Simba Materials Limited
We hope that this information on the characteristics and properties of ceramic raw materials, frits and oxides is useful to you, especially the way that they interact with each other to create helpful and not so helpful results.
information is provided in good faith, however like all ceramic calculations it
needs to be viewed in perspective. Always remember that glazes are made of
materials that have chemical, mineralogical and physical properties and you
cannot ignore any of these or the way that all materials are affected by others
present and many other factors such as age of material, time left standing,
contamination from dust, refractories, the weather and of course Murphys law.
Al2O3 Calcined and Al2O3.3H2O or Al2(OH)6 Hydrated aluminas are fine
granular white powders that have good flow properties increasing glaze
viscosity, firing range and resistance to crystallisation. This material has a
very high melting temperature. It is important that the powder is very fine. The
hydrated version of alumina stays in suspension better in glaze slurries and has
better adhesive qualities than calcined. Also using hydrated alumina in glazes
and glass can promote the fining operation of coalescing and removing finely
dispersed gas bubbles. Small additions of fine alumina hydrate added to a glaze
can also enhance the colour of Cr-Al pinks. Larger additions (15% of glaze) of
fine material can impart matt and opaque effects if the glaze is able to take it
into solution (sourcing alumina from kaolin and frits may be more practical).
Also used in Batt Wash.
Sb2O3. Antimony oxide is used as an opacifier in porcelain enamel (mainly
leadless but it has been replaced to an extent by titania) and ceramic glazes.
However it can give a yellowish colour (1-2%) if the glaze contains lead, due to
the precipitation of yellow lead antimonate (known as Naples yellow). Antimony
is also used as a yellow body stain in combination with rutile or titanium. It
is slightly fluxing in higher temperature glazes.
BALL CLAY Fine secondary or tertiary clay mined in Devon and Dorset. Usually grey or blue in colour. It is very plastic and fuses relatively easily with greater density than china clay. Called ball clays because they used to be rolled into balls for loading onto transport. Not normally used on its own due to its high shrinkage - up to 17% so it is usually mixed with china clay
BALL CLAY - Blue - Puraflo AK 32% Al2O3, 50% SiO2, 1.1% Fe2O3, 2.1% K/Na2O. A Devon ball clay, very plastic, clean burning ball clay. recommended for glazes needing ball clay as an ingredient.
BALL CLAY - Hymod AT 29% Al2O3, 55% SiO2, 2.3% Fe2O3, 3.6% K/Na2O3. A Dorset ball clay with a high iron content, high strength, useful in the production of warm coloured textured bodies at stoneware temperatures. At earthenware temperatures the colour tends to be deeper.
BALL CLAY - Hyplas 71 20% Al2O3, 70% SiO2, 0.8% Fe2O3, 2.3% K/Na2O3. A high siliceous Devon ball clay giving good plasticity with medium strength and usefully low iron content.
CARBONATE BaCO3. Used in casting slips at low levels
to prevent scumming (up to 0.5%).
Used in high temperature glazes as a flux and produces matt and semi-matt
surfaces at earthenware temperatures. At higher temperatures it can make a
distinctive turquoise with copper. In bodies it seems to impart better
translucency but can lead to weakness and excessive shrinkage.
Al2O 2H2O SiO2. A clay mineral with incredibly small particle sizes. Must
be added in the dry state. This, in combination with the active chemistry on the
surface of the particles (that makes them hold onto water), makes it the most
plastic and impermeable common clay material used in ceramics. Anyone who uses
this material should have their eyes open to its advantages and disadvantages.
Binder: Bentonite binds particles together in ceramic bodies to make them
stronger in the green or dry state (up to 5%). Its minute particles fill voids
between others to produce a more dense mass with more points of contact. Adding
bentonite to glazes also imparts better dry strength and a harder and more
durable surface. 1 part bentonite can plasticize a
body as much as 10 parts kaolin. Bentonitic bodies are stronger in the dry form but
dry slower, crack more and fire darker with potential iron specks (get a super
fine ground grade). 1-2% bentonite can greatly improve glaze suspension by
gelling it. In addition it will harden the dry layer. Coarser varieties can
impart some glaze speck. If a glaze already contains more than 15% clay (kaolin,
ball clay) you should not need more than 1% bentonite.
