Glazed Pottery Throughout the Middle and Modern Ages in Northern Spain

Abstract

A total of forty samples of medieval and modern glazed pottery from northern Spain were studied. Chemical and microstructural analyses of the glazes were performed by scanning electron microscopy coupled with electron dispersive spectroscopy (SEM-EDX), while the chemical composition of the pottery bodies and slips were determined by X-ray Fluorescence (XRF). The glazes studied come from the Santa Barbara Hill site (Tudela), the Treviño Castle site (Treviño), the Vega workshop (Burgos) and the Torrentejo village (Labastida) and correspond to transparent glazes and opaque white glazes. Transparent glazes were lead glazes with variable PbO content. Opaque white glazes were lead-tin and lead–alkaline–tin glazes. The glaze was mainly applied to a pre-fired body made of local clays, but the glazes of the Santa Barbara Hills pottery (Tudela) were applied to raw bodies. The microstructure of the interfaces indicates a single firing process for the glazed pottery from Tudela and a double firing process in the rest of the sites. Some correlation are identified between the use of specific clays to produce different glaze colours. White opaque glazes are applied to calcium-rich clays. Similarly, calcium-rich clays were used to produce dark green transparent glazes, while clays and slips aluminium–rich were used to produce light green and light honey glazes. Iron was also identified as the main colouring agent, although copper was also used. The white glazes were opacified by the addition of cassiterite and sometimes quartz and feldspar. The glazed pottery was mainly of local origin, but the identification of some non-local pottery at all sites suggests a pottery trade.

1. Introduction

The Middle Ages in the Iberian Peninsula refers to the historical period that began in the year 476, when the last West Roman emperor was removed, and ended in 1492, when the last Muslim territory, Granada, was reconquered by the Christian kings [1].

In contrast to other European territories, the medieval period in Spain was marked by the Islamic occupation of the territory between 711 and 1492. The Muslims conquered most of the Iberian Peninsula except for a small strip in the north where small Christian kingdoms were created [2,3]. In the early 11th century, the fragmentation of the Cordovan Caliphate into smaller kingdoms and the strengthening of the Christian state disrupted the previous balance, and the northern kingdoms expanded and conquered the Muslim territories (Al-Andalus) [2].

The early medieval Muslim territory was divided into three administrative and military frontiers or marches. The Upper March corresponded roughly to the Ebro valley and the adjacent Mediterranean coast, and Saragossa was the capital of the Mach both politically and economically. Besides the Upper March was a frontier between the Muslim and Christian territories from the 8th century to the early 11th century. The Middle March was the centre of the three marches, and the administrative centre was first at Toledo and later at Medinaceli (Soria). The Lower March included Portugal and Badajoz, with the capital at Mérida (Badajoz).

Al-Andalus experienced a period of splendour in art, architecture, science, medicine and literature, introducing complex technologies used in the Eastern Mediterranean [4].

Pottery manufacturing underwent significant technical innovation with the introduction of new uses, forms, painted decorations, and glazes. The biggest change in the production of pottery in the Islamic world was the introduction of glazed pottery. The first glazed pottery was produced in the Near East and Egypt in Late Antiquity, and the glazes were of the alkali–lime–silica type [5,6,7,8]. The great change in glazing was the introduction of lead glazes and tin opaque glazes. Lead glazes were known in the West since Roman times but were used extensively in medieval Europe throughout the Islamic world. However, glazing technology in the Iberian Peninsula arrived significantly later than in other Islamic Mediterranean regions. The first glazed pottery in Al-Andalus appeared in the middle of the 9th century [9,10,11].

Glazed pottery was reported at several archaeological sites in the north of the Iberian Peninsula [12,13,14,15,16,17]. However, few studies have analysed the glazes from a chemical–mineralogical point of view [18,19,20].

The present work studies glazes from the 8th to the 17th centuries in different cultural contexts at the northern region in the Iberian Peninsula. The site with the oldest glazed pottery is Santa Barbara Hill (Tudela, Navarre), with glazes from the pre-Islamic, Islamic, and Christian periods. The glazed pottery from the sites of the Treviño Castle (Treviño, Burgos) and the Vega pottery workshop (14th–15th centuries) (Burgos, Burgos) corresponds to some of the first glazed pottery manufactured also in the Christian context. The most recent glazes come from the Torrentejo village (Labastida, Álava) date from the 16th to 17th centuries and are enclosed in the Christian cultural context.

The aim of this work is to study glazes through chemical and mineralogical analysis in order to understand glazing techniques in different ages and cultural periods in northern Spain.

2. Materials and Methods

2.1. Materials

A total of forty glazed ceramics dating from the 9th to the 17th centuries from several medieval to modern archaeological sites in northern Spain were studied. Figure 1 shows the location of archaeological sites.

Eighteen glazed potsherds from Santa Barbara Hill (SBH, Tudela, Navarre) from three different cultural periods were studied. The SBH glazed pottery was selected because it is the earliest of all the studied pottery and shows direct influence from Islamic glaze manufacturing methods. Two samples belong to the pre-Islamic period dated before the 9th century (SBH-PI), seven samples belong to the Islamic period (10th–11th centuries, named SBH-I) and nine samples belong to the Christian period (12th–13th centuries, named SBH-C) [22,23,24]. Most of the samples are glazed on both sides, except for two samples that are glazed only on the exterior surface corresponding to Mudejar style. Within the samples studied, six are opaque polychrome glazes on a white glaze, two from the Islamic period and four from the Christian period. The colours of the transparent glazes include light brown, dark brown, honey, light green and dark green-coloured regardless of the cultural period. The paste colour of the pottery is mainly creamy, although reddish and light grey bodies were also observed regardless of the cultural period.

The second group of studied glazed ceramic consists of six sherds from the 14th and 15th centuries from Treviño Castle (CTV, Treviño Country, Burgos) [25]. Two samples are glazed on both sides, and the remaining four are glazed on one side only, which corresponds to the interior surface for tableware (i.e., plates and bowls) and to the exterior for the pots.

