By David Pier

WHAT?
       The lanthanides are the elements numbered 58-71, 57-71 if lanthanum itself is included. They're in the top of the two rows of the section of the periodic table that is usually cut out and shown below the rest of the periodic table. This arrangement is done to make the table fit nicely on an 8.5x11 piece of paper. They always occur in mixed mineral ores with yttrium and scandium oxides. The lanthanides together with yttrium and scandium are known as the rare earths. Despite the name, many of these metals aren't actually very rare. Their mixed ores occur in abundance in several places in the world. Due to a recent feedback loop between increases in mining and improved separation technologies, the prices of all of these metals have come down dramatically. Some of them don't give nice colors or any color, some are more expensive than gold, but neodymium, praseodymium, and erbium give luscious new colors at affordable prices.

WHY?
       First, what does the word depth mean, when used in reference to glazes? It is vague, but generally it is applied to a group of glaze properties including transparency or translucency, high refraction, and variation in appearance due to variation in thickness of the glaze.
       Refraction is when the light changes direction when it passes into a material, why your arm looks bent when you stick it under water, and part of why diamonds (and glazes) sparkle. More refraction is usually considered more pleasing to the eye. More refractive glazes are also glossier. Since the lanthanide oxides are very dense, they are highly refractive, and they increase the refraction of any glaze they are added to, increasing sparkle and gloss.
       The color of transparent and translucent glazes intensifies with thickness, allowing the beautiful effects associated with celadon glazes (traditionally colored with dissolved iron oxide) such as highlighting incising and relief, and fading on edges. Transparency or translucency let you easily see the crackle in a glaze (called crazing if you don't like it). Pure metal oxides, including the lanthanides, can dissolve in molten glaze, just as food coloring dissolves in water, leaving it colored but not cloudy. This means the color will respond to the particular composition of the glaze, and the glaze can remain transparent or translucent if the colorant fully dissolves, which the lanthanides can do. Dissolved oxide colorants are also much more sensitive to small changes in the glaze, responding to base glaze composition, other colorants, firing and cooling conditions, and the clay body composition. This is in contrast to stains, which are particles of very particular multi-oxide composition that must remai n distinct within the glaze melt to retain their particular color. This means that stains opacity as they color, giving an effect similar to paint, an appearance lacking “depth.” Which would you rather have, Pepto-Bismol or Pink Lemonade?
       To summarize, since these newly available colorants are dense pure oxides, they can give a variety of responsive transparent or translucent colors with a depth unavailable from stains.

HOW?
       Just as each of the transition metal (e.g. copper, iron) colorants have their own demands, the lanthanide metal colorants have their own particulars, as a group, and as individuals.
       A group characteristic of the lanthanides is high density. This means that if you don't add the right suspension additives to your base glaze, these colorants will sink to the bottom of the glaze bucket. I use a combination of Veegum-T and CMC gum. Veegum-T offers the advantages of bentonite without the iron impurities. Since CMC is an organic gum material, it offers many advantages without affecting the fired glaze one bit (see “Additives for Glazes and Clay Bodies” by Jeff Zamek, December 1998 Ceramics Monthly). CMC takes some getting used to, since it increases the viscosity of the raw liquid glaze without making the glaze apply thicker to the bisque. Note the suggested specific gravities given in the recipes. Following them will help assure proper application.
       All of the lanthanide oxides have very high melting and boiling points, so that they do not volatilize significantly under any normal firing conditions.
       Although all of the recipes I present here are high-fire, I have seen many examples of these oxides used successfully in low-fire, including Egyptian paste and raku glazes. The considerations are the same as for use in high-fire glazes.
       These oxides are hygroscopic, meaning that they absorb atmospheric moisture. This means that if you do not keep them in tightly sealed containers before weighing, your results will change slightly over time. If you buy a large quantity, you might consider dividing it into several small jars or zipper-seal bags. You might notice a slight change in color due to the absorption of water.
       My current understanding is that a reduction atmosphere does not directly affect these lanthanides. They are, however, sensitive to trace amounts of transition metals, particularly iron, and the oxidation states of these transition metals. The net result is that the atmosphere affects their appearance. Praseodymium might be an exception, its color shifted to a greener, less yellow color by reduction, but it is difficult to distinguish affects of direct reduction from affects of trace impurities.
       My crossover tests with the traditional oxides is far from complete, but I have found a few interesting things.
       Nd (Neodymium): The oxide is a blue/lavender powder. It is the strongest of these three colorants, giving a good aqua color in most glaze bases at 1-2%. At 4-7% it gives a pleasant lavender/grape color that appears as a different color under different artificial light sources. It is very strongly affected by iron, particularly in oxidation, giving a reliable perfect neutral transparent gray. Avoid trace iron impurities to achieve the brightest blue and lavender colors.
       Pr (Praseodymium): The oxide is a dark brown color. It gives a small range of vibrant lime green colors 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 color. Other combinations with transition metals have given either beige or no effect.
       Er (Erbium): The oxide is a light baby pink color. It is the most expensive, the densest, and the weakest of these three colorants, but the only way 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 color 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 color in the presence of iron traces in reduction.