Firing cracks, explosions: Bentonite slows down water penetration. Not
only does a bentonite-containing clay body dry slower but it does not dry as
completely. Although ware might look dry it is not, several percent water
tightly-bound between bentonite particles remains. If ware is not
temperature-dried before being fired there is a risk that water will not be able
to escape fast enough during firing and ware will crack, fracture under steam
Obtained from metallic bismuth and is soluble in acids but insoluble in
water. It gives a pearly luster to glazes and glasses, especially in reduction
(and raku) firing. It is an ingredient in luster colours. It is a very very
BONE ASH CA3(PO4)2. Bone ash has traditionally been added to porcelain to achieve a high degree of translucency (hence the name 'bone china'). The manufacture of bone china is difficult to master because the clays are non-plastic, ware is unstable in the kiln, and it is difficult to burn consistently to the body's narrow firing range. Up to 1-2% bone ash can be used in enamels for opacification or milky character (more will usually cause pinholes). In glazes, as with enamels, too much or too high a temperature will cause blistering. Bone ash or calcium phosphate are used to opacify opal glass (1-3%) because the P2O5 content forms colourless compounds with iron impurities. Not suitable for slip casting.
BORATE FRIT 1030-1180oC. SiO2
17.9%, B2O3 50.3%, MgO 0.1%, Al2O3 4.9%, K2O 0.3%, CaO 26.5%. Insoluble
colemanite frit for use where recipes specify greater than 5% colemanite. Can be
used as a base in lead free glazes.
CALCIUM CHLORIDE CaCl2 6H2O. Calcium Chloride is used as a suspending agent in glazes 0.05%. It works well with bentonite. Small additions (0.1-0.3%) to glaze slurries also produce gelling qualities that make it easier to apply coatings of even thickness. Vinegar has a similar effect although calcium chloride is said to work better. This material is the key to being able to apply a glaze to non-porous porcelain bisque ware.
Al2O3.2SiO2.2H2O.General purpose Kaolin. Because kaolinite mineral has a much larger particle size than ball clay and bentonite materials, blending it with them in bodies can produce a good cross section of ultimate particle sizes imparting enhanced working and drying properties. Kaoilns speed up casting rates in slurry bodies and drying rates in all bodies. Many porcelains contain only kaolin as their clay component but is has a relatively low plasticity and so is usually used (less than 50%) in combination with ball clays, bentonites and other plasticizers. It is also the most common glaze suspender (15-20%). GROLLEG is lower in iron and a little stronger, so is whiter and more suitable for casting.
OXIDE A very versatile stain which disperses well
through a glaze, normally producing a green colour but giving reds and yellows
in lead glazes, and pink in the presence of tin.
chrome reacts with normally inert tin to produce chrome-tin pink colors whiting
and alumina are usually used instead of tin to lighten and clarify chrome green
glazes. Chrome-tin pinks are much more consistent if the combination is
premelted (i.e. commercial stain) and if the glaze is high in calcium or
strontium, and free of zinc. Strontium is most effective if a wide firing range
is desired (0.1-0.5% chrome, 4-10% tin).
opacifier 1-2% is often added to chrome glazes to stabilize them and prevent
Purple - Chrome-tin pinks move toward purple in glazes with significant boron.
One glaze with 3.3 SiO2, 0.27 Al2O3, 0.2 B2O3, 0.15 Li2O, 0.5 CaO, 0.1 MgO, 0.15
Na2O employed 5% tin oxide, 0.6% cobalt carbonate, 0.17% chrome oxide to produce
a good purple at cone 6.
Green - Chrome is a classic green coluorant for recipes in oxidation and
reduction at all temperatures. However, the shades it produces can be opaque,
uninteresting. In the presence of CaO, the color moves toward grass green.
Green Peacock - Drab chrome greens can be moved toward peacock green with
the addition of cobalt oxide
(1% each gives bright color, some MgO needed
also). This works in zinc free boron and soda glazes.
Brown - Chrome in zinc glazes tends to form brown zinc chromate.
Orange - Chrome in high lead glazes forms yellow lead chromate. Zinc and
chrome tend to produce orange.
Black - Chrome is a constituent in almost all black oxidation colours. It is
used up to 40% in Cr-Co-Fe blacks and as high as 65% in Cu-Cr blacks.
Pink / Maroon - Chrome and tin are a widely used combination to produce
pinks in zinc free glazes with at least 10% CaO and low MgO (alkaline glazes
work well). Many stains are based on this system and typically have around 20-30
times as much tin oxide as chrome oxide. Tin would typically be around 4-5%.
Chinese Red - Below 950C in high lead, low alumina glazes, chrome will
produce reds to ranges, often with a crystalline surface. The addition of soda
will move the colour toward yellow.