The third set of glazed pottery corresponds to sherds from the Vega Pottery Workshop (VPW, Burgos) [26] and includes fourteen samples from the late 14th and 15th centuries. The glazed pottery from this workshop is often glazed on white slip and rarely on a reddish body. Almost all of them are glazed on one side only, on the inside for open shapes (plates and bowls) and on the outside for closed shapes (jars and pots). All glazes are transparent but usually do not produce a homogeneous pure colour over the whole piece. The honey-coloured glazes are the most common colours, with less frequent light green and dark green-coloured glazes. In addition, the distinctive use of white slip increases the colour variety of the glazes. The colour of the glaze can differ from green on white slip to a honey-coloured glaze on a reddish body without slip (Figure 2). The colour of the body is often reddish, although whitish bodies were also found in a few samples.

The last set of glazed pottery consists of twelve sherds from a workshop in the village of Torrentejo (T, Labastida, Álava) [27,28]. The glazes from this site are both transparent and tin opaque glazes. The transparent glazes are only applied to the outside of the pottery and are honey and green in colour, whereas the opaque white tin glazes are observed on both sides of the pottery. The colour of the body includes whitish, pinkish grey, greyish-white and pink colours.

2.2. Methods

The chemical composition of pottery bodies was analysed by means of X-ray fluorescence (XRF) using a wavelength dispersive X-ray fluorescence (WDXRF) equipped with a PANalytical Axios Advanced PW4400 XRF spectrometer (Malvern Panalytical Ltd., Worcestershire, UK) (4 kW Rh anode SST-mAX X-ray tube). LODs for major elements are around 0.01 wt.% and ~5 ppm for trace elements. Fused beads were obtained by melting the bulk powder samples with lithium borate flux (Spectromelt A12, Merck, Darmstadt, Germany) in a 20:1 ratio at 1200 °C for 3 min in Pt/Au crucibles using a PANalytical Perl’X3. The loss on ignition (LOI) was calculated after heating the bulk powder samples at 1050 °C for one hour. To avoid glaze chemical contribution prior to crushing, all glaze was removed using a dental microdrill (MF-Perfecta) with a PM CX72 tungsten carbide rugby ball shape bur.

The elemental analysis of glazes was performed on polished thin sections using a Scanning Electron Microscopy (SEM) JEOL JSM-6400 (JEOL, Tokyo, Japan) operating with an INCA EDX detector X-sight Series Si (Li) Oxford pentaFET microanalysis system. The samples were coated with carbon before analyses. Two types of measurements were performed: (1) wide area determinations when the glaze was inclusion-free, and (2) spot analyses for inclusions. The chemical composition of the glazes was determined by analysing 3 or 4 areas totalling approximately 300–400 µm². The detection limit of the EDS analysis was in the range of 0.1–1.0 wt%. The analyses were normalised to 100 wt% as oxides and then averaged.

3. Results

Since the studied samples are diachronic and correspond to different cultural periods, the results are presented site by site.

3.1. Santa Barbara Hill

The EDS chemical composition allows us to distinguish several groups of glazes. The data highlight the distinctions between lead-glazed and alkali-glazed pottery.

The alkali glazes display an average composition of ~11 wt% in alkali oxides (pre-Islamic pottery SBH-PI-10a and SBH-PI-10b). The two samples differ in their alkali composition. Sample SBH-PI-10a contains a higher proportion of Na₂O (10 wt%) than K₂O (1.6 wt%), whereas sample SBH-PI-10b exhibits a greater concentration of K₂O (4.8 wt%) than Na₂O (6.3 wt%). The differences in chemical composition are also observed in other elements including Al₂O₃ and CaO (

Table 1. SEM-EDS average chemical composition of glazes of Santa Barbara Hill pottery.

Glaze	Colour	Period	Side	SiO2	Al2O3	FeOt	MnO	MgO	CaO	Na2O	K2O	TiO2	P2O5	SnO	CuO	PbO
SBH-10a	B. brown	PI		72.8	5.3	1.5	0.7	0.8	6.5	10.1	1.6	0.3	0.2	0.2	-	0.2
SBH-10b	B. brown	PI		69.0	10.0	3.3	0.7	1.2	4.2	6.3	4.8	0.5	0.2	0.1	-	0.1
SBH-M-1	White	C	In	37.8	2.1	1.0	6.3	0.7	2.8	1.1	2.7	0.2	0.1	1.0	-	44.6
SBH-M-1	Honey	C	Out	35.9	3.6	1.8	0.1	0.7	3.5	0.9	2.7	0.1	-	0.7	-	50.0
SBH-M-2	White	C	In	47.0	2.7	0.9	3.7	0.5	1.7	1.3	3.1	0.1	0.1	2.8	-	36.0
SBH-M-2	White	C	In	41.5	1.9	0.7	5.3	0.4	1.6	1.0	2.4	0.1	0.1	3.2	-	41.5
SBH-M-2	Honey	C	Out	35.9	4.0	2.4	0.1	0.4	3.4	1.1	2.5	0.6	-	0.1	-	49.6
SBH-M-3	Honey	C		35.8	7.2	2.3	0.1	0.9	3.7	0.7	1.8	0.3	-	0.1	-	47.4
SBH-M-4	L. green	C	In	35.3	3.9	1.7	-	0.5	1.9	0.4	1.3	0.2	-	0.1	-	55.2
SBH-M-4	L. green	C	Out	27.1	5.3	1.7	0.1	0.4	2.9	0.4	1.0	0.2	-	-	-	61.4
SBH-M-5	D. green	C	In	39.7	4.2	2.6	-	0.9	3.2	1.0	2.1	0.2	-	-	-	45.4
SBH-M-5	D. green	C	Out	38.6	4.6	2.3	0.1	0.9	3.4	1.0	2.0	0.2	-	0.1	-	46.3
SBH-M-7	L. brown	C	In	43.2	6.3	4.6	0.1	0.6	4.6	0.7	2.9	0.2	-	0.2	-	36.8
SBH-M-7	L. brown	C	Out	47.8	9.1	4.3	-	0.5	3.0	0.9	4.1	0.4	-	-	-	29.8
SBH-M-8	White	C	Out	31.2	4.6	1.7	-	0.5	2.5	0.3	1.1	0.3	-	2.0	1.1	57.3
SBH-M-9	White	C	Out	43.8	4.0	1.7	1.2	0.4	2.0	1.5	3.2	0.1	0.1	2.2	2.4	37.5
SBH-I-28a	White	I	In	33.3	4.7	2.0	0.1	0.8	4.3	0.8	2.2	0.2	0.2	0.2	1.3	49.8
SBH-I-28b	White	I	In	38.0	5.7	1.6	0.1	0.8	4.7	1.6	2.7	0.2	0.1	2.3	1.3	41.5
SBH-I-29a	Honey	I		40.3	6.6	1.9	0.1	1.5	2.4	0.1	2.3	0.3	0.4	0.1	-	43.2
SBH-I-29b	Honey	I		38.6	7.6	1.6	0.1	1.7	2.7	0.1	1.8	0.2	1.3	0.1	-	44.5
SBH-I-30	L. green	I	In	32.5	5.2	2.9	0.1	0.8	5.4	1.3	2.2	0.3	0.2	-	-	48.8
SBH-I-30	L. green	I	Out	30.7	4.6	3.0	0.2	0.8	4.9	1.4	1.8	0.5	0.2	0.3	-	51.3
SBH-I-31	L. green	I	In	34.4	7.0	3.0	0.1	0.9	5.9	0.5	1.8	0.4	0.1	0.1	-	46.2
SBH-I-31	L. green	I	Out	32.9	6.7	3.0	0.1	0.8	5.4	0.4	1.7	0.3	0.2	0.1	-	48.7
SBH-I-32	L. green	I	In	36.3	3.7	1.8	-	0.5	1.3	-	1.5	0.2	-	0.1	-	55.8
SBH-I-32	L. green	I	Out	32.8	4.3	2.1	0.1	0.6	2.6	0.6	1.7	0.2	-	0.1	-	55.9

