Techno File: Lanthanide Oxides Linda Bloomfield and David Pier

Appears in the March 2023 issue of Ceramics Monthly.

Lanthanide metal oxides make up most of what are commonly known as the “rare earths.” Despite their name, many of these are not all that rare, and relatively recent discoveries in places such as Sweden have exposed large deposits. Several of the lanthanide oxides give dramatic, vibrant colors in a wide firing range.

In the News

Rare Earth Mines

In early January 2023, the Swedish state-owned mining company, LKAB, announced the discovery of more than 1 million tons of rare earth metals, Europe’s biggest deposit to date. Rare earth elements are currently not mined in Europe, which has left the region dependent on imports from elsewhere.

Such minerals are essential to many high-tech manufacturing processes and are used in electric vehicles, wind turbines, portable electronics, etc.—in addition to use as coloring oxides by ceramic manufacturers. The discovery could lead to a significant step forward in producing the necessary raw materials crucial to a green transition. Sourced from www.reuters.com, January 13, 2023.

Recycling Lanthanides

Rare earth oxides are expected to grow in demand due to their use in magnets, electronics, and green initiatives including wind turbines and hybrid cars (rare earths are an essential part of renewable energy technologies needed to get the world to a low- or zero-carbon future). 

Mining these minerals is both cost and environmentally prohibitive. Recycling rare earth elements is also labor intensive, but scientists are trying to make it easier. Methods being researched include robots dismantling small electronics to separate out the rare earths, the use of copper salts to pull rare earths from discarded magnets, and employing microscopic bacteria to naturally produce organic acids (rather than hydrochloric acid) to extract lanthanides from various mixed metals. Sourced from www.sciencenews.org, January 20, 2023.

Rare Earth Metals

The rare earth elements cerium (Ce), praseodymium (Pr), neodymium (Nd), holmium (Ho), and erbium (Er) are used to color glass and, in the case of praseodymium, to make a yellow stain for glazes. They were originally found mixed together and were difficult to separate as they have very similar chemical properties. Neodymium (meaning “new twin”) is found mixed with praseodymium (meaning “green twin,” from the Greek word prasios didymos), and the pure oxides give respectively a violet and green color in glazes. The rare earths are also known as the lanthanides as they follow the element lanthanum in the periodic table (shown below). They are refractory and have the theoretical formulas CeO₂, PrO₂, Nd₂O₃, and Er₂O₃. They are weak colorants but can be used in glazes on porcelain. The insoluble oxides are not toxic, although the firing fumes should be avoided. Some of the other lanthanides (samarium, europium, terbium, dysprosium, and thulium) can give bright fluorescent glaze colors, but only under ultraviolet light. —Linda Bloomfield

Advantages

The primary advantage of using pure oxide colorants over stains is the possibility of transparency or translucency. In order for a stain to maintain its color, it necessarily remains as a distinct particulate suspended in the glass, thereby opacifying as it colors. Pure oxides can color while leaving the glaze otherwise transparent, as long as they fully dissolve in the molten glass, which the lanthanides easily do.

Transparent glazes possess what is often called depth, which includes the intensification of color with increasing thickness, allowing such beautiful effects as highlighting incising or relief and fading on edges. Transparency lets you easily see the crackle in a glaze (it’s called crackle if you like it, or crazing if you don’t). Dissolved oxide colorants are also much more sensitive to small changes in the glaze—responding to other colorants, firing and cooling conditions, and the clay body. In other words, dissolved colorants are responsible for all of the subtle effects that make glazes so unlike paints. A glaze colored with a pink stain will look much like pink paint, while a glaze colored with a pink oxide will look something like pink sugar candy. Of course, it is necessary to apply it to a light-colored clay body to prevent muddying of these colors.

The lanthanide oxides are very dense, which may cause some glaze-batch suspension problems but improve the fired results. Denser glazes have higher refraction (light-bending ability), making their effect on incident light more like diamond and lead crystal, and less like plastic. —David Pier

Toxicity

Because of their unfamiliarity and exotic names, some potters suspect the lanthanide oxides may be toxic or even radioactive. Not so. With the exception of promethium (which doesn’t exist naturally) and thorium (which is actually an actinide and is removed during separation of the various elements), they absolutely are not radioactive. And while the lanthanides have not been researched as much as the commonly used metal oxides, they have low acute toxicity and almost no evidence of chronic toxicity. In fact, the lanthanides are less toxic than copper, cobalt, nickel, zinc, and other commonly used transition metal colorants. Of course, it’s always important to check the materials safety data sheets (MSDS). None of the lanthanides volatilize significantly, even at cone 10, allowing the formulation of extremely hard and durable glazes. —David Pier

the authors Linda Bloomfield, author of the book, Science for Potters; and David Pier, author of the article, “Using Rare Earth Colorants,” Ceramics Monthly, September 2002. Both published by The American Ceramic Society.