Yellow - Chrome in high lead glazes forms yellow lead chromate. Alkalies are
recommended in the base glaze. Added zinc can extend the range to orange. In
other types of glazes, less than 0.5% chrome oxide will give yellowish or yellow
CoCO3. Cobalt Carbonate -A pinkish tan powder. It is a strong colourant
and almost always produces blue in glazes. The carbonate form of cobalt is very
fine grained and disperses better and gives more evenly distributed colour than
cobalt oxide. However, as with any carbonate, it produces gases as it decomposes
and these can cause pinholes or blisters in glazes. Also the carbonate form
contains less cobalt per gram, therefore colours are less intense than the oxide
Co3O4. Cobalt Oxide is a metallic oxide that produces blue in glazes at
all temperatures. Black Cobalt Oxide is the principle source of CoO used in
glazes, glass, and enamels. Cobalt is the most powerful ceramic colourant and it
is stable in most systems. It is also useful as a body and slip stain. Cobalt
will often produce glaze specking if it is not thoroughly sieved or ball milled
to distribute the particles. Cobalt carbonate tends to disperse better in glazes
to give even blue coloration because it is not as powerful and produces some
glaze blistering problems. A high cobalt stain is also an alternative.
- Violet, Lilac - In magnesia glazes a
colour range is from violet to lilac is possible.
Blue - The shade of blue can, however, be affected in many ways by the presence
of different oxides. Cobalt is powerful and often less than 1% will give strong
colour. If the colour needs to be toned down, additions of iron, titanium,
rutile and nickel may work.
Soft Blue - Often calcined with alumina and lime for soft underglaze colours.
Stains often employ mixes of alumina, cobalt, and zinc for softer blue colours.
Yellow - Used in combination with manganese and selenium to mask excess
yellow coloration (yellow plus blue gives green which is masked by the pink of
- Blue Slate - Combinations with iron and manganese can give a slate
- Blue-green - With
barium shades of blue-green are possible.
- Blue-black - With
chrome and manganese blue-black and black are common.
Blue-green - With chrome and copper, cobalt can yield tints from
pure cobalt blue, to greenish-blue, to the green of chromium. These effects work
best when silica is not too high and there is adequate alumina.
Purple - With
manganese (i.e. 1-3% cobalt carb, 3-5% manganese carb), purples and violets can
be made. Less cobalt will lighten the colour. This effect works well in magnesia
glazes. In high magnesia glazes, 1-2% cobalt alone will give purple. Add tin to
move the colour toward lavender.
- Lavender, Purple, Violet, Pink - With
adequate SiO2 and high MgO (0.4 molar), purple, violet, lavender, and pinks can
be made using 1% or more CoO. Mimimizing boron, alumina, and KNaO will help
prevent it from turning blue. Note that the high MgO will generally make the
glaze matte and it could suffer some ill effects associated with excessive MgO.
Red - With MgO, SiO2, and B2O3, red, violet, lavender, and
pinks can be made.
B2O3 43.9%, CaO 26%, SiO2 4.5%. A popular natural source of insoluble
boron for many decades. Gerstley Borate contained significant amounts of
colemanite. Pure colemanite, however, is much higher in B2O3 than Gerstley
Borate. Be sure to screen out any materials coarser than 200 mesh, or ball
mill the glaze. Gum or other binders also help.
Acting as a powerful flux it can intensify the effect of colouring oxides and
can increase craze resistance in glazes however
Acting as a powerful flux it can intensify the effect of colouring oxides and can increase craze resistance in glazes however
- Green - Under normal oxidizing conditions the CuO molecule remains unchanged
and produces clear green colours in glazes. The shade of copper greens can vary
with firing rate and soaking changes. The best colours are generally obtained
with fast firing and little soaking. Copper in calcium/magnesium glazes gives a green very
different from that produced with lead.
- Blue-green - Fluoride,
when used with copper, can produce blue green colours.
Red - Copper is well-known for its ability to produce
blood-red and fire-red colours in steady reduction atmosphere firings where CuO
is altered to Cu2O. Bright red colours are usually achieved with very small
amounts of copper (i.e. 0.2-0.5%) in a low alumina base with at least .4 molar
equivalents of CaO and plenty of the alkalis. Tin oxide will enhance colour. Use
of silicon carbide in oxidation (2%) can produce red.
Purple - Purple copper reduction glazes are the result of a mixture of
copper in its green oxidized and red reduced forms. This effect appears most
frequently in high lime glazes or where early stages of firing are oxidizing or
latter stages are light or neutral. The
use of boron in a copper red reduction glaze will give a purple hue.
Turquoise - In copper red glazes, barium additions in a high
feldspar base will produce turquoise to deep blue depending on how much copper
is added. Lithium contributes to the colour also. Combinations of Black CuO with
tin or zircon will give turquoise or blue-greens when the glaze is alkaline (KNaO)
and low alumina. Look for a frit with this profile for best results. Glazes of
this type often craze.