FeOt corresponds to total iron as FeO. Colours: B. brown—bright brown; White—white opaque; L. green—light green; D. green—dark green; and L. brown—light brown. Period: PI—pre-Islamic; C—Christian; I—Islamic; and M—Mudejar.

).

The lead glazes are made of a variable mixture of lead and silica [29]. Two groups of lead glazes are distinguished: opaque white glazes and transparent glazes. Likewise, principal component analysis shows the chemical composition is consistent with the alkali glazes, white opaque glazes and transparent glazes (Figure 3). White opaque glazes show SnO₂ content ranging from 2.0 wt% to 3.2 wt%. Tin occurs heterogeneously distributed in the glaze, forming cassiterite microcrystals and sometimes small clusters of crystallites. Chemical data for white glazes of Islamic production show higher contents of MgO, Al₂O₃, and CaO than those of the Christian productions. Over the white glazes, green and black/brown-coloured decorations can be seen. The black line decoration on the Christian glazed pottery (samples SBH-M-1, SBH-M-2 and SBH-M-9) was achieved by adding a mixture of manganese oxides (e.g., pyrolusite and psilomelane). Copper (1–2 wt% CuO) gives the green colour to the decoration on SBH-M-8, SBH-M-9, SBH-I-28a, and SBH-I-28b.

The transparent glazes show little chemical variation; however, the glaze colour exhibits a greater variability for the Christian pottery glazes than those of Islamic pottery (Figure 3). In the latter, only green and honey-coloured glazes are identified, while light and dark brown are also found in the Christian glazes. The colour of the transparent glazes does not seem to be determined by a specific colouring agent; only iron has a high content. Iron is a colouring agent providing different colouring effects depending on the oxidation state, from a yellow/brown colour under oxidising firing conditions to green/blue under reducing conditions [30]. Green Islamic glazes show high lead content, 51 wt% PbO, and relatively large amounts of calcium (5 wt% CaO) and iron (3 wt% FeO). Meanwhile, the honey-coloured glazes have lower FeO and PbO contents, ~1.6 wt% and 44 wt%, respectively. Green Christian glazes have lower iron content than Islamic glazes (~3 wt% FeO), and lead content varies according to the colour of the pottery body. Glazes on red pottery show high-lead contents (60 wt% PbO), while glazes on cream-coloured pottery have 45 wt% PbO. The highest iron content (~4.5 wt% FeO) and lowest lead content (33 wt% PbO) are dark brown glazes.

Regarding the chemical composition of the pottery bodies, principal component analysis allows us to distinguish three groups (Figure 3). High silica and alumina contents (65 wt% SiO₂ and 25 wt% Al₂O₃) characterise the pre-Islamic alkaline glazed pottery (

Table 2. XRF chemical composition of the pottery bodies from Santa Barbara Hill.

Body	Colour	Period	SiO2	Al2O3	Fe2O3t	MnO	MgO	CaO	Na2O	K2O	TiO2	P2O5	LOI
SBH-1	Creamy	C	47.32	18.31	5.81	0.07	4.51	15.76	2.14	1.87	0.69	0.23	1.94
SBH-2	Reddish	C	48.60	20.41	6.25	0.08	3.69	11.47	2.48	1.84	0.72	0.18	2.02
SBH-3	Creamy	C	50.21	20.17	6.42	0.08	3.60	11.54	1.91	2.51	0.75	0.18	1.47
SBH-4	Reddish	C	47.96	19.44	6.06	0.08	3.70	12.74	1.24	3.62	0.72	0.22	4.30
SBH-5	Creamy	C	49.23	19.29	5.94	0.07	4.53	13.09	2.33	1.61	0.70	0.17	2.10
SBH-6	Creamy	C	47.33	18.86	5.69	0.07	5.37	14.36	2.10	2.11	0.67	0.17	2.34
SBH-7	Creamy	C	49.42	19.71	6.18	0.08	3.92	12.36	1.64	2.78	0.73	0.17	1.26
SBH-8	Creamy	C	50.07	19.86	6.18	0.08	3.64	11.14	2.08	2.72	0.74	0.30	2.52
SBH-9	Creamy	C	49.49	20.16	6.40	0.07	3.47	11.10	2.17	2.75	0.74	0.46	2.65
SBH-10B	Reddish	PI	65.70	24.21	4.15	0.03	0.71	1.34	0.78	1.90	0.74	0.15	1.23
SBH-10A	Reddish	PI	64.03	25.12	3.19	0.03	0.65	1.42	0.68	1.84	0.78	0.13	1.31
SBH-28A	Greyish	I	48.38	17.74	5.62	0.07	4.56	15.12	1.38	2.69	0.66	0.23	2.62
SBH-28B	Greyish	I	57.74	15.11	5.43	0.08	2.91	13.15	1.09	1.84	0.72	0.23	1.45
SBH-29A	Reddish	I	56.73	16.99	5.22	0.05	5.02	6.69	0.36	4.63	0.77	0.27	0.88
SBH-29B	Reddish	I	56.58	16.68	5.17	0.05	5.19	7.18	0.33	4.63	0.76	0.28	1.88
SBH-30	Creamy	I	47.73	17.86	5.59	0.07	4.68	15.60	1.41	2.63	0.68	0.20	2.15
SBH-31	Creamy	I	48.61	18.66	5.77	0.09	3.31	13.46	1.81	2.35	0.70	0.18	4.13

Fe₂O₃t correspond to total iron as Fe₂O₃. Period: PI—pre-Islamic; C—Christian; and I—Islamic.

