Rare Earth Colorants
Article by Max Campbell and Chris Keane

Introduction
We will be introducing you to a new range of glaze colorants that offer exciting new possibilities. They offer lemonade pinks, purple blues and acid greens. The colorants are based on oxides of the rare earth metals, neodymium, praseodymium and erbium. As the oxides are soluble in the glaze we can achieve transparent colored glazes. Our preliminary testing suggests that the colors can be fired at earthenware, mid-fire or stoneware temperatures, as well as in oxidation and reduction without significantly affecting the color achieved.

The use of rare earth colorants in ceramics is a recent development as the materials are only now available at reasonable prices.

Investigation of the colorants is only at the starting point and our work is only a preliminary look.

What are the Rare Earths?
The periodic table is the way scientists set out the elements, grouping them in columns of materials having similar nature, i.e. Li, Na, P etc are all metals having a single electron in the outer shell. When the table was established there was a group of elements that did not really fit, so rather than changing the model they were separated off. At the time they were considered rare, hence the name, Rare Earths.

In reality they are not particularly rare, although, ore bodies large enough to mine are not particularly common. Cerium is thought to comprise 60 ppm in the earths crust, whereas, copper is at 50 ppm. Yttrium is though to comprise 33 ppm and Neodymium 28 ppm.

The lanthanides are more readily available as we have found more uses for them, consequently, we have developed more economic refining processes. In addition, they are now available at lower purity levels, i.e. 99% for ceramics, whereas, other applications may require 99.9999%.

We now come across the lanthanides in a wide range of applications:

• More powerful motors can be produced with neodymium permanent magnets.
• Tantalum is used in the electronics industry especially mobile phones.
• Large quantities of neodymium and praseodymium are use as glass colorants.
• Cerium oxide is used in catalytic converts for petrol cars.
• Yttrium oxide in oxygen probes.
• Erbium in optical devices such as night vision goggles, laser beams and fibre optics.

Praseodymium Oxide
Praseodymium was discovered in 1885 by Aver von Welsbach. Its name comes from the Greek prasios meaning green and didymos meaning twin.

It comes as a black powder with a particle size in the range 5 to 15 microns. The ceramics grade is 99% pure and its primary uses are for coloring glass and ceramic glazes. We believe that it has been used in stains such as yellow 28CY4.

In glazes it produces transparent green colors.

Neodymium Oxide
Neodymium was only discovered in 1925 by Aver von Welsbach. It name comes from the Greek neos meaning new and didymos meaning twin.

It is available as a blue gray powder with a particle size range of 1 to 10 micron. The ceramics grade is 99% pure and its primary uses are coloring glass and ceramic glazes. In glazes it produces blue to lavender colors.

Erbium Oxide
Erbium was discovered in 1843 by Carl Mosander and named after Ytterby in Sweden (?).

It comes as pink powder with a particle size in the range 5 to 15 microns. It is used in optical devices, laser beams, phosphorescent materials, as well as, a colorant for glass and ceramic glazes.

In glazes it produces a stunning pink color.

Development Program
Our objective was to develop an over view of the performance of the oxides as glaze colorants. We wanted to assess their usefulness to potters and whether we should proceed with them. We only had limited quantities of the oxides available and had to make every gram count.

We undertook several simple preliminary test demonstrating that the oxides could be fire at earthenware, mid-fire and stoneware. Having satisfied ourselves that the work was worthwhile we proceeded in toe assessment program.

Step 1
We looked at the interaction of the colorants with a variety of commonly used glaze fluxes. Glazes high in sodium, barium, calcium, magnesium, lithium, boron, zinc were prepared. We did not bother to produce a high quality glaze as our interest was only in the effect on the colorants. We also used two commercial glazes, one a clear stoneware and the other a clear mid fire.

The colored glaze trials were brushed onto bisqued white stoneware tiles and fired to 1280C in an electric kiln.

Irrespective of the flux used we did not observe any significant change in the color achieved.

Step 2
Using the clear stoneware base we added oxide in the range 2.5 to 50%. Again the trials were fired in an electric kiln to stoneware.

Increasing the percentage of oxide only increased the intensity of the color.

Step 3
The literature suggested that small quantities of iron could intensify the color achieved.

Again using the clear stoneware base we added 10% of the Neodymium oxide to produce GLAZE A. A batch of the stoneware glaze was blended with the following quantities of traditional glaze colorants, GLAZES B1 to B6
Iron, 1%
Nickel, 1%
Tin, 3%
Copper, 1%
Cobalt, 0.5%
Vanadium, 2%

The glaze mixtures were used to produce line blends as follows:

Glaze A
8 parts Glaze A, 1 part Glaze B
8 parts Glaze A, 3 parts glaze B
8 parts glaze A, 5 parts glaze B
8 parts glaze A, 8 parts glaze B
Glaze B

This was repeated for each oxide and each traditional colorant.

In this trial the colored glaze was applied to bisqued Southern Ice tiles and fired in an electric kiln to stoneware. A selection was fired in reduction in a gas kiln.

Firing in reduction does not appear to significantly effect the color achieved.

We did not observe that any of the colorants had an unexpected effect on the oxides. Ideally we would have maintained the same level of oxide in each trial, but we were limited to the material available.

The trials provide a starting point for further development.

Step 4
The lanthanide oxides were mixed together to investigate the colors that could be achieved. We applied our aesthetic judgment and did not mix the pink and green oxides.

Again the clear stoneware was used as a base and 10% of the lanthanide oxide was added. The blended glazes were mixed in the following ratios.

4 parts Erbium glaze
3 parts erbium glaze and 1 part neodymium
2 parts erbium glaze and 2 parts neodymium
1 part erbium glaze and 3 parts neodymium
4 parts neodymium

The sequence was repeated for the neodymium praseodymium blend.

Again the glazes were applied to Southern Ice tiles and fired to stoneware in an electric kiln.

A stunning array of colors resulted

Safety
There are several important factors to be considered, the oxides are hydroscopic, i.e. they will absorb moisture from the atmosphere. They will also absorb carbon dioxide, consequently, they should be stored in air tight containers.

All chemicals must be treated with respect and care. Good hygiene should be adopted.

Eye irritation has been observed in rabbits with dosages of 100mg.

The LD 50 level for rats is 5 gms per kilogram of body weight, i.e. 50% of the rats died at that level of ingestion. To put that in perspective that dosage would require a person weighing 75kg to consumer 375 gm. We do not know what dosage killed the first rat.

The oxides are considered non hazardous for transport by road, ship or plane.

All the information you may require can be obtained at the TOXNET web site.

Conclusion
An exciting range of rare earth glaze colorants - neodymium oxide, praseodymium oxide and erbium oxide offer the opportunity to decorate with transparent glazes in colors that could not be achieved from other sources, especially at stoneware temperatures.

The colors achieved appear to be unaffected by the glaze flux used, firing temperature or whether it is fired in oxidation or reduction.

The oxides can be mixed to produce an even broader pallet of color.

Our investigation has only touched the surface we have not considered crystalline glazes or raku firing or whatever variation you can imagine.

Rare earth colorants may be obtained in the USA from Laguna Clay and in Australia from Clayworks.