- Green Yellowish - K2O
can turn a copper glaze yellowish. If Na2O or PbO are present, K2O should not
exceed 0.15 equivalent.
Blue - Copper in a barium/zinc/sodium glaze gives a blue.
Colour can also be enhanced by lithia. Tin and copper can produce turquoise to
robin's egg blue
Metallic - Large
amounts of copper (7%) in a glaze give metallic and even graphite effects.
COPPER CARBONATE (
CuCO3) is bulkier than the oxide form, thus it tends to disperse better to give
more even results. It is also more chemically reactive than the oxide form and
thus melts better. As such, it is ideal for use in brush work where minimal
speck is required. However it produces gases as it decomposes and these can
cause pinholes or blisters in glazes. Also the carbonate form contains less
copper per gram, therefore colours are less intense than the oxide form.
Supplies of green copper carbonate often vary in colour and density. Despite
variations in the physical appearance of the material, the amount of contained
copper metal remains essentially constant but the ability to stay in suspension
can be different from one manufacturer to another and so the ceramic grade must
always be used.
normally produces green colours in amounts to 5% whereafter it moves toward
black. In reduction firing, it turns to Cu2O and gives vibrant red hues. Above
1025C copper becomes increasingly volatile and its crystalline structure breaks
down. At 1325C CuO melts. This can affect the colour of other glazed pieces in
the kiln. Glazes containing copper can change significantly because of loss of
copper. Some potters alternate between reduction and oxidation, and even put a
dish filled with copper carbonate in the centre of the kiln to minimize this
phenomenon. It can act as a strong flux. It is the most stable form of oxidized
copper ( Black Cuprous oxide oxidizes to Red cupric oxide in normal firings).
The oxide form of copper can give a speckled colour in glazes whereas the
carbonate form will give a more uniform effect.
K2O 3.8%, Na2O 4%, Al2O3 15.3%, SiO2 69.5%. A secondary flux in
earthenware temperature glazes, and an alternative to feldspar at higher
temperatures, giving greater fired strength. A decomposed granite however it is
not a fusible as feldspar due to its high silica content.
Low iron and high silica content
Low iron and high silica contentpromotes whiteness and transparency and so particularly useful in porcelain and similar white bodies.
CaO3 31.4%, MgO 20.8%. Dolomite as a ceramic material is a uniform
calcium magnesium carbonate. In ceramic glazes it is used as a source of
magnesia and calcia to act as a secondary flux. Other than talc, dolomite is the
principle source of MgO in high temperature raw glazes but can be used as low as
1060oC. 'Dolomite matte' stoneware glazes, for example, are highly prized for
their pleasant 'silky' surface texture. Above 5% it begins to opacify eventually
making a matt glaze. Dolomite by itself is refractory, but when combined with
the typical oxides in a glaze (especially boron) it readily enters the melt.
Er2O3. Erbium oxide is a light baby pink colour. It is an expensive,
dense, and weak colourant, but one of the very few ways you will ever get a
transparent pink. Erbium oxide's density means it is absolutely essential that
you use CMC gum. Erbium oxide gives its best pink colour at concentrations of
8-10%, but it is difficult to get more than 8% to fully dissolve in the melt. It
has given a more lavender colour in the presence of iron traces when in
FERRO FRIT 3110
1000 - 1060oC. Soft low alumina sodium borosilicate frit for glazes.
Often used in crystal and crackle glazes. This frit can be very useful to reduce
the feldspar content in glazes (since many high feldspar glazes have low clay
content and therefore poor slurry suspension properties and dried hardness). The
chemistry of this frit is similar to feldspar (but with low alumina and CaO in
addition to the alkali fluxes). That means if you substitute this for at least
part of the feldspar you can increase the kaolin (to supply the alumina) and
thereby improve slurry properties. In addition you will be able to reduce the
amount of troublesome calcium carbonate. Helps in the production of copper blues
and manganese purples and raku glazes.
FLINT SiO2. A major source of calcined silica for glazes and clay bodies (15%). It increases the fired temperature and craze resistance of glazes and its low expansion and contraction helps to stabilise the glaze. It is added to bodies to reduce shrinkage in drying and firing and to give a certain rigidity and eliminate crazing.
FRITS Glaze materials that have been melted together and then ground to an appropriate particle size - this reduces the handling of toxic materials such as lead by converting them into a silicate. Many materials, particularly carbonates are soluble in water and by fritting them first this prevents this problem. It also removes many volatiles which would have created firing problems. The tend to have a consistent formula and so give predicatbale and relieable results. See Calcium Borate, Standard Borax, Lead Bi-silicate and Sesquisilicate and HAF
FRIT 860 - 1060oC.