). The Christian pottery SBH-M-29a and SBH-M-29b have high K₂O contents (4.6 wt%) and relatively low calcium content (~6 wt% CaO). The remaining pottery, both Islamic and Christian, has high calcium (11–15 wt% CaO) and low silica contents (47–50 wt% SiO₂).

Backscattered electron images reveal that the alkali glazes show no interface between the body and the glaze, only a change in microstructure. No microcrystals are present in the glaze of sample SBH-PI-10a, but rounded quartz grains are observed in some areas at the glaze/body interface. On the contrary, the glaze of sample SBH-PI-10b shows two bands of neoformed microcrystals, one comprising pyroxene and wollastonite close to the body, and the other consisting of prismatic silica crystals close to the outer surface of the pottery (Figure 4). Similar textural features are observed in high-temperature silica polymorphs such as tridymite [31,32,33]. In both samples, the glaze is bubble-free, and the alteration of the glaze is limited to microcracks perpendicular to the surface.

The lead glazes do hardly show weathering and corrosion, except for the white opaque Islamic samples (SBH-I-28a and SBH-I-28b) displaying weathering on the surface and along microcracks. In the case of opaque white glazes, the interface shows an extension of 5–10 µm. It is defined by well-crystallised leucite or lead–potassium feldspar microcrystals forming near the interface, depending on the PbO/SiO₂ ratio of the glaze. Thus, leucite crystals form in silica-poor glazes (35 wt% SiO₂), whereas lead–potassium feldspars form in silica-rich glazes (47 wt% SiO₂) (Figure 5). Isolated microcrystals or clusters of tin oxide are also observed in these glazes. Near the black decoration, well-crystallised manganese colouring particles occur: both manganese oxides and manganese-tin silicates and manganese-calcium silicates. Additionally, microcrystals of calcium pyroxene throughout the glaze occur.

In the transparent glazes, the neoformed crystals are similar to those in white opaque glazes. However, the size of the interface increases to about 30 µm in Islamic pottery and to 50–100 µm in Christian pottery. In both pottery groups, the interface is formed by leucite or acicular potassium feldspar crystals. Additionally, single crystals or clusters of pyroxene crystals appear dispersed in the glaze.

3.2. Treviño Castle

The glazes found at Treviño Castle are lead glazes, including both opaque white glazes and transparent glazes. It should be noted that most of the pottery was glazed on just one side. Only two samples of opaque white-glazed pottery were identified, displaying distinct chemical compositions (

Table 3. SEM-EDS average chemical composition of glazes and XRF chemical composition of the pottery bodies from Treviño Castle.

Glaze	Colour	SiO2	Al2O3	FeOt	MnO	MgO	CaO	Na2O	K2O	TiO2	P2O5	SnO	PbO
CTV-9	White	40.0	3.3	0.4	-	0.2	0.9	1.4	1.6	0.1	1.9	1.3	49.0
CTV-10	L. brown	35.5	5.4	1.8	-	0.5	1.1	0.3	0.8	0.4	1.8	-	53.6
CTV-11	Honey	29.8	2.8	3.2	-	0.2	0.9	0.3	0.3	0.1	1.4	-	61.4
CTV-15	L. brown	32.3	6.5	1.6	-	0.4	2.3	0.3	1.1	0.2	1.3	-	54.1
CTV-16	L. brown	32.1	5.3	2.9	-	0.1	1.6	-	0.9	0.2	1.2	-	55.8
CTV-22	White	43.2	4.4	0.1	-	0.2	1.1	0.6	3.4	0.1	1.9	4.5	40.6
Body	Colour	SiO2	Al2O3	Fe2O3t	MnO	MgO	CaO	Na2O	K2O	TiO2	P2O5	LOI
CTV-9	Reddish	64.87	17.60	3.51	0.07	2.34	6.47	0.11	4.19	0.48	0.35	0.96
CTV-10	Reddish	68.02	16.24	3.80	0.05	2.18	4.61	0.14	3.49	0.50	0.39	0.42
CTV-11	Creamy	67.04	24.72	1.58	0.02	0.70	1.25	0.15	3.07	0.70	0.37	0.64
CTV-15	P. orange	57.29	24.98	2.85	0.03	1.12	7.07	0.24	4.40	0.63	0.29	0.72
CTV-16	P. orange	65.42	23.11	1.98	0.03	0.66	3.70	0.21	3.98	0.62	0.34	0.57
CTV-22	Creamy	52.75	17.79	2.63	0.04	4.23	18.72	0.40	1.65	0.59	0.28	0.70

FeOt corresponds to total iron as FeO. Fe₂O₃t correspond to total iron as Fe₂O₃. Glaze colours: White—white opaque; L. brown—light brown. Body colour: P. orange—pinkish orange.

). One sample (CTV-22) shows a SnO₂ content of 4.6 wt%, higher contents of silica and potassium (43.2 wt% SiO₂; 3.4 wt% K₂O) and lower contents of lead (40 wt% PbO). By contrast, the second sample (CTV-9) displays lower tin (1.3 wt% SnO₂), silica (40.0 wt% SiO₂) and potassium (1.6 wt% K₂O) contents, while the levels of lead are higher (49.0 wt% PbO). The transparent glazes are typically light brown, displaying a similar chemical composition with abundant lead content (53.5–55.7 wt% PbO). However, sample CTV-11 exhibiting a honey-coloured glaze shows the highest lead (61.42 wt% PbO) and iron (3.17 wt% FeO) contents, accompanied by lower silica and alumina contents (Table 3). The different iron contents could explain the colour variation between brown- and honey-coloured glazes.

