Digitalfire Ceramic Oxides Directory

Cr₂O₃ (Chrome Oxide)

Family	Colorant
Weight	152.000

MLPT - Melting Point (MP)

2257-2427C (From The Oxide Handbook)

Notes

-Amphoteric chrome oxide is the only stable oxide of chromium metal and can be used at all temperatures to 1200C (after which it can volatilize somewhat).

-Chromium is a 'fast' color, meaning it produces its characteristic green in slow or fast and oxidizing or reducing firing. It is also used in paints and dyes.

-Chromium is used in the glass industry to make green glass (up to 1%). Antimony is sometimes used as a reducing agent to ensure an emerald green.

-Chromium is not very soluble in glass and does not form silicates or combine with fluxes readily unless compounds are finely ground and dispersed and amounts are not excessive (1% will dissolve in most glazes).

-Zircon opacifier 1-2% is often added to chrome glazes to stabilize them and prevent brown edges. Amounts up to 3% in a glaze recipe gives opacity and greyish green coloration.

-Chrome oxide can be used as a body stain in amounts to 5% to give grey-green.

-Drab chrome greens can be moved toward peacock green with the addition of cobalt oxide (1% each gives bright color). This works in boron and soda glazes.

-Chrome in zinc glazes tends to form brown zinc chromate.

-Because 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).

-Chromium oxide is added to enamels for green where borax and zinc are used to increase the brilliance of the color. However, chrome in ground coat enamels tends to react with the metal to cause blistering.

Mechanisms

Glaze Color - 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.
Glaze Color - Green
Chrome is a classic green colorant for recipes in oxidation and reduction at all temperatures. However, the shades it produces can be opaque, dull, and uninteresting. In the presence of CaO, the color moves toward grass green.
Glaze Color - 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.
Glaze Color - Brown
Chrome in zinc glazes tends to form the stable crystalline compound, zinc chromate (ZnCr2O4)
Glaze Color - Orange
Chrome in high lead glazes forms yellow lead chromate. Zinc and chrome tend to produce orange.
Glaze Color - Black
Chrome is a constituent in almost all black oxidation colors. It is used up to 40% in Cr-Co-Fe blacks and as high as 65% in Cu-Cr blacks.
Glaze Color - Pink to 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%.
Glaze Color - Red Chinese
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 color toward yellow.
Glaze Color - 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 green tints.

Out Bound Links

(Materials - Material source) Chrome Oxide - Cr2O3
Chromium (III) Oxide, Cr2O3, Chromium Oxide
(Materials - Material source) Potassium Dichromate - K2Cr2O7 - A bright red-orange crystalline powder
(Materials - Material source) Stain (green) - Greens (CrCa)
(URLs) Chrome Oxide at Wikipedia
http://en.wikipedia.org/wiki/Chromium(III)_oxide
(Articles)
Formulating a Clear Glaze Compatible with Chrome-Tin Stains
In ceramics color is often a matter of chemistry, that is, the host glaze must be compatible and hav...

In Bound Links

(Oxides - Related) CrO3 - Chromium Trioxide

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Digitalfire Ceramic Oxides Directory

Cr2O3 (Chrome Oxide)

Notes

Cr₂O₃ (Chrome Oxide)