Typically SiO2 52.5%, B2O3 3.4%, Al2O3 5.2%, Na2O3 18.6%, K2O 10.3%, CaO 2.9%. A
high alkaline (Soda and Potash) version of a borax frit. High expansion rate
making them suitable for crackle glazes. Helps create turquoise copper blues and
purple/brown manganese in glazes.
ILMENITE FeO TiO2. Like rutile, ilmenite is quite variable in nature. You can tell the difference between granular rutile and granular ilmenite by doing a smear test against an abrasive surface (i.e. an unglazed white tile). The rutile will be tan or brown, the ilmenite will be black or dark brown. Ilmenite can be used in small amounts (-1%) to produce dark brown specks in bodies and specialized glazes. It is also used in combination with rutile to develop characteristic rutile break glazes; it seeds crystals in titania glazes.
Fe2O3. Red iron oxide is the most common colourant in ceramics and has
the highest amount of iron. It is available commercially as a soft and very fine
powder made by grinding ore material or heat processing ferrous/ferric sulphate
or ferric hydroxide. During firing all irons normally decompose and produce
similar colours in glazes and clay bodies (although they have differing amounts
of Fe metal per gram of powder).
black iron oxides (FeO) have a higher iron iron content and gives darker shades
usually dark browns and can produce speckles and crystals as an aventurine
Actual Yellow Ochre iron oxides (Fe2O3) are around 85% Fe2O3 and about 12% LOI with some impurities (e.g. SiO2, CaO). In ceramics, yellow irons are used where its raw colour or other raw properties are important to the manufacturing process or colour of the unfired product. It can produce yellowish honey colours (3%) but generally gives shades of brown (up to 8%) especially in lead glazes. It has excellent hiding power, absorbs ultraviolet light, is compatible with a broad range of vehicles, disperses well in aqueous and solvent systems, does not contain heavy metals
FRIT 900 - 1100oC.
SiO2 29%, Al2O3 12%, PbO 59%. A good clear, general purpose lead frit for
developing rich, bright surfaces and has a good reaction to colours
LEAD SESQUISILICATE FRIT 860 - 1080oC. As above but specially coated to reduce solubility.
Li2CO3. Lithium Carbonate is the best source of lithium oxide for glazes.
It is slightly soluble. It is unusual to see more than 5% lithium carbonate in
glaze because it promotes devitrifaction. Because of the low expansion of Li2O,
high lithium glazes tend to shiver. There are certain basic properties of
lithium which are of interest in ceramics. Since lithium has a very small ionic
radius in comparison to the other alkali metals, it has a higher field strength.
Low expansion coefficients are generally imparted to ceramic compositions
containing lithia. Lithium carbonate is a very strong flux (also true of lithium
fluoride). It is the lightest, smallest, and most reactive flux. In addition to
being soluble, lithium carbonate produces gases as it decomposes and these can
cause pinholes or blisters in glazes. In frits and glazes, lithia is used to
reduce the viscosity and thereby increase the fluidity of the coatings. This
reduces maturing times and lowers firing temperatures. 1% additions can increase
glaze gloss to a marked degree and slightly greater amounts (3%) can reduce
melting temperature by several cones and affect surface tension of the melt
Blue - Lithia can produce blue effects with copper.
Pink - Lithia can produce pinks and warm blues with cobalt.
Variegation - Lithia
contributes to mottled and flow effects when used in small amounts (-1%).
MgCO3. In high temperature glazes it acts as a flux (beginning action
about 1170C) producing viscous melts of high surface tension and opaque and
matte glazes. Like CaO, its melting action drastically accelerates at high
temperatures. The surface tension of MgO-containing melts is less of a problem
in reduction. Zircon and Magnesia melt at 2800C making them the highest melting
oxides. Remarkably, MgO readily forms eutectics with other oxides to melt at
surprisingly low temperatures.
is well known for the pleasant vellum 'fatty matte' and 'hares fur' tactile and
visual effects that it produces around 1200C, especially in reduction firing
(dolomite matte). The mechanism is phase separation of the suddenly melting MgO,
but MgO can also produce matte effects at lower temperatures as a refractory
MnCO3. Above 1080C, half of the oxygen disassociates to produce MnO, a
flux that immediately reacts with silica to produce violet colours in the
absence of alumina, browns in its presence. Thus if it is being used in glazes
fired below 1080C it should be considered as MnO2, if above it should be taken
as 81.5 MnO and 18.5 LOI.
Black - Manganese
and cobalt mixture produce black. Iron can also be used. For example, a mix of 8
iron, 4 manganese dioxide and 0.5 cobalt make a raw black stain.
