The rare-earth oxides are not particularly rare, but they can be expensive and have only recently become widely available. They are scandium, yttrium, and the heavy metals found near the bottom of the periodic table, in the lanthanide series (in the pink section) (1). Of the fifteen metals in the lanthanide series, only four are widely used for coloring glazes.
These are cerium, praseodymium, neodymium, and erbium. Cerium oxide can be used as an opacifier at low temperatures and gives a tan-yellow or orange color in glazes, particularly when combined with titanium or rutile. Praseodymium oxide is black but gives pale green in glazes. It’s combined with zirconium silicate to make a yellow stain. Neodymium oxide is pale lavender and gives a pale violet color in glazes, although it looks blue under fluorescent light. Erbium oxide is the heaviest of the four and is pale pink.
Other rare-earth oxides—samarium, europium, terbium, dysprosium, and thulium—can be used to make bright fluorescent glazes, although the colors (fluorescent orange, pink, green, and blue) can only be seen under ultraviolet light and the glazes look colorless in daylight. The rare-earth oxides settle quickly in the glaze bucket and require the use of bentonite or CMC gum in the glaze. They’re refractory and aren’t toxic. They’re quite weak colorants and 5–10% can be added to a glaze. They’re best used in alkaline glazes (which increase their solubility) on porcelain.
*Note: Glazes containing barium carbonate should be restricted to use on surfaces that do not come into contact with food or beverages.
One-gallon pitcher with 12-ounce cups, cast porcelain, with Unreal Blue Celadon and Unreal Green Celadon glazes ﬁred to cone 8–9 in reduction, and Day-Glo Pink Lemonade and Day-Glo Lime Jell-O glazes ﬁred to cone 8–9 in oxidation.
Recipes and test tiles by David Pier originally appeared in Ceramics Monthly, September 2002.
Excerpted form Colour in Glazes by Linda Bloomfield, available in the Ceramic Arts Daily Shop.