ Straw yellow, yellow brown to brown.
RD ‑ Cold colors; .5 to
1 percent gives Celadon blue to blue‑green.
Lead
RD ‑ Warm yellow, amber, tan, honey, brown, reddish brown to
deep red; saturate
iron; 7 to 10 percent in low alumina gives aventurine; with tin
gives creams
and rusts.
RD ‑ High leads not
reduced.
Boron
OX ‑ Dull tans and browns.
RD ‑ Blue‑green
celadons and red plum In saturated irons 8 to 12 %.
High
Magnesia OX ‑
Usually browns.
RD ‑ Small amounts
gives gray‑green matts and yellow‑green at 10 %.
High
Zinc
‑
Dull muddy browns in 10 to 18
percent zinc at all temperatures and
atmosphers.
High
Barium OX ‑ Similar to the
alkaline. Rich yellow‑green, yellow to effects, cold yellow to
greens; small percents for brown.
RD ‑ High calcium and
barium based transparent glazes give delicate blue‑green
to turquoise.
Metallic
Dominant Firing Atmosphere Resulting
color(s)
Colorant
Flux
Oxidation (OX) or Reduction (RD)
IRON CHROMATE
‑
Used with all fluxing agents. In all conditions and temperatures produces
grays, browns, and iron blues. Often a base for making black glazes and
stains.
MANGANESE
Alkaline
OX ‑ Violet, grape purple to red‑blue plum; some alkalines
give browns or violet
browns at all temperature and atmospheres.
Lead
OX‑ Reddish brown to brownish violet (threaded with light and dark
specks); In
large amounts will blister.
RD‑ Not reduced.
Boron
OX ‑ Reddish
purple‑brown to reddish brown at all temperatures and atmosphers.
High Magnesia
‑
Lighter or pastel like effects of browns and violet‑browns at all
temperatures
and atmospheres.
High Zinc
‑
Browns at ail temperatures and atmospheres.
High Barium
OX ‑ Delicate pink, violet, reddish violets, to red wine.
RD ‑ Violet, blue, to brown.
Barium and Alkaline
‑
At low temperatures gives pink.
NICKEL
Alkaline
OX ‑ Gray, brown, or greens; with barium gives violet to
red‑brown. Refractory In
large amounts.
Lead/Boron/or
Magnesia ‑ Grays and browns at all temperatures and atmospheres.
High Zinc
‑
Sometimes blue to violet; grays and browns at all temperatures and atmo
spheres.
High Barium OX
‑ Gray‑browns.
RD ‑ With a small amount of zinc will give red‑violet to
blues.
RUTILE
Alkaline OX
‑Iron type colors of pale tans or grays.
RD ‑ Varies from tan, light blue straw to light brown to orange.
Crystalline
effects.
Lead
OX ‑ Smooth and even colors of straw to tan.
RD ‑ Not reduced.
Boron
‑
Very active streaking and mottling; tans to yellows at all temperatures
and
atmospheres.
High Magnesia ‑
Mottled and matt tans at all temperatures and atmospheres.
High Zinc
‑
Low alumina and slow cooling for crystalline, crystal, and rutlie/zinc
malts;
dull tans at all temperatures and atmospheres.
High Barium OX
‑ Tans to yellows.
RD ‑ Pale blues.
TIN
Chrome based OX/RD
‑ Pinks.
VANADIUM
Alkaline OX
‑ Various yellows; best at 8 to 10 percent at medium to low temperatures.
RD ‑ Not reduced.
Lead
OX‑ Pasty pale yellow to bright yellow.
RD‑ Not reduced.
Boron/Zinc/or
Magnesia
‑
Pasty yellows or grays at medium to low
temperatures.
RD ‑ Not reduced.
High Barium OX
‑ Strong yellows at medium to low temperatures.
RD ‑ Can be lightly reduced.
IRON based
minerals
Clay minerals containing a high percentage of Iron
including rutile, raw sienna, sienna, burnt sienna, raw umber, umber, burnt
umber, and yellow ochre. These are forms of
ochreous earth. Albany slip and barnard clay are more glaze/clay like. The
result is translucent or opaque due to the clay Impurities and the Iron
generates typical Iron colors of tans, browns, blacks, and iron blues in all the
glaze conditions.