- Purple, Violet - Purple colours can be produced in glazes of high
alkali (KNaO) and low alumina, especially in combinations with cobalt (look for
a frit with this profile for best results).
- When added
to terra cotta bodies in amounts around 5% manganese dioxide will produce dark
gray to black firing bodies.
Metallic - Large amounts of manganese can produce metallic
effects in a glaze. However, these glazes must not be used on food surfaces.
Al2O3 37%, SiO2 48%. A calcined china clay or aluminium silicate used as
a fine powder to increase the firing temperature of glazes as it introduces
alumina and silica; it reduces the tendency to crawl in glazes with high clay
contents. Large sizes are used as grog for white bodies and to open the texture
of bodies; it induces mechanical stability and resistance to thermal shock
through the development of mullite crystals.
K2O 9.1%, Na2O 7%, Al2O3 24.9%, SiO2 56%. A beneficiated mineral similar
to but more fusible than feldspar, it has lower silica and higher soda and
potassium levels and so may be used when a lower melting temperature is
required. It has a fairly narrow vitrification band typically used in vitreous
NiO3. Gives brownish greens through to grey colours (1-3%). In reducing
conditions a yellow or blue may be obtained in high zinc stoneware glazes
0.15 nickel with 0.15 zinc oxide gives a brown, 0.25 zinc a reddish purple, and
0.35 zinc a dark blue.
K2O 0.2%, Na2O 1.6%, Li2O 4%, Al2O3 15.7%, SiO2 76.1%. A secondary
Lithia, alumina, silica bearing flux for use in high temperature bodies such as
porcelain and high temperature glazes. It can be used to alter colour response
and to reduce thermal expansion lowering maturing temperatures without
shortening of firing range, especially when used as a replacement for feldspar.
K2O 11.3%, Na2O 3.2%, Al2O3 18.5%, SiO2 65.8%. One of the most important
materials for medium and high temperature ceramic glazes. Potash feldspars are
not usually as pure and white as soda spars. Glazes high in feldspar (35% or
more) are likely to produce crazing problems. 'Flux saturated' glazes with more
than 50% feldspar may be unbalanced and lack adequate glass former or alumina. SODA
FELDSPAR (K2O 2.8%, Na2O 8.5%, Al2O3 18.5%,
SiO2 69.5%) is more suitable at lower temperatures. If recipe only states
Feldspar it usually means Potash Feldspar. FFF Finnish Floatation Feldspar (K2O
7.5%, Na2O 3.2%, Al2O3 18.5%, SiO2 67.5%) is a high quality product half
way between Soda and Potash Feldspar. (The
name Felspar is sometimes used but Feldspar is traditionally the correct name
and is used throughout Europe).
Pr6O11. Praseodymium Oxide gives a small range of vibrant lime green
colours in oxidation and reduction at concentrations of 5-8%. In small amounts
(0.65%) in reduction with a trace of iron, gives a bright spring green colour.
SiO2. Quartz sand is often used in bodies as grog for texture and to
increase thermal expansion and craze resistance. Powdered quartz is used in
glazes and bodies also. Quartz of very fine particle size (-400 mesh) will
typically enter the feldspathic melt or convert to cristobalite during firing if
fluxes are lacking, coarse powdered grades help to 'squeeze' glazes into fit.
Intermediate sizes (200-300 mesh) seem to be best however, since their greater
surface area exerts more compressive squeeze per unit.
TiO2 +. Rutile is the mineral name for natural crystals of titanium
dioxide. In nature rutile is always contaminated by up to 15% other minerals
(especially iron but also things like tantalum, niobium, chromium and tin). The
term 'rutile' is thus generally understood to refer to the brown powder into
which these minerals are ground and industry accepts up to 15% contaminants and
yet still calls it rutile (below 85% titanium is called ilmenite). Rutile is
considered an impure form of titanium whereas ilmenite is considered as FeTiO3.
ceramic glazes rutile is more often considered a variegator than a colorant. As
little as 2% can impart significant effects in stoneware glazes. It is normally
used in combination with a wide range of metal oxide and stain colorants to
produce surfaces that are much more visually interesting. In glazes with high
melt fluidity (e.g. having high boron), large amounts of rutile (e.g. 8%) can be
quite stunning. The rutile encourages the development of micro-crystals and
rivulets. Since rutile contains significant iron its use in combination with
other colourants will often muddy the colour that they would otherwise have or
alter it if they are sensitive to the presense of iron. Even though rutile
generally makes up less than 5% of stoneware glazes that employ it, they are
often called 'rutile glazes' in recognition of its dramatic contribution.
SCMC GLAZE BINDER Glazes being brush applied generally incorporate up to 5% of a glaze binder. Also useful in 0.5 to 2% addition to prevent colour transference.
