Techno File: Gemstones and Glazes Ryan Coppage, PhD

Appears in the September 2017 issue of Ceramics Monthly.

Gemstones give us inspiration for glaze design: from green emeralds and red rubies, to blue aquamarines and golden beryls, we unintentionally emulate these gemstones in our glaze formulations. Knowing this, we can begin to design glazes to reflect the faceted gem specimens we see in museums. 

Defining the Terms

Beryl: A beryllium aluminum silicate gemstone base that allows for golden, blue, red, pink, and green semi-precious gems. 

Corundum: A crystalline form of alumina, Al₂O₃, that is best known in the form of both rubies and sapphires.

Crystal Field Theory: A theoretical model that describes the splitting of d orbitals by anion neighbors in a coordination environment around a transition metal, absorption of light as a function of those energy differences, and the presence of transmitted or reflected wavelengths of light that are not absorbed by the metal center.

d-Orbital Splitting: When transition metal atoms are surrounded by a material, which inadvertently splits the energy levels of the orbitals.

Oxidation States of Metals (Upper, Lower, Intermediate): The charge of the metal colorant, which is effectively the atomic number of the metal (protons) minus the number of electrons; thus, if a metal is missing 2 electrons, it will have a charge of +2. This charge dictates the color and can be controlled (and changed) by the firing environment. Iron appears blue in small quantities (or black, in large ones) with a +2 oxidation state (reduction firing) and amber/red/brown with a +3 oxidation state (oxidation firing).

Gemstone Color

Typically, a gemstone family possesses a base formula—just as a glaze has a base formula. The base formula is clear or milky without metal impurities to provide it with color. A perfect example of this is beryl, beryllium aluminum cyclosilicate, with the repeat formula Be₃Al₂(SiO₃)₆. Additionally, another common and ideal example is corundum, a crystalline form of aluminum oxide with the repeat unit Al₂O₃. These are bare, clear gemstones in their raw form. In order for them to possess color—the variety of colors that we are timelessly drawn to—they must have a small metal impurity, which controls the color and intensity of color in the gemstone.

When a small impurity of metal is present in one of these gemstone bases, like beryl (1), it is suspended in the glaze matrix and is surrounded by those repeat formulas. The electronegativity and structure of the gemstone base around each metal center interact with the d-orbital electrons of the metal, causing the energy levels to separate and split via crystal field theory. This difference in energy levels can then allow for absorption of light—absorption in the visible spectrum, wherein we see all of the light that is not absorbed after it reflects back, resulting in reflected color.

Crystal field theory has long been studied and used to explain gemstone color—specifically to characterize how rubies and emeralds both have chromium impurities, but are so contrasting in appearance. Therein, glazes work effectively the same way as gemstones and allow for inspiration from natural mineral colors, such that we can recreate vibrant color profiles on our ceramic surfaces.

Beryl, Color, and Glazes

As far as gemstones are concerned, Beryl exists as a nearly perfect glaze base. When beryl has an iron impurity, it possesses either a gold color as heliodor or a soft blue as aquamarine (2). Interestingly, the only difference in color is the charge of the iron. It is Fe²⁺ for blue aquamarine and Fe³⁺ for the gold/amber colored heliodor. By using a clear base with a small iron impurity in both oxidation and reduction atmospheres, heliodor and aquamarine gemstones can be emulated in glaze form (like Gold Beryl glaze or Babu Celadon glaze, respectively), purely dependent on firing conditions. Using 2% red iron oxide in a cone 6 oxidation firing produces the Fe³⁺ heliodor amber celadon color and using 2% yellow iron oxide through a reduction firing yields the Fe²⁺ blue celadon colors.

For the rock hounds out there, manganese is notorious for producing pinks and raspberry colors in morganite and red emerald, though the effect is incredibly rare. Normally, manganese dioxide is a component of brown/black slips, but it is used in somewhat higher weight percentages. While red emerald is very difficult to reproduce in a glaze (volunteers??), morganite can be recreated in a glaze by adding 0.5–1.0% manganese dioxide to Fa’s Crystalline Base glaze and firing to cone 6 (3). Notably, when adding more than 2% manganese dioxide, the color shifted more mauve, transitioning to a soft brown with a pink tint. Only at these very small concentrations was a soft pink visible. Additionally, note that the crystalline base runs, but also forms small pink crystals, which can be grown or further developed with more complicated firing schedules.

Emeralds and Rubies

The most notable gemstone in the beryl family is the famous emerald, for which movies have been made, plenty of jewelry fabricated, and millions of dollars spent—all for some chromium impurity in a chunk of beryl. Because of the nature of beryl, chromium impurities produce a very vibrant emerald green color (4). The chromium oxide is green, and in a regular glaze base, chromium produces an emerald green at 0.2% content. When that same chromium impurity is present in corundum (crystalline alumina), a vibrant red is produced that we all associate with rubies (4). The difference here is the environment around the chromium, how the gemstone environment interacts with the d-orbitals of the chromium, and how much the energy levels split and affect light absorption.

The same series of colors can be replicated in a non-alkaline, highly fluid glaze base. At 0.15% chromium oxide and no tin oxide, a very literal emerald green can be produced. Upon the addition of both chromium oxide and 5–6% tin oxide to that same base, we have changed the crystal field of the glaze base to more closely resemble that of corundum, resulting in a vibrant ruby red.

Limitations

Ultimately, not all gemstones can be reproduced as glazes. We are limited to the upper and lower oxidation states of metals for oxidation and reduction firing conditions. Intermediate oxidation states are more difficult and would require an uncanny degree of precision. My personal inspiration comes from trips to the natural gemstone exhibit at the Smithsonian National Museum of Natural History in Washington, DC. The most vibrant gemstones are those containing antimony, arsenic, and even lead/manganese that are on display at the Smithsonian Mineral Gallery. If only they weren’t toxic!

the author Ryan Coppage is currently Chemistry Faculty at the University of Richmond. He occasionally teaches the Japanese Ceramics and Glaze Design class at the Visual Arts Center of Richmond and still makes far too many pots. To learn more, www.ryancoppage.com.

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