METALLICS
Color, as we know it, Is an effect produced on the
eye and its associated nerves by various wave lengths. Light reflected or
directed to the eye stimulate the different color cones of the retina creating
the perception of the various colors. When light strikes an object, some of It
Is absorbed and some Is reflected while the color of a solid or transparent
object Is determined by the wavelength of the light transmitted. Pigments
(metallics) are mixed with the glaze, overglaze, engobe, enamel, and such to
give color. Often times the metailica are oxides and carbonates, but other forms
are used including carbides, chlorides, nitrates, phosphates, silicates,
sulfides, and sulfates. It should be noted that paint pigments and ceramic
colorants are different In that paint pigments respond to color mixing, while
ceramic colorants respond to both heat and chemistry involved.
Examples of non pigment results of using metallics
in ceramics follow. Iron of 1 percent of iron added to a glaze fired reduction =
celadon and 14 percent = brick red‑black at C/11 and In oxidation, 4
percent = light tan, 6 percent = brown, and 10 percent = black; 4 percent iron
and 6 percent rutile = blue. Copper at 1 percent in oxidation = light green but
= bright blood red in reduction. Chrome at 2 percent chrome at C/OS =
orange‑red but = dull green at C/8. These are not unusual examples for
many factors affect the resulting color Including: temperature; acid/alkaline
composition of glaze; type of flux; amount or lack of zinc, barium, magnesium,
or tin; location In kiln; underfired or overfired; thickness of application;
composition of clay body; atmosphere (oxidation, neutral, and reduction)
control; other glaze over, next to, or adjacent; cooling cycle; length of
firing; and whim of the kiln gods.
Opacify Is the state of
being opaque and there are various levels starting with opaque (no light passing
through), translucent (some light passing), to transparent (clear) with various
in‑betweens. Distinct levels of opacity is achieved by introducing
differing amounts of opacifiers like zircon, zircopax, tin, zinc, and at low
temperature antimony for white and/or metallics for colors. The following chart
lists the mineral, use, color, and any remarks on metallics.
FIGURE 11 ‑ COLOR AND OPACIFIER CHART
Mineral Use
Color Remarks
Antimony oxide
Lead glaze Yellow
Primarily an opacifier
Sb203
Iron/lead glaze Orange
Stain Yellow
With rutile or titanium
Stain/glaze Pale green
Chrome oxide and lead
Barium chromate
Overglaze Lemon yellow
All low fire
BaCr04
Borax glaze Lemon
yellow, pale green
Alkaline glaze Lemon yellow, pale green
Cadmium sulfide
Glaze colorant Yellow, orange Unreliable
above C/010
CdS
Borax glaze Red
Used with 2096 selenium
Stain Yellow, orange,
red Up to 832oF with selenium
and
sulfur
Chromium oxide
Stain, underglaze, Chrome green Turns
brown If zinc present
Cr203
glaze, overglaze
Tin
glaze Pink
Has blue cast If zinc is used
Stain,
glaze Green‑blue, olive green
Addition of cobalt, nickel, Iron,
and/or copper
High
lead, low Chinese red
Low temperatures
alumina glaze
High
lead & sodium
Yellows Low temperatures
High
zinc and/ Browns
All temperatures
potassium
Cobalt oxide
Stain, underglaze, Blue,
violet Very powerful colorant
co
03
glaze, overglaze
Co 63
Underglaze
Black With Iron chromate
Boric
glaze Pink, pink‑violet
Low temperature with Mg0
Sodium
glaze Green
C/15 glaze
COLOR AND OPACIFIER CHART
Mineral
Use
Color
Remarks
Copper oxide
Lead glaze
Grass green
Large amounts yield gun metal
Cu0
Alkaline glaze Turquoise
All temperatures
Glaze
Red, purple, blue Reducing
atmosphere with 1% tin
Tin
glaze Turquoise or robin's
egg blue
Zinc
glaze Grass green
Zinc In glaze Increases brilliance
of color
Stain
Green, blue Low alumina stains
Gold Paste,
liquid, or Gold
Low temperature
Au
powder for overglaze
Gold chloride Glaze
Gold luster Low temperature,
reduced
AuCl3
Glaze Purple, rose, red
Low temperature with tin
Iron chromate
Underglaze
Brown
With manganese or zinc at low
FeCr04
temperature
Underglaze, body
Black
With Cu, Co, Mg0
stain
Overglaze