A medium silica sand used for either placing or as a grog, helping glaze fit and
making the body more refractory.
Silver nitrate is highly soluble and forms Ag+ ions when dissolved in
water. This ion transfers to ware as adhered material and when fired and reduced
the Ag metal gives a gold sheen. It is possible to dry blend the nitrate and
carbonate forms and putting these into a glaze to get a better gold sheen. The
Nitrate is hazardous the Carbonate is not. 100 grams of Carbonate is equivalent
to 123 grams Nitrate.
SODIUM CARBONATE Na2CO3. In ceramics, a common use of soda ash is as a soluble deflocculant in ceramic slips and glazes. It works well in combination with sodium silicate to produce slips that do not gel too quickly and whose rheology can be adjusted for changes in the hardness of the water. Higher soda ash in proportion to sodium silicate will produce a slip that gives a softer cast (stays wet longer). The total soda ash and sodium silicate amount should be tuned to create a slip that will eventually gel if left to stand. This thixotropic behavior will prevent it from settling.
low solubility glazes it will weaken their resistance to acidic attacks.
SODIUM SILICATE Na2 SiO2. The most popular deflocculant used in casting slips for many years. It is nearly always used with soda ash (when employed alone it can make a slip 'stringy' and thixotropic). The material is effective, reliable and inexpensive. However, it attacks the plaster in molds much more than more modern deflocculants and it is easier to over-deflocculate a slip with sodium silicate. The 75TW with Bone China and Terracotta, the 140TW with everything else.
FRIT 900 - 1140oC.
Often used in the production of earthenware glazes when a lead-free glaze is
required. Slight milkiness, especially at low temperatures, may be evident over
red clays, and the colour response with oxides etc is not as vivid as with lead
SrCO3. A flux typically used in glazes at stoneware temperatures (above
1090oC) with similar effects to whiting and zinc oxide but with less pinholing
and as an alternative to lead oxide. It
has been used in glazes at low temperatures as low as cone 01 giving a gloss
mirror finish with good heat shock resistance. Excessive additions will
precipitate a crystalline matt surface.
Typically MgO 32/33%, SiO2 46%. No talcs have the theoretical chemistry,
the most common impurities are CaO (up to 8%) and Al2O3 (up to 9%). Along with
dolomite, and to a less extent magnesium carbonate, it is an important source of
MgO flux for bodies and glazes. Dolomite and magnesium carbonate have high loss
on ignitions which can produce glaze bubbles, blisters and pinholes, talc is
much less of a problem in this respect.
SnO2. Tin oxide has long been used to opacify glazes (make transparents
opaque) at all temperatures. Hand decorated tin glazed earthenware of the
1700/1800s is the most famous use of tin in glazes (delftware-England,
faience-France, maiolica-Italy). While many potters are keeping this tradition
alive today most now use zircon based opacifiers instead.
any discussion about the use of tin oxide as an opacifier ends up comparing it
with zircon products: Twice as much zircon is required to produce the same level
of opacity. Like zircon, tin melts at very high temperatures and thus does not
go into solution in typical glaze melts. Zircon will stiffen the glaze melt more
than tin. Zircon is likely produce a harder glaze surface. Zircon will reduce
the thermal expansion of the glaze more than tin. The quality of the white
colour is different (tin tends to be more of a blue white, zircon a yellowish
white). Tin is very expensive, this is likely to be the main reason for its much
more limited use as an opacifier today. Zircon tends to have less of an effect
on the development of metal oxide colors (e.g. tin reacts with chrome to make
pink). Tin can react with titanium and rutile to variegate the glaze.
opacifiers also tend to variegate the glaze. Copper red glazes require tin, with
iron in oxidation tin makes a warmer shade of brown than zirconium does.
Pink - Chrome
and tin are the most well known way to produce pink. For example, 7.5% tin and
0.5 chrome oxide will produce pink. Many Cr-Sn stains are available to make many
shades of pink. However this mechanism requires that the glaze chemistry be
right (e.g. no zinc, boron not excessive).
White - The quality of colour tends to be a 'soft-bluish
white'. As little as 4-7% can produce brilliant white, although it is more
typical to use 8-10% for full opacity. However, be aware that even tiny amounts
of chrome in the kiln will volatilize and combine with the tin to produce pink
- Variegation -
Tin/Iron Effects - Tin
reacts with iron in fluid glazes to produce variegated surfaces. A good example
is the Albany Slip 85, Tin 4, Lithium 11 glaze for cone 6.