The chemical composition of the pottery bodies can be grouped into three chemical groups. The first group includes CTV-9 and CTV-10 samples enriched in potassium and magnesium and slightly richer in calcium, showing a chemical composition similar to the average of the mature argillaceous sediments [34,35,36]. The second group is distinguished by high alumina content (25 wt% Al₂O₃), including samples CTV-11, CTV-15 and CTV-16. The third group, which formed sample CTV-22, is distinguished by low silica content (54 wt% SiO₂) and unusually high calcium content (18.7 wt% CaO).

Backscattered electron images of white-glazed pottery show that the glaze/body interface is sharp without neoformed minerals. However, lead diffusion into the body is observed in sample CTV-9, unlike sample CTV-22. The BSE images show the contrasting chemical composition of the pottery body; thus, the white colour shows the diffusion of lead from the glaze into the body (Figure 6). Microcrystals and clusters of cassiterite (tin oxide) and some undissolved rounded grains of quartz and mica are scattered throughout the glaze. Bubbles and microcracks are common and promote the weathering of the glaze. In contrast, transparent glazed pottery shows a thicker (up to 80 µm) and irregular interface. The interface consists of neoformed lead–potassium feldspar acicular crystals, calcium pyroxenes and rare wollastonite scattered throughout the glaze. Unlike the opaque white glazes, bubbles are rare or absent. Lead diffusion in all transparent glazes is similar to opaque white glazes.

3.3. Vega Pottery Workshop

The glazed pottery of the Vega pottery workshops are transparent lead glazes. Principal Component Analysis (PCA) of the EDS chemical composition allows us to distinguish four groups of glazes (Figure 7). Two groups of glazes are characterised by high lead contents (50–64 wt% PbO) and correspond to green and light brown to honey-coloured glazes. The green glazes also have high CuO contents (2–5 wt%), while the brownish glazes have high iron contents (2.6–4.4 wt% FeO). The other two groups are characterised by low lead content (32–40 wt% PbO) and high calcium content (~4.8 wt% CaO) and include honey-coloured glazes with high zinc content (~3.5 wt% ZnO) and dark green glazes with high iron content (~5 wt% FeO) and no zinc.

The green colour of the lead-rich glazes is due to the copper, while the dark green colour of the lead-poor glazes is due to the iron in oxidising firing conditions (

Table 4. SEM-EDS average chemical composition of glazes from the Vega Pottery Workshop.

Glaze	Colour	Slip	SiO2	Al2O3	FeOt	MnO	MgO	CaO	Na2O	K2O	TiO2	CuO	ZnO	PbO
VPW-15	L. Honey		31.1	2.8	3.7	-	0.1	0.3	0.1	0.6	0.2	0.1	0.1	61.4
VPW-17	Green		30.8	7.1	1.1	0.1	0.4	1.0	-	0.3	0.3	2.1	0.6	56.2
VPW-18	L. Honey		38.5	4.5	5.1	0.1	0.7	4.4	0.4	2.3	0.3	0.2	3.3	40.4
VPW-19	L. Honey		45.8	4.1	5.0	-	0.7	4.6	0.5	3.1	0.3	0.2	3.8	32.2
VPW-20	L. Brown		32.8	4.1	4.2	0.1	0.3	1.4	0.1	1.2	0.2	0.2	0.1	55.8
VPW-22	L. Brown		30.7	3.7	4.4	-	0.4	2.6	0.1	0.9	0.3	0.2	-	57.0
VPW-23	D. Green	Clayed	43.4	7.0	4.9	0.1	0.7	4.8	0.1	2.3	0.4	0.1	0.1	36.3
VPW-24	D. Green	Clayed	45.7	6.6	4.4	-	0.8	4.8	-	2.4	0.4	0.2	0.1	34.8
VPW-25	D. Green		41.4	7.3	5.5	-	0.8	5.1	-	2.2	0.6	0.1	-	37.2
VPW-27	Green	White	26.4	6.4	0.7	-	0.2	0.7	0.1	0.3	0.3	2.6	-	62.5
VPW-29	Honey	White	36.1	5.8	3.5	-	0.4	1.8	0.2	1.5	0.3	0.1	-	50.4
VPW-33	Green	White	31.8	4.9	1.2	0.1	0.2	0.4	0.1	0.6	0.3	2.3	-	61.0
VPW-34	L. Honey	White	27.7	1.8	2.2	-	0.1	0.4	0.7	0.4	0.2	0.1	6.5	59.9
VPW-35	L. Honey	White	28.3	3.7	2.6	-	0.1	0.5	0.1	0.6	0.2	0.1	-	63.8

FeOt corresponds to total iron as FeO. Glaze colours: L. Honey—light honey; and D. Green—dark green.

). The colour variation from light brown to light honey in lead-rich glazes is determined by the iron content, the colour of the pottery body, and the presence or absence of the white slip coating. The light brown glazes have high iron content (~4.3 wt% FeO) and occur on unslipped red-coloured pottery. The lightness of the honey-coloured glazes is related to the iron content (2.6 to 3.6 wt% FeO) and occurs in cream-coloured or white-slipped pottery. In the group of honey-coloured glazes, sample VPW-34 stands out with 6.5 wt% ZnO.

Principal component analysis of the pottery bodies allows us to distinguish three groups of chemical composition (Figure 7). Most of the samples have a chemical composition similar to the average of the shales [34] with average contents of 61.3 wt% SiO₂, 16 wt% Al₂O₃, 5.5 wt% FeO, 8.0 wt% CaO and 3.5 wt% K₂O and correspond to the red-coloured pottery (G1) (

Table 5. XRF chemical composition of the pottery bodies and SEM-EDS average chemical composition of the slips from the Vega pottery workshop.

