Exploring the Colors of Copper-Containing Pigments, Copper (II) Oxide and Malachite, and Their Origins in Ceramic Glazes

Abstract

The colors of copper-containing pigments, copper (II) oxide and malachite, and their origins in ceramic glazes were systematically examined over a wide firing temperature range using a suite of analytical and spectroscopy techniques including SEM, UV-Vis FORS, XRD, FTIR, and EPR to gain new insight into the structural and chemical transformations of the glaze during firing. The two colorants investigated were black copper (II) oxide (CuO) nanopowder and blue-green basic copper carbonate, or malachite (Cu2CO3(OH)2), both of which produce a final light blue color following firing. Additionally, silicon carbide (SiC) was used to locally reduce CuO to simulate firing glazes in a reductive environment and produce a final red color. At lower temperatures, malachite was found to decompose to form CuO at 550 °C, elucidating the reason that two different copper colorants could be used interchangeably to form the same “Robin’s Egg Blue” color. At 850 °C, a glaze sintering process occurred, resulting in the distribution of Cu2+ in a square planar geometry and an observed blue color. This structural change occurred at temperatures lower than the glaze’s melting point, indicating that complete vitrification of the glaze is not required for glaze coloration. Conversely, the reduction in Cu2+ to Cu+ through the addition of SiC did not occur until the glaze was fired above the melting temperature (1000 °C), signifying that high temperatures are required for the redox reaction to occur. This study sheds light on intermediate colorant-glaze interactions that are beneficial for understanding and predicting glaze coloring upon exposure to varying temperatures, and the results from this study can be applied to better-controlled glaze production for artists and a deeper appreciation of ceramic glaze chemistry and aesthetics.

Exploring the Role of Transition Metal Complexes in Artistic Coloration through a Bottom-Up Scientific Approach

“…The unique quality of transition metals that gives them the ability to produce these bright, visible wavelengths of light derives from their unfilled valence d-orbitals and ligand interactions, resulting in d-d orbital splitting and/or charge transfer [3,13,19]. The energetic interaction of visible light with the electrons in the d-orbitals of the central metal ion and at times with those of the ligands, as well as other variables that will be discussed, determine the magnitudes of the wavelengths that the gaps between such orbitals (Figure 1b,c) [20][21][22].…”

“…This phenomenon, called d-orbital splitting, is the most common coloration mechanism for transition metal complexes [34]. Consequently, valence electrons can transition from lower energy d-orbitals to those of higher energy when subjected to radiation [19]. When the energy difference between the split orbitals falls within the visible light range of the electromagnetic spectrum, these electronic transitions manifest as vibrant and distinct colors, complementary to the wavelengths of light absorbed [9].…”

“…The quantity of d-electrons as well as the nature, number, and geometric configuration of the ligands encircling the metal ion influence the magnitude of the energy gaps between their orbitals [19]. The electron density of the 5 d-orbitals is distributed along the axes for the dx 2 -y 2 and dz 2 orbitals, and between the axes for the dyz, dxy, and dxz orbitals [35].…”

“…Temperature plays a crucial role in influencing the coloration of final products involving transition metal complexes as coloring agents. The creation of ceramic art pieces with colored glaze coatings involves exposing the pieces to a range of temperatures that often exceed 1150 °C [19,52]. At high temperatures, significant structural changes occur in ceramic glazes which can interact with colorants, causing a change in appearance during the firing process.…”

“…Upon exceeding 1000 °C on the bisque disk, the glaze without colorant turned transparent and colorless. Both the 1% malachite and 1% CuO glazes shared a transparent, light blue appearance [19] (Figure 13). It was concluded that the colors of coppercontaining glazes are not due to the colorant in the physical mixture of the raw glaze but rather to a change in overall glaze structure and transition metal complexation as the firing temperature approaches and exceeds 850 °C.…”