Lanthanum, the least rare-earth metal, does not behave like a normal colorant, or even like other rare earth metals. Instead of providing color to a glaze, it acts as an opacifier. Find out why and how to use it.
Define the Terms
Color Doping: The addition of a transition metal (colorant) into a crystalline glaze, to dictate the color of the crystals that grow in the glaze.
Crystalline Pottery: Ceramics fired with a glaze resulting in the precipitation of zinc silicate crystals along the surface.
F Orbitals: The fourth electron orbital, preceded by s, p, and d orbitals, which defines the expected energy and location of a particular electron. These orbitals have a clover shape, surrounding the nucleus of an element, and first appear in elements in the lanthanide series.
F Orbital Splitting: A change in energy when a ligand bonds with a transition metal, as suggested in crystal field theory.
Lanthanum (La): The most prevalent of rare earth metals in the lanthanides, it is element 57 on the periodic table. La is a solid at room temperature that often exists as an oxide among other minerals.
Seeding: Adding a mixture of crystals into a crystalline base to nucleate (grow) more crystals within the base.
Lanthanum is the first element in the lanthanide series on the periodic table, which are often referred to as the rare earth metals. While not all of them are rare, these elements can be added to ceramic glazes in their oxide form to produce vibrant colors through a different mechanism than traditional glaze colors—f orbital splitting. This notoriously produces “highlighter” colors and results in dichroism in holmium and neodymium. Strangely, when lanthanum is added to crystalline glazes, it has a unique color doping process, in which the transition metal primarily acts as a colorant rather than lanthanum itself.
Lanthanum (1) also almost never exists in pure mineral deposit veins, but instead typically resides in monzonite and bastnaesite.
Crystalline pottery, in a simple definition, is a high-temperature chemical reaction involving silica and zinc oxide. Zinc silicate forms and starts to deposit and grow as crystals after the firing between 1400ºF (760ºC) and 2000ºF (1093ºC). As these grow out from a center seed point, the crystalline lattice can pick up metal colorant atoms from the surrounding available glaze, which take the place of other zinc silicate materials that would have locked into the crystalline lattice. The zinc silicate crystals are translucent white without the presence of other metals, but can be colored by adding transition metals such as cobalt, copper, manganese, iron, nickel, etc. The metals become trapped in the crystalline lattice as they form and grow. This coloration process is known as color doping, as small metal atoms change the appearance of the crystals.1, 2
Crystal Formation and Color Doping
Crystalline pottery most often uses Ferro frit 3110, zinc oxide, and silica. The added materials within the firing environment create zinc silicate, which will ultimately seed and deposit into crystals as the glaze cools. These crystalline seeds nucleate throughout the glaze surface during initial cooling. The zinc silicate deposits around the seeds, resulting in their continued growth during cooling cycles/holds, forming the notable crystals in crystalline pottery. It could be suggested (by crystal-field theory) that as zinc silicate crystallizes, transition metals, like lanthanum and cobalt, get picked up and lock into the crystalline structure, forming a crystal field around the pigment to produce color. When light hits the metal pigment, some light is absorbed while the remainder is transmitted and eventually reflected off the surface. The reflected light is rebuilt by the eye and perceived as a color in the visible light spectrum depending on the transition metal. One metal can produce a vast array of colors depending on the ligand bonds formed.2–4
One may expect lanthanum to behave like other transition metal colorants, due to its empty f-orbitals; however, lanthanum appears to diminish crystal growth and shifts crystal coloration from white to silver and bronze colors at increasing concentrations. Additionally, lanthanum inhibits both color and crystal size.
For the following glazes, 100g of a standard mid-range crystalline base was used and then lanthanum oxide was added in 1.0g increments, fired on a schedule previously published (“No-Grind Crystals” in the June/July/August 2022 issue of Ceramics Monthly), and the imaged with an Olympus SZX12 microscope and an Infinity8 camera. Results are present in figures 2 and 3, both for a neat crystalline glaze base and the glaze base colored with 1.0g cobalt oxide. Notable changes appear in the opacity of the glaze surface in addition to the crystal size, color, shape, and frequency.
Figure 2 shows first the presence of neat zinc oxide crystals in a standard mid-range crystalline base. The translucent/white crystals are the largest with no lanthanum oxide and form spiked circular patterns. As lanthanum oxide is added to the glaze, the crystals shrink in size, become more uniformly circular and start to turn silver. At 2.0g of lanthanum oxide, a silver color is observable on the outer edges of the crystals, moving inward. Above 2.0g of lanthanum oxide, opacity is observed in the glaze making the clay color no longer visible. Moreover, surface crystals shift to a silver color while decreasing in size, as lanthanum is added in increasing amounts.
Interestingly, the addition of lanthanum to a cobalt-containing crystalline glaze base provided very unexpected results. Instead of the glaze turning gray/silver as a function lanthanum addition, it turned brass/bronze around the periphery of the crystals and started to grow inward, overtaking the cobalt blue color, as seen in figure 3.
Figure 3 shows the unique color of lanthanum-doped crystals as cobalt is introduced to the base glaze. The light blue spiked crystals appear with no lanthanum oxide added to the base glaze. The crystals form in round spiked patterns throughout the surface of the glaze. As increasing amounts of lanthanum oxide is added, the crystals shrink in size and progressively change to a silver/bronze color—starting at the edges and moving to the interior of the crystal. Tile 1 has large vibrant blue crystals on a shiny blue background glaze. The silver/bronze coloration first starts to appear on the crystal edges at 1.0g of lanthanum oxide on tile 2. At 3.0g of lanthanum oxide added, the crystals are almost entirely silver/bronze with limited blue interior and continually shrink in size. Furthermore, the glaze starts to become less translucent and more opaque with each addition of lanthanum oxide; this suggests that lanthanum is both a crystal color doper and an opacifier. At 5.0g of lanthanum oxide, the larger circular crystals are entirely silver/bronze and only appear along the edges of the tile while the smaller crystals are spread throughout the glaze.
the authors Craig Caudill is a Science Leadership Scholar at the Jepson School of Leadership Studies, University of Richmond. He majors in leadership studies with a minor in chemistry.
Ryan Coppage is chemistry faculty at the University of Richmond. He fiddles with various glaze projects and makes a reasonable number of pots. To see more, visit www.RyanCoppage.com.
Acknowledgments go to Stacey Criswell, PhD from the University of Richmond for guidance and for offering the biological imaging suite for use. Also, to Michael Leopold, PhD from the University of Richmond for providing laboratory space.