Body	Paste	Colour	SiO2	Al2O3	Fe2O3t	MnO	MgO	CaO	Na2O	K2O	TiO2	P2O5	LOI
VPW-15	P6	Creamy	61.11	29.83	1.89	b.d.	0.44	1.45	b.d.	1.61	1.14	0.09	2.34
VPW-17	P6	Creamy	60.44	30.45	2.21	b.d.	0.42	1.38	b.d.	1.65	1.19	0.1	2.07
VPW-18	P7	Creamy	58.6	15.73	5.2	0.01	1.59	12.02	1.02	1.78	0.73	0.23	2.95
VPW-19	P7	Creamy	53.81	15.93	5.25	0.02	1.82	16.09	0.98	1.83	0.69	0.17	3.24
VPW-20	P1	Reddish	61.35	15.73	5.55	0.02	1.71	8.17	b.d.	3.47	0.76	0.21	2.92
VPW-22	P5	Reddish	63.42	15.45	5.61	0.03	1.37	6.77	b.d.	3.57	0.76	0.28	2.65
VPW-23	P1	Reddish	59.21	16.26	5.62	0.03	1.72	10.16	0.03	3.46	0.79	0.31	2.31
VPW-24	P1	Reddish	59.22	16.46	5.71	0.02	1.72	10.17	0.03	3.52	0.8	0.22	2.07
VPW-25	P5	Reddish	61.81	16.02	5.76	0.02	1.6	8.74	0.01	3.46	0.76	0.16	1.6
VPW-27	P1	Reddish	60.85	17.71	5.95	0.03	2.24	4.67	b.d.	3.89	0.82	0.27	3.39
VPW-29	P1	Reddish	62.53	16.07	5.02	0.02	1.5	7.36	b.d.	3.43	0.82	0.31	2.82
VPW-33	P1	Reddish	59.76	17.71	5.78	0.03	2.54	6.34	0.28	3.77	0.64	0.17	2.88
VPW-34	P1	Reddish	64.04	13.75	4.86	0.01	1.89	7.92	b.d.	3.31	0.74	0.19	3.09
VPW-35	P5	Reddish	61.11	14.60	5.00	0.01	1.38	9.59	0.02	3.23	0.76	0.38	3.71
Sample	Slip	SiO2	Al2O3	FeOt	MnO	MgO	CaO	Na2O	K2O	TiO2	P2O5	PbO
VPW-23	Clayey	59.8	16.5	5.0	0.1	1.4	10.5	0.2	3.2	0.8	0.1	2.5
VPW-24	Clayey	58.4	16.4	5.5	-	1.4	11.6	0.2	3.4	1.0	-	2.1
VPW-27	White	59.7	32.8	1.7	-	0.7	1.7	0.2	1.8	1.4	-	0.1
VPW-29	White	61.3	30.3	1.7	-	0.6	1.0	0.3	3.2	1.1	0.3	0.3
VPW-33	White	58.3	33.7	1.4	-	0.5	1.6	0.2	2.9	1.0	0.1	0.2
VPW-34	White	61.1	30.7	1.6	-	0.5	1.7	0.6	2.6	1.2	-	0.1
VPW-35	White	59.13	32.6	1.7	-	0.6	1.4	0.3	2.9	1.2	-	0.1

Fe₂O₃t correspond to total iron as Fe₂O₃; b.d.: below detection limit. FeOt corresponds to total iron as FeO.

). The second group (G2) consists of two creamy-coloured pottery with high alumina content (30.2 wt% Al₂O₃) and low calcium content (1.4 wt% CaO). The third group (G3) consists of two other samples with high calcium content (14 wt% CaO) and low silica and potassium content (56.4 wt% SiO₂, 1.6 wt% K₂O). The chemical composition of the slips can be divided into two distinct groups. The clay-like slip shows similarities to red-coloured pottery bodies, whereas the white slip displays similarities to aluminium-rich pastes (Figure 7).

The glazes are generally free of neoformed crystals, without signs of alteration in favour of microcracks, and show few bubbles. Glazes on clay-like slip and on reddish body pottery (P1 and P5 pastes and the G1 chemical group) show a thick interface of up to 100 µm formed by acicular lead–potassium feldspars and diffusion of lead inwards the body. In contrast, glazes on white slip-coated pottery show a very thin (10–15 µm) interface formed by tabular lead–potassium feldspars. Glazes on creamy coloured alumina-rich pastes (P6 paste and the G2 chemical group) also show a very thin (~10 µm) interface with lead–potassium feldspar and no or very limited lead diffusion in the body up to 15 µm. The glazes on creamy calcareous pottery (P7paste, G3 chemical group) show a thick interface of up to 100 µm formed by acicular lead–potassium feldspars. Pyroxene and some undissolved rounded quartz grains are observed in the glaze close to the interface (Figure 8).

3.4. Torrentejo Village

The set of sixteen glazed pottery from the Torrentejo village consisting of honey-coloured and green-coloured transparent and white opacified glazes was analysed. Some of the white opacified glazes show blue decoration, and glaze appears on one or both sides.

Transparent glazes are lead glazes, including seven samples with green and honey-coloured glazes. White opaque glazes include four samples of white glazes and four samples of greyish-white-coloured glazes (

Table 6. SEM-EDS average chemical composition of glazes from the Torrentejo site.

Glaze	Colour	Side	SiO2	Al2O3	FeOt	MnO	MgO	CaO	Na2O	K2O	TiO2	ZnO	SnO	PbO
T2	L. green		20.9	4.9	1.5	-	0.5	0.8	0.4	0.4	0.3	-	-	70.7
T5	D. green		34.0	5.1	2.7	-	0.7	0.4	0.1	1.1	0.5	0.1	-	55.5
T7d	Honey	Out	37.1	5.0	3.6	-	0.8	1.9	0.4	1.3	0.3	2.1	0.1	36.0
T10	Honey	In	35.002	3.661	2.547	0.04	0.439	0.883	0.327	0.772	0.216	1.774	0.105	54.986
T12i	Honey	In	34.1	8.2	2.9	-	1.2	1.9	0.3	1.4	0.5	1.7	0.1	32.2
T12d	Honey	Out	50.4	8.8	3.1	0.1	1.1	3.2	0.7	3.1	0.7	4.7	0.2	32.3
T13i	G. white	Out	53.2	2.4	0.3	-	0.2	1.4	0.4	5.9	0.1	0.1	4.5	32.5
T13d	G. white	In	52.9	2.4	0.2	0.1	0.2	1.5	0.5	5.9	0.1	0.1	4.1	30.1
T14	G. white		52.7	2.7	0.2	-	0.2	1.3	0.5	5.9	0.1	-	6.2	48.0
T15i	G. white	Out	53.3	2.5	0.1	-	0.1	0.7	0.4	5.8	0.1	-	2.7	23.8
T18i	White	In	46.0	3.7	0.4	-	0.2	1.3	1.2	5.1	0.2	0.1	5.9	34.3
T18d	White	Out	45.6	3.6	0.4	-	0.2	1.3	1.3	5.1	0.2	0.1	6.9	31.8
T20i	White	Out	50.1	2.8	0.3	-	0.1	1.8	0.4	7.6	0.1	-	4.6	32.1
T20d	White	In	46.5	2.8	0.3	-	0.2	2.5	0.4	7.4	0.1	0.1	7.3	30.4
T24	L. green		29.2	3.3	1.4	-	0.2	0.9	0.1	0.5	0.2	0.9	-	54.9
T26	L. green		31.5	3.3	1.1	-	0.2	0.3	-	0.6	0.2	0.1	-	67.7

FeOt correspond to total iron as FeO. Colours: L. green—light green; D. green—dark green; G. white—greyish white; and L. green—light green.