TiO2. Although titanium is the strongest white pigment known for many
uses, in ceramics the whiteness (and opacity) it imparts to glazes is due to its
tendency to crystallize during cooling. Although titanium dioxide is used in
glazes as an opacifier, it is not as effective and easy-to-use as tin oxide or
zircon. It can be used as an additive to enliven (variegate, crystallize) the
colour and texture of glazes (rutile works in a similar manner). In moderate
amounts it encourages strong melts, durable surfaces and rich visual textures
amounts below 1% titania can dissolve completely in a glaze melt. In slightly
greater amounts it can give a bluish-white flush to transparent glazes
(depending on their amount of alumina).
can be reduced to produce colours in keeping with the elements present. If
highly reduced it can yield a red, with iron the colour could be yellow, brown
or green. Other combinations can yield blues, greens, yellows. Titania is
oxygen-hungry and will quickly oxidize from its reduced state if given the
- Crystal Matte - Titanium
can be used in glazes to produce a matte surface with increasing amounts of
crystallization in amounts up to 25%. The effect works in most stoneware glazes
and is better when the glaze is slow cooled.
White - Titanium is a crystalline mineral and encourages
crystal development during cooling and freezing of the glaze melt. This
generally produces opacity. However, titanium opacified glazes have a much
different character than zircon or tin types. The latter produces a much more
even and bright white coloration. When used as an opacifier the batch amount can
range to 10% or more of the recipe. Lead greatly enhances the yellow at low
Variegation - Smaller
amounts of titanium dioxide (i.e. 5%) added to coloured or opacified recipes can
variegate the surface and make it more interesting (e.g. it alters the shape of
crystals, shade of colours). The more you use the greater the effect (up to
Red - In high fire matte glazes, iron oxide and titanium
can produce red colours.
V2O5 A colouring agent that produces yellows in amounts up to 10%,
especially when in combination with tin oxide. Its colour in generally weak, but
can be strengthened when fritted with tin and zirconia. Although yellows can be
prepared with antimony, vanadium is more stable at higher temperatures. The most
vibrant colour is obtained in leaded glazes.
CaCO3. A fine calcium carbonate (limestone) used as the principle source
of lime in glazes and also a flux at high temperatures. Also, contributes
hardness and durability. Under reducing conditions it assists in development of
the celadon colours; and can enhance salt glazes by enabling a thicker glaze.
Too much whiting in recipes will lead to dull and rough surfaces.
CaO47% SiO2%. The fibrous form of wollastonite can be very beneficial in
bodies. In low fired ceramics wollastonite reduces drying and firing shrinkage
and drying and firing warpage. It also promotes lower moisture and thermal
expansion in the fired product. It fires with no LOI and its fibres help vent
out gassing. These factors have made it a valuable component in tile bodies,
especially for fast fire. Vitreous and semi vitreous bodies can also show
reduced shrinkage with small additions (2-5%), however wollastonite becomes a
stronger flux as temperatures go above 1100C.
ZnO. Zinc oxide is a fluffy white to yellow/white powder with a very fine
particle size coupled with high surface area. It can be an active flux in
smaller amounts. It generally promotes crystalline effects and matteness/softness
in greater amounts. If too much is used (10%) the glaze surface can become dry
and the heavily crystalline surface can present problems with cutlery marking.
Also at high levels other surface defects like pitting, pinholing, blistering
and crawling can also occur (because its fine particle size contributes to glaze
shrinkage during drying and it pulls the glaze together during fusion). The use
of zinc in glazes is limited by its price, its hostility to the development of
certain colours and its tendency to make glazes more leachable in acids. Zinc oxide will produce opacity or whiteness, especially at
low temperatures, if the calcium content is low. It does not opacify as well in
boron glazes. It works well in combination with tin.
has a complicated colour response. It can have harmful and helpful effects on
blues, browns, greens, pinks and is not recommended with copper, iron, or
Enabler - Almost
all crystalline glazes are high in ZnO, its presence coupled with low alumina
and adequate SiO2 is the secret. The very fluid melt created is perfect for
growing a wide range of metallic zinc-silicate crystals.
- In larger
amounts ZnO can produce opacity or whiteness in glazes. It exhibits refractory
properties and can contribute to the development of a crystal mesh surface.
ZrSiO4. Zirconium silicate is extremely stable and will survive to very
high temperatures in a glaze melt without dissolving (although small amounts do
dissolve). 4-7% produces semi opaque, 8-10% usually produces full opacity but
some transparent glazes require even higher amounts; amounts beyond 20% reach
saturation where crystallization begins to occur. The exact amount needed varies
between different glaze types. It is thus most effective at low temperatures.
Tin oxide can be a more effective opacifier than zircon (it has various
advantages and disadvantages).
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