(Sources: Chrome Oxide, Potassium Dichromate)

-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.

Properties

• Fusion - 2257-2427C

  From The Oxide Handbook

• 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 brown zinc chromate.

• Surface Modifier - Orange

  Chrome in high lead glazes forms yellow lead chromate. Zinc and chrome tend to produce orange.

• Surface Modifier - 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.

• Surface Modifier - 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%.

• Surface Modifier - 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.

• Surface Modifier - 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.

Authors

• Tony Hansen (Owner)

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