Dark pink
Low temperature
Tin glaze
Dirty pink, pink‑brown
Low temperature
Iron oxide
Lead glaze
Straw yellow to light
Oxidizing atmosphere
FeO
brown
Leadless glaze
Brown to red‑brown
Borax
Goldstone
Saturated 12% plus
Opaque glaze
Cream
Matt glaze
Red‑orange
With titanium C/11‑13
Body stain
Red, yellow, brown
Celadon
Greens, blues, olives
Reducing atmospheres under 2%
Glaze
Blues Glaze
with lithium, barium,
magnesium
Glazes, stains
Blacks Saturated
over 10% with Co, Cu, Mg
Lead chromate
Basic flux in glaze
Coral, red
Low temperature
PbCr04
Body stain Green
Acid
flux In trans‑ Silver yellow Low
temperature
parent glaze
Stain,
glaze Pink
With tin, low temperature
Manganese oxide Body stain
Red, purple, brown, black
Mn02
Alkaline glaze Plum
Lead
glaze Light pink‑tan, dark
brown, red‑purple
Glaze,
stain Black
Large amounts or with Co, Cu
Underglaze
Brown With iron chromate
Glaze,
stain Intense black
With cobalt
Glaze
Violet Leadless
alkaline glaze or zinc and
calcium glaze
Nickel oxide Glaze
Blue, green, gray,
NIO
brown, yellow Unreliable, hard
to handle
Stain
Olive green With cobalt and
iron
Glaze
Gray‑blue High zinc
(20%) over C/4
Stain, glaze
Yellow, blue, greenish High
zinc/barium base
COLOR AND OPACIFIER CHART (Continued)
Mlnerat
Use
Color
Remarks
Platinum
Glaze
Steel gray luster
Preferable to silver
Pt
Overglaze, luster
Silver
Will not tarnish, available in liquid
form
Ruble
Stain, glazes
Ivory, yellow, brown
A crude titanium and Iron
TI02
(.2Fe0) Glazes
Brown, blue
Over 8 percent
Glazes Broken tans,
opaque 2 to 8 percent
Selenium Borax
glazes Pink or fire engine red
With cadmium
Se
Black Over
C/10
Silver
Paste or powder
Silver
Tarnishes easily, not available in
Ag
for overglaze
liquid form
Silver chloride
Glaze
Yellow, purple, silver
Low temperature, reducing
AgCl
luster
Tin oxide
Opaque glaze
Pink‑maroon
With chrome
Sn02
Glaze
White
Opacifier, 2 to 10%
Glaze
Yellow
With vanadium
Titanium oxide
Matt or crystalline
Light cream to dark Ivory
Oxidizing
1102
Glaze
Gray or blue
Over 5%, reducing
atmosphere
Uranium dioxide Lead glaze
Hot yellow, orange
Low temperature
U02
Alkaline
Cool yellow, green
Glaze
Yellow
High temperature, 8%
Glaze
Jet black or gray
Reducing atmosphere
Lead glaze
Red
Pb, Zn, SI base no Ca, B
Vanadium oxide Glaze
Yellow, bluish‑green
A substitute for uranium, large
Va02
amounts of
tin or Zr
Zinc oxide
Glaze
Off
white to cream
Adds whiteness and brilliance to
Zn02
some colors
Glaze
Matt, white At 5 % or more
Zircon oxide Glaze
White Made up as a silicate
frit
Zr0
(Drcon G, Superpax, Zircopax, Opax)
COLORING CLAY BODIES
Coloring
the clay body is a way to obtain a diversity to one's production assortment.
Metallics as
well as stains are mixed in clay to give color but
the range of color is less than glazes. Wedgwood produced
"marbleware" by mixing colored balls of clay together, slightly
wedging it, and when scraped It after throwing to produce the agate, marble, or
Zebra patterns. In slip casting two or more colored clays were poured at the
same time resulting in similar types of patterns. Metlach's factory produced
cameo by slip casting a basic shape in colored clay (blue, tan, brick, black)
and sprig molded design and figures In contrasting colored clay (cream, white).
These were placed on the basic shape In low relief producing cameo.
In making of porcelain, white earthenware, and white stoneware, the
purest of clays, fluxes, and silicas are used to avoid dirtying the whiteness of
the clay. Commercial clay bodies that are colored use natural clays containing iron, or manganese to
produce the gray, tan, brick red, brown, and chocc;ale colors. It Is cheaper to
use natural colored clays rather than adding metallics. The metallics red Iron
oxide, black Iron oxide, Iron chromate, magnesite, manganese dioxide, Ilmenite,
and granular iron are the most common additives and are the cheapest colorants.
Granular Iron, Ilmenite, and magneslte produces speckles while some sand and
grog have traces of iron resulting In scant speckling. At lower temperature a
colored clay may be brighter but darker at higher temperatures.
