White on blue: A study on underglaze-decorated ceramic tiles from 15th-16th-century Valencian and Sevillian productions

Highlights

• Multi-analytical characterisation of a unique historical ceramic technology.

• Underglaze decoration using an opaque white tin-glaze over the pigment layer.

• New features to distinguish Hispano-Moresque ceramic production centres.

Abstract

This study characterises and compares tin-opacified underglaze-decorated tiles from Valencian and Sevillian provenances. This technique, where the cobalt and manganese pigments are applied below an opaque white glaze, was used in the Iberian Peninsula between the 14th and early 16th centuries. The chemical and morphological characterisation of the glazes was performed by Optical Microscopy (OM), Scanning Electron Microscopy with Energy-Dispersive X-Ray Spectroscopy (SEM-EDS), µ-Raman Spectroscopy, and µ-Particle-Induced X-Ray Emission (µ-PIXE). Both the morphology of the glazes and their chemical composition allowed for the distinction between the two production centres. Sevillian glazes exhibit a thicker pigment layer, as well as higher SnO₂ and lower K₂O contents than the Valencian ones. Furthermore, the SEM analysis of cobalt pigment particles identified an interior nucleus rich in Co, Fe and Ni, and an exterior layer rich in Si, Ca, Mg and Na, suggesting that the pigment was used mixed with clay or sand.

Introduction

Underglaze implies that the painting layer of the ceramic object is located under a glaze layer, which acts like a varnish to fixate the pigments and to give a homogenous and brilliant finish to the surface. This technique has been documented in Islamic ceramics as early as the 9th century, i.e., in Raqqada (Ben Amara et al., 2011) and Palermo (Arcifa and Bagnera, 2018).

The mutual influence between Islam and China is believed to have driven the development of stonepaste (or fritware) technology, which provided a white background on which the pigments were directly applied and then covered with a transparent glaze, eliminating the need for a white slip or white-firing clays to achieve a porcelain-like appearance (Tite et al., 2011, Watson, 2004). In parallel, tin-opacified ceramics were also developed since the 8th century in Egypt and the Levant (Matin et al., 2018), and introduced in the Iberian Peninsula between the last quarter of the 9th century and the first quarter of the 10th century (Salinas and Pradell, 2018).

The Islamic presence in the Iberian Peninsula between the 8th and the 15th centuries provided a specific background for the development of important ceramic centres. Tin-opacified glazes were among the most significant innovations brought by the Islamic potters, along with the cobalt blue pigment, which became inseparable from the architectural tile until our days (Martínez Caviró, 1991). During the 14th century, a different type of underglaze painting was developed: instead of a transparent glaze (like the one used in stonepaste or fritware ceramics), this type used a white tin-opacified glaze over the painted decorations. Archaeological evidence has shown that in Valencian blue-and-white underglaze-decorated ceramics, the cobalt pigment was applied onto the raw ceramic object, which was then fired for the first time. Afterwards, a white lead–tin glaze frit was applied over the painted object, which was then was fired a second time (Coll Conesa, 2009a). This technology has also been documented in 14th-century Nasrid Granada, where ceramic fragments were found decorated with cobalt oxide prior to having been coated with a tin-glaze frit (García-Porras 2012).

The beginnings of this specific opaque underglaze technique or the reasons for its use are not clear. Their highlight was in 15th-century Valencia (Spain), already under Christian ruling, albeit with many Islamic potters in the region (Coll Conesa, 2009b). Matin et al. (2018) have shown that several early Islamic tin-opacified wares were overglaze-decorated, including green-and-brown and blue-and-white ceramics from Raqqa and Samarra, respectively. The closest Islamic parallel to the Hispano-Moresque underglaze technology documented unequivocally in the literature concerns Sicilian 9th-10th-century shards that were decorated with copper green and manganese brown under an opaque glaze, although this glaze is not tin-opacified. Instead, it contains quartz (SiO₂) crystals that render it opaque with a matt appearance (Testolini 2018).

In the Iberian Peninsula, an underglaze tin-opacified cobalt blue decoration has been documented in Granada (García-Porras 2012), Valencia (Roldán et al., 2006), Catalonia (Pradell et al., 2010), Teruel (Pérez-Arantegui et al., 2009b), and Seville (Pleguezuelo 2011). Contrary to blue-and-white ceramics, tin-opacified “green and brown” Islamic and Hispano-Moresque wares exhibit overglaze decorations throughout the Iberian Peninsula, except for one (so far) set of Catalonian ceramics dated from the 14th century (Salinas et al., 2017).

The Valencian and Sevillian regions were among the most important production centres of Hispano-Moresque ceramics. While in Valencia the underglaze technique was the most used one for decorating ceramic tiles, in Seville the cuerda seca and the arista techniques were more popular (Martínez Caviró, 1991). Nevertheless, during a recent study on Hispano-Moresque tiles (Coentro, 2017), an underglaze decoration was identified in shards attributed to a Sevillian production. Opposing to what has been written about Valencia, underglaze-decorated Sevillian ceramics were yet to be studied.

The main objective of this work is to compare Valencian and Sevillian tin-opacified underglaze-decorated architectural tiles in order to identify and characterise different underglaze decoration techniques from different provenances with tin-opacified glazes as a common feature. Furthermore, Co-rich inclusions are analised in detail by SEM-EDS, giving clues to the original raw material.