The Science of Ceramic Glaze Color

Anh Bui , Ryan Coppage Ceramics Art + Perception 108 2018 Technical Home

The colors we observe from surrounding objects are fascinating phenomena of their interaction with light. When a beam of light strikes an object, a portion of the light frequencies is absorbed by the object, whereas the unabsorbed wavelengths are reflected back to our eyes. The light receptors inside our eyes convey these messages to the brain, producing our perception of a color associated with the object.1,2 Light consists of a continuum of frequencies with corresponding wavelengths and energy levels. The relationship between energy and wavelength can be explained by the equation: E=hc⁄λ, in which λ is the wavelength value, E is the energy, c is the speed of light and h is Plank’s constant.3,4,5 The frequency value is reciprocally proportional to wavelength through the equation v=c⁄λ. (Figure 1a). As frequency increases for an incidence of light, shorter wavelengths are present with greater energy (Figure 1b).

Human eyes are only able to process light and color within a certain range of wavelengths (400-700 nm), the visible spectrum region. The colors detectable by our eyes in this region are red, orange, yellow, green, blue, and violet (from the longest to shortest wavelength).6 Colorless and transparent objects absorb wavelengths outside of the visible region, thus their light reflection is not recognizable as any color by our eyes. Black objects are perceived as possessing an absence of colors by our brain because they absorb all of the wavelengths in this region and reflect nothing back.

An object’s color absorbance and reflection properties are dictated by its molecular structure and composition.7 Atoms consist of a positively charged central core and a cloud of electrons which are negatively charged particles.8 Around the central core, electrons move in a region of space called orbitals, which possess different shapes and energy levels and are responsible for bonding between elements.8,9

In transition metals, there are five d-orbitals that are normally equal in energy.9 When a metal is suspended in a crystal field (a gemstone, glaze, or otherwise.) parts of the surrounding environment start to interact with those orbitals. As they are more (or less) negative or move in closer (or are farther away), the surrounding environment starts to change the energy spacing of the orbitals.10,11

Even though they both contain a chromium impurity for color, emeralds are constructed from beryllium aluminum silicate (beryl) as opposed to corundum.

Different geometries of these interactions form a range of energy spacing patterns (Figure 2) – mostly tetrahedral and octahedral. During orbital interaction, the five d-orbitals are split into two different sets of energy, wherein a gap between energy levels can exist.11

As light strikes an electron in a metal, it can be promoted from one of these lower energy levels to one of the higher ones, with the total energy of the promotion proportional to the wavelength (and color!) of the light absorbed (Figure 3).7,12 By inducing light of a specific range of wavelength on a substance’s surface using a spectrophotometer, light absorption of a substance can be measured. The value of this measurement is called the absorbance and corresponds to a specific amount of energy required for the electron transition.13 Across a continuum of wavelengths and corresponding absorbances/reflections for a metal in a crystal field, spectroscopic information can be collected to create a reflectance spectra – the reflected and absorbed color profile of the metal in its environment. Though this is theoretically described as a single transition with a very absolute energy gap, this transition event is not as narrow and absolute as expected.

There are several factors that contribute to the broadening of absorption events by transition metals. Through changes in vibrational and energy states of the particles and the interaction between these vibrations with the instrument itself, a single absorption even will take place across a range of wavelengths. The combination of these factors yield us a Voigt profile of the spectroscopic spectra, allowing for a more broad color range instead of one particular wavelength of absorption (Figure 4). 14,15

The colors we perceive are those not absorbed by the metal color centers – the reflected wavelengths of light whose energies do not correspond to that energy gap – as all remaining light that wasn’t absorbed.
The energy of this gap depends on the transition metal, the charge of the transition metal, the electronic nature of the crystal field (glaze base) around it, and their arrangement around the metal center.16 Transition metals have distinctive light absorption and reflection properties. Varying the charge on a metal center or the crystal field surrounding it changes the electronic interaction between them, thus changing the energy gap between orbitals and the wavelengths of light absorbed (and reflected back).16,17

The science of color is a common phenomenon in our everyday lives, from nearly every product available to the red corvette you watched speed around the corner. To that end, the pigment industry is projected to reach a net worth of 34.2 billion dollars by 2020.18

The constantly increasing demand in organic dyes and pigments reflects the growing desire of developing high performance color products that are environmentally friendly parallel to ever-increasing performance standards. A color market with a net worth in the billions of dollars warrants very specialized techniques and an incredible degree of understanding of the science of color.

