A Possible Natural and Inexpensive Substitute for Lapis Lazuli in the Frederick II Era: The Finding of Haüyne in Blue Lead-Tin Glazed Pottery from Melfi Castle (Italy)

Abstract

The blue color of glass and ceramic glazes produced in Apulia and Basilicata (Southern Italy) between the 13th and 14th centuries and connected to the Norman-Swabian Emperor Frederick II, has been, for a long time, under archaeometric investigation. On the one hand, it has usually been associated with lapis lazuli, due to the finding of the polysulphide blue chromophores typical of lazurite. Moreover, the observation that the mineral haüyne, which belongs to the sodalite group as well as lazurite, can be blue and/or can gain a blue color after heating, due to the same chromophores, has caused this automatic attribution to be questioned, and also considering that the mineral is characteristic of the rock haüynophyre of Melfi (Potenza, Southern Italy), a location of interest for glass and pottery findings. In this paper, for the first time, several haüyne crystals were found in the blue glaze of a ceramic dish found at Melfi Castle, leading to the hypothesis that, in this case, the local haüyne-bearing source could have been used as the coloring raw material. The discovery was possible thanks to SEM-EDS and Raman analyses that, respectively, highlighted the typical numerous presence of very fine sulphur-based inclusions in the crystals and the characteristic Raman signal of blue haüyne. This study was also focused on the composition of the crystals inclusions, aided by SEM-EDS and Raman maps, since the original very fine pyrrhotite was transformed into Cu and Pb phases (copper sulphates, copper sulphides, and lead oxide) due to reactions with cations that had mobilized from the glaze, while the migration of Si from the glass allowed the transformation of the rim of the haüyne, a silica-undersaturated mineral, into a corona of small euhedral and neomorphic Pb-rich feldspars, a silica-saturated phase.

Keywords: Frederick II; blue; haüyne; lapis lazuli; medieval blue tin-lead glazed ceramic.

Figure 1

M22 sample from Melfi (upper, left); in its stratigraphic section, in the red-bordered inset, the blue zone with two blue sodalite crystals is easily recognizable (upper, right) and SEM-BSE image of the blue lead-tin glaze in which two sodalite crystals are present (bottom, left). In the red-bordered inset (bottom, right) a crystal that shows very fine opaque mineral inclusions, typical of the Vulture rocks haüyne, is present.

Figure 2

Reflected light optical microscope (OM) images of the M22 sample with the lead-tin glaze blue zone in which numerous haüyne crystals are easily recognizable; in the yellow-bordered inset SEM-BSE images of the sample are reported.

Figure 3

(left) SEM-ED spectrum of an haüyne crystal found in the lead-tin glaze of the M22 sample (data represent the compositions of different haüyne crystals); (right) SEM-ED spectrum of the lead-tin glaze of the M22 sample (the microanalyses, performed on raster of 320 × 60 μm with relative time acquisition, of different glaze areas are also reported).

Figure 4

Composition of some haüyne from Monte Vulture outcrops and of the haüyne present in the lead-tin glaze of the M22 ceramic sample; lazurite of two samples (Sar-e Sang area of Afghanistan and Condoriaco area of Chile) of the Earth Science Museum of the Bari University and the mean of heated lazurite crystals of a Melfi (PZ) blue gilded and enameled glassware [1] are also plotted for comparison in the K₂O-Na₂O-CaO diagram. Lines separating the fields of sodalite (S), nosean (N), and haüyne-lazurite (H and L, respectively) are from Lessing and Grout [22].

Figure 5

Haüyne compositions from phonolite and haüynophyre volcanics of the Monte Vulture area (Toppo San Paolo, VU1521, and Melfi, VU1523, samples) both heated at 750 °C as compared with the composition of the small crystals found in the lead-tin glaze of the M22 ceramic sample; as suggested in [23] for lazurite, the haüyne chemical data are plotted atomically with respect to S⁶⁺, S²⁻, and Cl⁻. Lazurite field is delimited taking into account the data reported in [1,23], in which are also plotted the analyses of two samples (Sar-e Sang outcrops of Afghanistan and Condoriaco outcrops of Chile) from the Earth Science Museum of the Bari University and the heated lazurite crystals of a Melfi (PZ) blue gilded and enameled glassware [1].

Figure 6

SEM BS electron images of a haüyne crystal and the relative more representative elemental X-ray maps.

Figure 7

Representative Raman spectra extracted from the hyperspectra acquired on the mapped areas on haüyne crystals 3 (a–e) and 5 (a–h).

Figure 8

Representative baseline-subtracted Raman spectrum acquired on haüyne crystals.