).

PCA of the chemical composition of the glazes shows the differences between the samples. The transparent glazes are divided into high lead glazes (>55 wt% PbO) and lead glazes (<55 wt% PbO) (Figure 9). The later lead glazes show higher aluminium (≈8 wt% Al₂O₃), calcium (1.9–3.2 wt% CaO), iron (2.9–3.12 wt% FeO_t) and zinc (1.65–4.7 wt% ZnO) contents, while high lead glazes are lower in aluminium (≈5 wt% Al₂O₃), calcium (0.3–0.8 wt% CaO), iron (1–2.7 wt% FeO_t) and lower zinc (0–1.7 wt% ZnO) contents. High lead glazes are green, while lead glazes are honey-coloured glazes. In addition, honey-coloured glazes showed higher zinc content (1.65–4.7 wt% ZnO) (Figure 9). Both lead glazes T12i and T12d show different compositions with the highest aluminium and zinc contents and include some iron-zinc microcrystals. In the high lead group, sample T26 glaze shows rounded galena and quartz inclusions with embayments as a result of the poor dissolution of the frits during glaze manufacture (Figure 10) [37].

The white opacified glazes are well grouped at the negative side of PC1 (Figure 9). These opaque glazes are lead-alkali glazes and fall into two compositional groups based on tin content. The first group has 5.4–7.25 wt% SnO₂, low lead content (32–36 wt% PbO) with Pb/Sn ratios varying between 4.4 and 6.9. The alkali content varies between 6.4 and 8.1 wt% (Na₂O + K₂O). The second group is characterised by a lower tin content (2.6–6.2 wt% SnO₂), slightly lower lead content (30–34 wt% PbO) with higher 4.8–12 Pb/Sn ratios and slightly lower alkali (6.1–6.4 wt% Na₂O + K₂O) content. These differences are reflected in the glaze colour being the first group’s glaze colour white, whereas the second group’s colour is greyish-white. In addition, although the opacity was provided by the appearance of tin-oxide microcrystals dispersed or clustered in the glazes that occur in both groups, the occurrence of crushed quartz and feldspar inclusions and bubbles throughout the glaze characterised the greyish-white glazes (Figure 10). The microtextures of the K-feldspars are variable, some of them showing textures of absorption by the glaze. Moreover, mineral relics are observed, such as cassiterite or galena. The latter is an important raw material in the production of lead for glazing, the ore of the lead (Figure 10).

Based on the PCA of the chemical composition of pottery bodies (

Table 7. XRF chemical composition of the pottery bodies from the Torrentejo workshop.

Body	Colour	SiO2	Al2O3	Fe2O3t	MnO	MgO	CaO	Na2O	K2O	TiO2	P2O5	LOI
T2	P. grey	57.16	21.55	2.77	0.01	0.82	3.57	0.05	2.78	0.94	0.07	9.99
T5	Red	62.55	16.34	6.52	0.07	1.42	0.59	0.12	3.90	0.93	0.06	7.61
T7	Creamy	62.06	21.36	2.74	0.01	0.69	1.99	0.06	4.61	0.82	0.25	5.60
T12	Creamy	53.98	15.67	4.04	0.02	4.96	7.83	0.05	3.19	0.73	0.10	9.56
T13	Pinkish	44.19	13.33	4.87	0.05	1.35	22.12	0.18	2.07	0.68	0.11	10.75
T14	Pinkish	43.89	13.09	4.68	0.05	1.39	20.35	0.18	1.99	0.64	0.09	13.47
T15	Creamy	46.28	13.15	4.72	0.06	1.40	21.76	0.24	1.49	0.69	0.11	9.68
T18	Grey	48.32	15.55	5.35	0.02	1.37	14.79	0.37	1.73	0.74	0.09	11.59
T20	Creamy	44.08	13.52	4.05	0.01	1.94	23.84	0.40	1.31	0.61	0.09	10.17
T24	Creamy	60.66	19.42	3.30	0.01	0.83	3.11	0.06	3.11	0.90	0.09	8.33
T26	Orange	53.09	20.64	3.62	0.01	1.05	8.12	0.15	3.99	0.74	0.12	8.08

Fe₂O₃t correspond to total iron as Fe₂O₃. Colour: P. grey—pinkish grey.

), the samples can be differentiated into three compositional groups (Figure 9). The first group is characterised by high calcium content (14.7–23.8 wt% CaO) and corresponds to the light-coloured bodies (creamy, grey, and creamy pinkish) of white opaque glazes. The second group is characterised by slightly high aluminium content (15.6–21.55 wt% Al₂O₃) and potassium content (3.59 wt% K₂O on average) and shows more chemical variability. The T12 sample has the lowest aluminium content and the highest magnesium and calcium contents and stands out from the rest of the Torrentejo samples. The third paste group is distinguished by the highest iron content (6.5 wt% Fe₂O₃t) as indicated by the reddish colour of the body.

Backscattered electron images show that the glaze/body boundary interface, regardless of the nature of the pottery body or glaze colour, is sharp, sometimes undulated and irregular and very thin where potassium-lead crystallites are observed. Only sample T12 shows a broad interface and the development of lead–potassium feldspars and diopside crystallites, which extend through the exterior of the glaze. White opaque glazes bearing pottery show a very thin (10–15 µm) interface formed by tabular calcium silicates and pyroxenes, indicating that most of the glaze pottery was previously fired before glaze application.