FIGURE 12 ‑ COLORANTS IN CLAY BODY
Metallics
Percent
Oxidation Firing
Reduction Firing
Antimony oxide, sulfide
2
‑ 8%
Off white
Cream
Albany slip
5
‑ 15 %
Cream to brown
Cream to brown
Barnard clay
5
‑ 15 %
Cream to chocolate
Cream to chocolate
Brick, red (30 mesh)
1
‑ 10 %
Light tan to dark specks
Light tan to dark specks
Cerium oxide
4
‑ 10 %
Warm pink
Pinkish
Chrome oxide
1
‑ 5 %
Light green to chrome green
Grayed light green to gray olive
Copper oxide, carbonate
1
‑ 5 %
Light green to olive green
Light green, red, green‑black
Cobalt oxide, carbonate
.6 ‑ 4 %
Light blue to dark blue
Light blue to dark blue
Iimenite, powder
1
‑ 8 %
Cream to light brown
Buff to grayed brown
Ilmenite, granular
.5 ‑ 4 %
Few to heavy specks
Few to heavy specks
iron red, black
2
‑ 10 %
Pink to dark brick red
Pinkish to medium brown
Iron chromate
1
‑ 8 %
Light tan to medium brown
Light grayed tan to grayed
brown
Magneslte, granular
.5 ‑ 4 %
Light to heavy specks
Increased size of specks
Manganese dioxide
1
‑ 8 %
Light tan with specks to
Light grayed tan with medium
brown with specks
specks to grayed brown
Nickel oxide, carbonate
1
‑ 6 °/a
Light tan to dark tan
Light tan to grayed tan
Potassium carbonate
2
‑ 7 %
Light green to medium olive
Light green to dark green
Ochre, yellow
1
‑ 8 %
Light tan to medium tan
Light tan to cool medium tan
Rutlle, ground
1
‑ 8 %
Light tan to medium brown
Light grayed tan to grayed
brown
Rutlle, granular
.5 ‑ 6 %
Few to heavy speckling
Few to heavy speckling
Tin oxide
2
‑ 10 %
White and opaque
Whiter and opaque
Uranium oxide, sulfate
2
‑ 6 °/a
Pink to tanish
Light gray to gray
Vanadium pentoxide
1
‑ 5 %
Slight tan to gray
Slight tan to grayed olive green
Zirconium (commercial
1
‑ 10 %
White and opaque
Whiter and opaque
opaclfiers) ‑ Zircopax, Opax
COLORANTS USED AS WASHES FOR
PIT FIRING,
SAGGAR, AND OPEN FIRING
Mixing
colorants with water and then brushing, spraying, atomizing, spattering, or
sponging to wet, damp, dry, bisque ware, and some cases over glazed ware giving
random patterns on a pot will produce distinct effects. These effects range from matt black, luster,
mother‑of‑pearl (unsmoked), rainbow, bright or metallic spots to
subtle pastel colors. The colorants, mostly metaliics, are mixed with water (and
small amount of glue), and screened In 80 to 100 mesh screen. The result depends
upon the firing, amount of reduction, temperature, and thickness of application.
The following is a basis for testing.
A.
Copper carbonate 50
D.
Yellow ochre
70
G. Potash dichromate
60
Red Iron oxide
50
Copper carbonate 30
Copper
carbonate 20
100
100
Rutile
20
100
B.
Copper carbonate 60
E. Copper carbonate 90
Red Iron oxide
30
Cobalt carbonate
10
H.
Red Iron oxide
70
Cobalt carbonate
10
100
Copper carbonate
30
100
100
F.
Copper sulfate*
100
C. Silver nitrate*
100
* Fumes are dangerous
COLORANTS IN GLAZES
Cobalt = green. chrome = green, tin = white, and
the list goes on. By grouping several colorants together the resulting color
can be modified giving more interest, variety, and choices. In particular many
glazes for electric firing are bland but they can be given more interesting
effects by grouping. The following colorants combinations are added to base
glaze (white, translucent, or transparent) that can be further developed.
FIGURE 13 ‑ COLORANT GROUPINGS FOR GLAZES
Colorant Percent
Colorant Percent
Colorant Percent
Broken
Blue
Broken Brown
Cobalt carbonate
.'
Red Iron oxide
1.0
Dark Broken Greens
Rutile
1.5
Manganese carbonate 4.0
Manganese dioxide
2.0
Granular Ilmenlte .3
Rutile 2.0
Chrome oxide 1.0
Copper carbonate 4.0
Orange
Brown
Speckled White
Red Iron oxide
2.5 Tin oxide
2.0
Black
Tin oxide
1.0 Granular magnetite .5 Black Iron
oxide 7.0
Rutile 2.0
Cobalt oxide 1.0
Granular magnetite
.4
Broken Rust
Manganese dioxide
3.0
Red Iron oxide 8.0 Granular
magnetite .3
Broken
Greens
Manganese dioxide 1.0
Chrome oxide
1.0 Zircopax
4.0
Copper carbonate
3.0
Rutile 4.0
OPACIFIERS
Opacity occurs when a
glaze contains suspended particles of a material that have a different
refractive Index (the ability to bend light) from the glaze matrix. These
particles cause the light rays to be diffused or the surface of the glaze has
crystals that reflect the light causing the glaze to be opaque. One major way to
render a glaze opaque Is to suspend undissolved particles or crystals on or In
the glaze. Gas bubbles suspended In the glaze can also make the glaze opaque.
