Dating Alhambra stuccoes

Conservation scientists, conservators and art historians can often obtain useful clues for the dating and authentication of art objects by identifying the materials used on them.  For example, when pigment analysis is carried out on a museum object, it can reveal the presence of date-marker compounds, i.e. materials which have a first date of manufacture or, for other reasons, are unlikely to have been used for the decoration of a work of art produced at a particular time and place. This concept is exemplified by the analysis of five painted stuccoes from the Victoria and Albert Museum collection of medieval Hispano-Islamic panels (see Figure 1).  The stuccoes are from the Alhambra Palace in Granada, Spain, and supposedly date from the 14th century. ¹.Pigment analysis was requested by Victor Borges, V&A Sculpture conservator as an aid to dating.  Samples of the pigmented areas of the stuccoes were first analysed by Raman microscopy, a non-destructive, non-intrusive technique which is particularly suitable for the identification of pigments (see figures 2 and 3).  Subsequently, the samples were mounted as dispersions and examined by polarised light microscopy. ² The identification of the pigments present on the stucco fragments as well as the evaluation of the shape and size of the pigment particles provided clues about the date of the fragments (see Table 1). ³

In contrast, modern pigments such as Prussian blue and lead chromate (also known as chrome yellow) were detected on stucco fragment A.10-1913 (see Figure 3).  This indicates that either the whole fragment is a modern replica or the fragment is original, but was re-painted at some point in the 19th century.  This 'terminus post quem' derives from the fact that Prussian blue was first synthesised in 1704, but it was not commercially available until the 1720s; on the other hand lead chromate, also known as chrome yellow, was first synthesised at the beginning of the 19th century.  One of the fragments (A.10-1913) also shows traces of synthetic, wet-process vermilion, characterised by uniformly sized, small, rounded particles.

The analysis of pigments and materials found on these stucco panels from the Alhambra Palace in Granada, Spain, gave sufficient clues to hypothesise that one of the panels was not from the 14th century, as originally thought.  Gypsum, and in one instance, calcite were detected in the stucco mortar, and traditional materials such as lazurite, cinnabar, and carbon black were detected on stucco fragments supposedly still covered by the original decoration.  Pigments such as Prussian blue and lead chromate were found on one stucco fragment, which was therefore thought to be either an original fragment repainted at some point in the 19th century or  a copy made in the 19th century. The latter hypothesis is supported by observations collected during the conservation treatment of the stucco fragments. ¹

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