4. Discussion

This study shows some interesting manufacture aspects of glazed pottery in the northern region of the Iberian Peninsula. Considering the white glazes, the SEM-EDX results indicate different groups (Figure 11) related to the raw material and/or different opacification manufacturing techniques. The chemical data and SEM images indicate that the tin was used in all white opacified glazes. Cassiterite was observed as an inclusion in Torrentejo white opaque glazes as a relic raw mineral of the frit. The Islamic and Christian white glazes from the Santa Barbara Hill site show similar chemical composition with low alkalis and high lead contents. The two white opaque glazes from the Treviño Castle chemical are chemically similar to the Santa Barbara Hill white glazes. The more recent Torrentejo white glazes are alkali-lead glazes and are characterised by high potassium content due to the addition of quartz and feldspars. The decoration over the white glazes is only observed in Santa Barbara Hill and Torrentejo pottery. The black decoration on the Santa Barbara Hill glazes is obtained with manganese, while the blue decoration on the Torrentejo glazes is due to cobalt.

The white glazes from Tudela, Treviño and Torrentejo sites are compared with the nearby workshops of Najera and Logroño (La Rioja) and other further sites of Muel (Saragossa) and Teruel (Aragon) (Figure 11). The selected sites were chosen for comparison because of their documented close relationship over time with the sites under study [20,38]. The white glazes from Torrentejo show similarities to white glazes from Logroño, which are rich in tin content and quartz–feldspar inclusions [37]. The white glazes from Tudela show similar alkali ratio contents to glazes from Teruel, Najera and Muel but lower Sn/Pb ratios. The glazes from Treviño Castle show roughly similar chemical characteristics to glazes from Tudela and Muel, suggesting regional trade [25,39].

The data from this study show two methods of opacification: the Islamic method of adding cassiterite and the more modern method of adding quartz and feldspar to cassiterite. These two methods of opacification were also observed in the regional glazes mentioned above [40,41,42].

The transparent glazes are the most common in all the studied sites. Hierarchical clustering analysis was used to determine the chemical affinities between the glazes. The cluster analysis shows two main branches, Cluster A and Cluster B, defined by the lead content (Figure 12).

Group A is defined by low lead and high silica, potassium and calcium contents and includes SBH (Santa Barbara Hill) and VPW (Vega Pottery Workshop) glazes in green and honey colours. These transparent glazes were applied to calcium-rich creamy bodies and to calcium-rich slip. Group A is divided into two subsets grouping the glazes according to the sites. The colouring agent used to obtain the green colour in SBH glazes is copper, while, in the VPW glazes, it is iron under reducing firing conditions.

Cluster B groups transparent high lead glazes and includes light brown, honey, light honey, and green-coloured glazes. Most of the subgroups group glazes from different sites, but only the CTV (Treviño Castle) glazes are grouped in one subgroup, suggesting fairly standardised glaze manufacturing. Torrentejo glazes are high lead glazes, and the distribution into different subgroups is roughly based on the colour of the glazes. The SBH high lead glazes are also divided into different subgroups and include from both Islamic and Mudejar styles, suggesting different manufacturing methods within the same site. The VPW glazes are mainly divided into two subgroups, one group of honey and light brown colours and the other group of mostly green colours. The subgroup of honey and light brown glazes is on unslipped pottery, while the subgroup of green-coloured glazes is on white slip and light honey on an aluminium-rich body.

Cluster B groups glazed pottery from a long timeframe, and the multiple subgroups could be due to changes in raw materials, recipes and technology. In fact, the glaze on slip technology observed in VPW honey-coloured glazes occurs only in the northern Meseta during the Christian period. Glazes on slips of different composition, calcareous clay and aluminium-rich, are used to obtain dark green glazes and honey-coloured glazes, respectively. In addition, the use of slip also enhances the decoration, resulting in high quality pottery.

The microstructure of the interface between body and glaze is related to the manufacturing technology, thus providing information about the firing process and the chemical composition of the body and glaze [8,37,43]. Depending on the composition of the body, the nature of the microcrystals formed in the interface varies. The Tudela glazes show a variable thickness of the interface formed by microcrystals of lead–potassium feldspar and leucite, depending on the chemical composition of the glaze. Dispersed calcium pyroxene in the glaze suggests the diffusion of calcium from the body into the glaze and indicate a single firing [37]. Treviño Castle glazes usually show thin interface with no or few microcrystals, indicating a double firing process for the pottery. The body/glaze or slip/glaze interfaces of Vega glazed pottery are usually thin, although thicker interfaces are observed in some dark green and honey-coloured glazed pottery. Thick interfaces with needle-shaped microcrystals of lead–potassium feldspar occur when the glaze is applied to unfired bodies, suggesting a single firing. The Torrentejo glazes show thin interfaces indicate the glaze is applied over a pre-fired body, suggesting a double firing. Only sample T12 shows a thicker interface with profuse microcrystals pointing to a single firing.

A certain relationship between the chemical composition of the pottery and the colour of the glazes is observed. The dark green and light honey glazes are obtained over calcium-rich pottery or slip. Similarly, the opaque white glazes are mainly applied over calcium-rich pottery in all the studied cultural periods. Brown glazes are mainly found on aluminium-rich pastes.

5. Conclusions

The chemical–mineralogical analysis of historical glazes has led to an understanding of medieval and modern glaze techniques at several sites in northern Spain.

In this study, only two samples with alkaline glazes were found at the Santa Barbara Hill (Tudela site), which, according to the archaeological data, predate the Islamic invasion. The remaining glazed pottery was lead-glazed, a glazing method introduced to the Iberian Peninsula by the Muslims during the Middle Age.

Two groups of glazes were identified: white opaque glazes and transparent glazes, although both types do not always appear at all sites. Two methods of manufacturing opaque white glazes were identified, one by adding tin oxides (cassiterite) and the other by adding tin oxides mixed with quartz and feldspar.

Two groups of transparent glazes were distinguished according to lead content. The first group corresponds to glazes with a low lead content found only at the Tudela and Vega sites. The second group had a higher lead content and was more geographically widespread, as it was found in all the sites studied.

The interface features indicate some glazes were applied to the raw body, indicating a single firing process as is observed in the samples, showing the diffusion of lead from the glaze into the body. However, most of the pottery showed a well-defined interface, indicating that glaze was applied to a previously fired body. These two types of interface were also observed in the slipped pottery of the Vega workshop (Burgos), indicating that the slip and glaze were applied to a raw body or to a pre-fired slip and body.