The amount of opacifiers entered In the glaze will determine the degree of
opacity. Often, too high a percent of opacifiers will render the surface rough
or act as a refractory material.
Cost of materials Is a
consideration for tin oxide is expensive and to make a glaze white requires
about 5 percent. The same whiteness using zirconium based opacifiers takes 10
percent with the total cost cheaper for zirconium stain. The other factor Is
that tin Is a warmer white compared to the stain a cooler white. A cheaper and
good compromise is 2 percent tin and 5 percent zirconium stain. Chun and other
glazes rely on trapped minute bubbles for the translucent opalescence, but a too
thin glaze application, prolonged glaze soaking, over firing, or refiring will
cause the bubbles to dissipate.
The following chart
shows various opacifiers, the resulting color, percentage, temperatures used,
atmosphere, and a guide for the glaze (acid, neutral, alkaline), soaking
period, and glaze thickness.
FIGURE 14
‑ OPACIFIER CHART
Qpacifier
Color
Percent
Temperature
Atmosphere
AIR BUBBLES
Whitish
Any
Either
Chun and similar glazes rely on trapped minute bubbles to produce
translucent and
opalescence. Bone ash Is the major mineral us= ~ for this type of glaze.
ANTIMONY
Weak yellow‑white
8 ‑ 12 % Under C/1
Oxidation
Low and medium temperatures, when used with lead, forms lead antimonate,
a yellow
opaque color.
BONE ASH
(see air bubbles)
CALCIUM FLUORIDE
Off white
5 ‑ 8 % Up to C/11
Either
Disadvantages ‑ May blister or pinhole in some glazes
COMMERCIAL
FRITTED OPACIFIERS:
Calcium zircon silicate, magnesium zircon silicate,
zinc zircon silicate, Zircopax, Uttrox, Superpax, Opax
Whites
4 ‑ 18 % Any
Either
RUTILE
Buff to brown
1 ‑ 12 % Any
Either
Advantages ‑ will produce crystal and crystalline finishes to glaze
Disadvantages ‑ may dissolve in melt; pulls out color from the clay
body; has a ten
dency to produce matt finish; may cause mottling of colors; subdues
colors; gives
yellow color to the glaze.
TIN OXIDE
Pure white
3 ‑ 8 % Any
Either
Advantages ‑ dependable, does not dissolve In melt, provides the
whitest and
cleanliest colors.
Disadvantages ‑ pink tinge near chrome or copper In firing,
expensive, in heavy
reduction gives gray cloudiness to the glaze and colors
TITANIUM
Weak blue‑white
8 ‑ 12 % Any
Either
Advantages ‑ will produce crystal and crystalline finishes.
Disadvantages ‑ will dissolve In melt; precipitates out of the
glaze during cooling,
thus must cool slowly; pulls out color from the clay body; has a tendency
to produce
matt finish; may cause mottling of colors; subdues colors.
ZINC OXIDE
Whitish to yellow
4 ‑ 12 % Any
Either
ZIRCONIUM
White
%‑ 12 % Any
Either
Light gray‑white up to 750 C, then whiter over this temperature
(Zircon G, Superpax,
Zircopax, Opax). Gray‑white color up to 1300° C, more white over
this temperature.
Advantages ‑ Dependable, little dissolves in melt, provides white
and clean colors,
does not accept pink tinges like tin oxide.
Disadvantages ‑ In heavy reduction gives slight gray cloudiness to the glaze and
colors.
FIGURE 15 ‑ RELATIVE REFRACTIVE OPACIFIER INDEX
Opacifier
Refractive Index
Ocacifier
Refractive Index
Opaciner
Refractive Index
Air
1.00
'Lead antimonate
2.20
Titanium oxide
2.56
Antimony oxide
2.10
Rullite
1.65
Wollastonite
1.61
Arsenic oxide
2.20
Quartz
1.55
Zircon
1.96
Calcium phosphate
1.63
Sodium fluoride
1.34
Zircon based frit
1.88
Cristobalite
1.48
Tin oxide
2.00
Zirconium oxide
2.20