In mineralogy, the colors in gemstones can be explained by their material makeup and the identity of the transition metal impurity in them.19 The various colors in cuprite and malachite are explained by differences in oxidation states of copper ions in their crystal structures.20 The presence of Cu+1 in cuprite yields a green light absorption, which corresponds to the gemstone’s distinctive vibrant red color. Increasing the charge on copper from +1 in cuprite to +2 in copper carbonate yields a larger charge density and allows the ligands to be closer to a Cu+2 than to a Cu+1 ion center.21,22 The smaller distance between ligands and metal central ions causes a greater repulsion force between them during their interaction and creates a smaller energy gap, explaining the longer wavelength absorption in copper carbonate and the reflection of color green in malachite.21,22

Both rubies and sapphires contain corundum, also known as crystalline alumina, Al2O3; however, their colors are vastly different from each other. The blue color in sapphire is a result of the interaction between the alumina and ions of iron and titanium, whereas the red color of rubies is derived from trace amounts of chromium in the corundum structure.23,24 Similarly, the difference between emeralds and rubies is also very slight. Even though they both contain a chromium impurity for color, emeralds are constructed from beryllium aluminum silicate (beryl) as opposed to corundum.

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Image Captions

Figure 1a: The relationship between energy and wavelength.
Figure 1b: The visible spectrum with wavelength values (in nm) and corresponding colors.
Figure 2: Splitting patterns in energy levels of d-orbitals with respect to the co-ordination geometries of the metal compound.
Figure 3: Light absorption stimulates electron promotion from lower to higher energy level.
Figure 4: Broadening of the spectroscopic spectrum.
Figure 5: Reflectance colour profile of a) cobalt compound in glaze and b) chromium compound (with tin oxide) in glaze.
Figure 9: Color profile of manganese dioxide. Samples show increasing concentration from left to right.
Figure 6: Celadon base recipe.
Figure 7: Color profile of cobalt carbonate.
Figure 8: Color profile of copper carbonate. Samples show increasing concentration from left to right.
Figure 9: Color profile of manganese dioxide. Samples show increasing concentration from left to right.
Figure 12: Chromium base recipe.
Figure 11: Color profile of copper carbonate. Reduction or oxidation firing results in different glaze color.
Figure 13: Color profile of chromium oxide. 0.15% Chromium Dioxide (left tile) produces an emearald green. The addition of tin dioxide changed the reflectance color to ruby red.

by Anh Bui , Ryan Coppage

Anh is currently a senior at the University of Richmond, completing a chemistry degree, actively participating in biology research, and has previously worked with ceramic glaze design and chemistry lab development pedagogy. She believes that education plays a truly essential role in shaping students’ perspectives about the scientific process. By developing laboratory curriculum to be more guided-inquiry based and complementary to lectures, a more successful education system can potentially inspire students to embrace science in their daily lives. And by being mindful of the scientific approach in the world, we can better understand how science impacts other facets of our lives, art included!

Do you know those annoying kids that always ask, "Why?" Well, that was me, except I really wanted to know EVERYTHING -- always. I needed to know how and why everything worked. It infuriated my elementary school teachers. And in addition to this, I wanted everyone else to know as well. Nothing could be taken at face value. People needed to make informed decisions. This was the mentality of 8-year-old Ryan. From this beginning, I chased chemistry -- through a BS, MS, and Ph.D. -- as it explained how the world worked to my always inquisitive mind. And alongside chemistry, I found and became utterly infatuated with the marriage of aesthetic and function via ceramics -- how an art can become completely utilitarian and useful.