X-ray powder microdiffraction in the analysis of art works

Šímová V.¹, Bezdička P¹., Hradilová J.², Bayerová T.³, Hradil D.¹

¹Institute of Inorganic Chemistry AS CR, 250 68 Řež, Czech Republic.

²Academy of Arts, U Akademie 4, 172 22 Praha 7, Czech Republic

³University of Applied Arts, Salzgries 14, A-1010 Vienna, Austria

e-mail: veronika@iic.cas.cz

Analysis of samples smaller than 1 mm has always been a problem for powder diffractometry. X-ray powder microdiffraction is a new laboratory technique, extremely quickly developing in last three years, and becoming available mainly due to hardware development of conventional powder diffractometers. Common X-ray tube and monocapillary producing a quasiparallel beam combined with a sensitive/fast solid-state detector enable direct phase analysis from 0.1 mm spots using a traditional diffractometer in less than 1 day. It is possible to analyse a selected place on a solid sample with irregular shape.

The microdiffraction was applied to analyses of colour layers of art works, e.g. canvas and wall painting and polychromy on wood. Materials research of colour layers helps to date artworks and identify repaints, to study painting techniques of different authors and historical periods. Powder diffraction is extremely well suited to distinguish inorganic pigments of different natural provenance and to reveal secondary mineralization deteriorating artworks.

The fragments with characteristic dimensions 1x1x0.2 mm were examined by a combination of SEM/EDX and optical microscopy (fig. 1). Microdiffraction was used to confirm the presence of mineral pigments assumed indirectly on the base of their elemental composition, to identify the minerals in earthy pigments and, in the case of wall paintings, to describe the phase composition of secondary salt efflorescences.

Figure 1: Optical microphotograph of fragment of Funeral Crown of Charles IV

In the case of polychrome funeral insignias from the Prague castle tombs, the microdiffraction was able to directly identify the structural forms and a method of mediaeval preparation of two different lead-tin yellows used to paint the royal crown of Charles IV. (Fig. 2). In the first fragment, the lead-tin yellow of type I (Pb₂SnO₄) was identified. In the second fragment, SEM/EDX indicated an excess of tin and the presence of silicon, which is typical for the lead-tin yellow of type II, i.e. type I recrystallized in the flux with SiO₂. However, type II was not confirmed by powder X-ray microdiffraction, the excess Sn was found as cassiterite and Si as quartz.

Figure 2: Diffractogram of fragment of Funeral Crown of Charles IV

In the case of Renaissance wall paintings from Malta, the microdiffraction was able to distinguish among different crystalline salts built by common elements (Ca, Mg, K, S, O, N, H) and to indicate those formed as a result of previous restoration of the painting. (Fig. 3).

Figure 3: Diffractogram of fragment of Renaissance wall painting from Malta

Based on microdiffraction measurements, bole grounds of Baroque paintings were distinguished according to their mineralogy: kaolinite, illite, smectites, hematite (a-Fe₂O₃), goethite (a-FeOOH), and jarosite (basic ferric sulphate) were found as a result of different natural genesis and further treatment of earthy pigments used.

Acknowledgements:

V. Šímová was supported by a Grant Agency of AS CR (project number B1032401), J. Hradilová, D. Hradil and P. Bezdička acknowledge a support by Grant Agency of CR (project number 203/04/2091). Financial support by the Ministry of Education of CR (project number LN00A028) is also gratefully acknowledged.