Rare Earth Colorants
Article by Max Campbell and Chris Keane
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
- Tantalum is used in the electronics industry especially mobile
- Large quantities of neodymium and praseodymium are use as glass
- 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 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 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 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.
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
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.
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.
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.
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
The glaze mixtures were used to produce line blends as follows:
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
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
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
The trials provide a starting point for further development.
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
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
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
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
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
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
Rare earth colorants may be obtained in the USA from Laguna
Clay and in Australia from Clayworks.