BUT WHAT ABOUT TOXICITY?
     The exotic names and odd placement in the periodic table lead people to imagine radioactivity and heavy metal poisoning. In reality, the pure lanthanide oxides are not radioactive and have very low acute and chronic toxicity. It might seem too good to be true, but the lanthanide oxides pose significantly less of a toxic risk than the commonly used transition metal oxides such as copper. They also improve the strength and acid resistance of the glaze. This doesn't mean that they're harmless. Avoid ingestion, use proper respiratory protection, and read the MSDS.

THE LATEST RECIPES

 

Revised PIER'S PURE LUX-DELUX Cone 8-9 Oxidation Base Glaze (v.3.2003)
Ferro Frit FB-284-M	22.8
Ferro Frit 3110	5
Low Iron Spodumene	12.7
Barium Carbonate	14.5
Zinc Oxide	2
6 Tile Kaolin	13.5
325 mesh Silica	27.5
Zircopax	0.8
Veegum-T	1.4
	--- 100%
Copper Carbonate	0.02
CMC gum	0.40
For PIER'S PURE LUX-DELUX PINK LEMONADE Add:
Erbium Oxide	8.0
For PIER'S PURE LUX-DELUX LIME CANDY Add:
Praseodymium Oxide	7.0
For PIER'S PURE LUX-DELUX GRAPE CANDY Add:
Neodymium Oxide	5.5
Additional 6 Tile Kaolin	1.0

Recommended specific gravity for Pink Lemonade: 1.55 g/ml
Recommended specific gravity for Lime and Grape: 1.50 g/ml

Notes:

• Takes two coats to look its best, be sure to allow sufficient drying time between coats to avoid water saturation of the bisque.
• Slow firing for the last few cones helps clear most of the bubbles out.
• I use an Orton small D10 in the kiln-sitter, which, with a slow firing schedule, is equivalent to a large visual D9, tip just touching the shelf.
• CMC gum is almost essential for suspension, particularly for erbium oxide.
• The copper carbonate is present as a preservative for the CMC gum.
• At cone 9, the glaze is a bit runny, but perfectly clear. At cone 8, the glaze is pretty stable, but not as fully transparent as at cone 9.
• This revision does not craze on my porcelain body. If you want crazing (crackle), try increasing the amount of frit 3110 and decreasing frit 3289, or going back to my first published recipes
• The barium content of these glazes is high for two reasons. (1) Barium has a positive effect on the solubility(=transparency) of the lanthanides in the glass. (2) Barium gives glossier, more lustrous glazes due to its high index of refraction. I have subjected these glazes to strong acid and base conditions far in excess of any the glaze would normally encounter, and there was no measurable leaching. These glazes are safe if mixed properly and fired to maturity.
• As discussed in the main body of this article, the lanthanide colors are not extremely sensitive to the choice of fluxing oxides. The fluxes used in this recipe are chosen for their positive effect on lanthanide solubility(=transparency) and other glaze properties such as fit, hardness, and gloss.

 

Cone 5-6 OXIDATION BASE GLAZE (v.3.2003)
Ferro Frit FB-284-M	24.4
Ferro Frit 3110	5.3
Low Iron Spodumene	13.6
6 Tile Kaolin	7.1
Barium Carbonate	15.5
Zinc Oxide	2.1
325 mesh Silica	29.6
Zircopax	0.8
Veegum-T	1.5
	--- 100%
Copper Carbonate	0.02
CMC gum	0.40

Notes:

• Colorant additions and recommended specific gravities are the same as for the D8-9 base glaze. These glazes are not shown in the articles, but the colors are identical to the D8-9 oxidation glazes.
• I use an Orton small D6 in the kiln-sitter, which, with a slow firing schedule, is equivalent to a large visual D5, tip just touching the shelf.
• All other information given in the notes on the D8-9 glazes apply to these D5-6 glazes.

 

Cone 9-11 CELADON TYPE GLAZES
Custer Feldspar	27.5
Grolleg Kaolin	20.0
Wollastonite	26.0
325 Mesh Silica	24.0
Zircopax	1.0
Veegum-T	1.5
	--- 100%
CMC gum	0.40
Copper Carbonate	0.02
For PIER'S UNREAL GREEN CELADON (Reduction) Add:
Praseodymium Oxide	0.65
For PIER'S UNREAL BLUE CELADON (Reduction) Add:
Neodymium Oxide	1.1
For PIER'S BLUE-GREEN CELADON (Reduction) Add:
Spanish Red Iron Oxide (or any red iron, ball milled)	0.07
For PIER'S APPARENTLY LAVENDER CELADON (Oxidation) Add:
Neodymium Oxide	4.5

Notes:

• Trouble free, usually craze-free, very reliable glaze base.
• Recommended specific gravity for all four: 1.5 g/ml
• You can get away without using the CMC gum is these glazes, but it helps with even and thick application, as well as suspension.
• Erbium and higher concentrations of Praseodymium are not used to their best advantage in this glaze base (dull colors, partially due to lower solubility of the lanthanides in this glass).
• Apply fairly thick, two to four coats, allowing time for bisque to dry between coats.
• If your wollastonite has an appreciable iron impurity, you can get an even cleaner color from the neodymium glazes (Blue and Lavender) if you make the following replacement: (26.0% Wollastonite)=(22.4% whiting + 13.4% 325 mesh silica).
• The “apparently” in Apparently Lavender Celadon refers to neodymium's interesting property of dramatically changing color (from lavender to green to bright blue to bright purple) under different lighting types.