A Nondestructive Methodology for the Study of Colored Enamels: Insights into Manufacturing and Weathering Processes

Abstract

We studied ancient enamels on gilded copper from a collection of archeological horse harness pendants of the Museo Instituto Valencia de Don Juan (Madrid, Spain) to test the benefits of a new, nondestructive analytical methodology based on chemometric analysis (i.e., Principal Component Analysis, PCA) on micro‐ATR‐FTIR spectral data and chemical quantification using SEM‐EDS. The novelty of this approach was threefold: (i) PCA allowed the discrimination of the different harness pendants of known origin and attributed to the 14th and 15th centuries according to the chemical complex composition, nanostructure, glass weathering, and/or coloring mechanisms of each colored enamel, separately (i.e., red, purple, blue, and white), (ii) it is a cheap, easily available and nondestructive methodology that enables us to (iii) draw archeological conclusions about the quality of the manufacturing process, reassess the chronology of these objects and attempt to attribute them to different workshops according to the different traditional recipes identified. In particular, the enamels were made of alkali and/or alkaline earth lead‐glass with a wide range of chemical compounds in the form of pigments or opacifiers. Two types of coloring mechanisms were identified, colloidal particles such as copper‐ruby for red enamels, and ionic mechanisms such as Fe(II) and Co(II) to achieve a blue pigments; Mn(III) in the purple pigment; and two kind of white enamels were identified, i.e., tin oxide as an opacifier and uranium oxide. In addition, we established the reason for the poor state of conservation of some of the enamels by means of the identification of depolymerization and ion exchanges, well‐known harmful effects of glass weathering, and finally a chronology was assigned for some of these pieces according to the enamel composition.

New insights in technology characterization of medieval Valencia glazes

“…Moreover, it is worth noting that glaze recipes are variable, both in terms of raw materials and available technologies among cultures, historical periods, and workshops. [16,[20][21][22][23][24][25] In most cases, glazes are produced by specific and controlled smelting (from 600°C to 1450°C) and cooling methods for different compounds. Glazes are generally formed by the addition of feldspar-rich sandstones and clays with variable amounts of oxides, such as alumina (Al 2 O 3 ), calcium, sodium, potassium and magnesium oxides (i.e., CaO, Na 2 O, K 2 O and MgO), and iron oxides (Fe 2 O 3 or Fe 3 O 4 ), which contribute to diverse glaze functions and properties.…”

“…Glazes are generally formed by the addition of feldspar-rich sandstones and clays with variable amounts of oxides, such as alumina (Al 2 O 3 ), calcium, sodium, potassium and magnesium oxides (i.e., CaO, Na 2 O, K 2 O and MgO), and iron oxides (Fe 2 O 3 or Fe 3 O 4 ), which contribute to diverse glaze functions and properties. [2,20,22,24,26] Other substances, such as opacifiers and fluxes, are used to obtain specific characteristics. [23] For instance, the traditional opacifier tin oxide (SnO 2, cassiterite) increases optical properties, while the main fluxes such as lead oxides (from diverse lead sources) avoid glaze devitrification and improve color development.…”

“…[23] For instance, the traditional opacifier tin oxide (SnO 2, cassiterite) increases optical properties, while the main fluxes such as lead oxides (from diverse lead sources) avoid glaze devitrification and improve color development. [2,18,[20][21][22][27][28][29] Another finishing ceramic decoration is the so-called luster, produced by metallic oxides such as silver and/or copper in an overglaze finish. Luster is obtained at low temperature (<600°C) under a reducing atmosphere to attain metallic sheen (effect of iridescence).…”

“…[2,4,5,10,[15][16][17][18]20,24,29,30] The analytical characterization of glazes and luster has been performed using an array of analytical techniques. [3][4][5]10,19,20,22,23,25,26,29,31,32] Particularly, Fourier-transformed infrared spectroscopy (FTIR) and Raman microscopy (RM) are exceptionally apt to explain their complex composition, offering molecular information (in a non-destructive way with high spatial resolution) that retrieve knowledge about production process. [20,23,25,26,[31][32][33] Specifically the polymerization index (Ip) and different spectral components (Q n for stretching components and Q n' for bending ones) derived from Raman spectra are used to clarify glass composition and nanostructure such that they serve as a parameter for unraveling firing temperatures.…”

“…[3][4][5]10,19,20,22,23,25,26,29,31,32] Particularly, Fourier-transformed infrared spectroscopy (FTIR) and Raman microscopy (RM) are exceptionally apt to explain their complex composition, offering molecular information (in a non-destructive way with high spatial resolution) that retrieve knowledge about production process. [20,23,25,26,[31][32][33] Specifically the polymerization index (Ip) and different spectral components (Q n for stretching components and Q n' for bending ones) derived from Raman spectra are used to clarify glass composition and nanostructure such that they serve as a parameter for unraveling firing temperatures. [21,22] However, in spite of the well documented archeological and scientific benefits obtained from applying vibrational techniques to characterize Muslim-glazed, Italianglazed, Ottoman-glazed, Iznik-glazed, Majolica-glazed or Vietnamese-glazed ceramics, among others and to our knowledge, glazes and lusters of Valencian ceramics have not been studied by means of RM.…”