In situ Raman spectroscopic investigations of the adorning gemstones on the reliquary Heinrich's Cross from the treasury of Basel Cathedral

An in situ non-destructive study of the reliquary cross called Heinrich's cross dating from the late Middle Ages (first quarter of the 11th century) was performed using a mobile Raman microspectrometer in order to identify the adorning gemstones. Heinrich's cross, today kept in the Museum of Applied Arts (Kunstgewerbemuseum, KGM), Berlin, is considered to be one of the most precious sanctuaries of the treasury of Basel Cathedral. It bears 68 gemstones that are mounted at the both sides of the cross. Silver pearls alternate with gemstones. Analyses were made with a fibre-optic System-100 Raman-analyser (Renishaw), directly in the museum, without the need to remove the object. Strengths and weaknesses of the use of a mobile Raman microspectrometer for the investigation of art and archaeological objects within the museum are discussed since only relatively few analyses using mobile spectrometers have so far been performed in situ. In this study, the determination of the nature of the gems generally confirms the identification based on previous optical observations. Most of the mounted materials were found to be glass paste or quartz varieties. Some garnets, sapphires and interestingly rubies were also identified. The use of rubies is rare in the Middle Ages. Generally, green, blue and dark-coloured stones were difficult to identify using this Raman set-up (lambda = 785 nm) because of fluorescence phenomena, and 11 gems could not be identified unambiguously. This study shows the importance of coupling optical observations with Raman spectroscopy for gemstone identification. These results together with those of other fine objects originating from the same treasury show that mainly quartz and glass paste were used on very valuable and outstanding religious items from the Middle Ages to the Gothic period. However, it could not be determined if they represent gems replaced during later times or original ones that were mainly chosen for their colour and not necessarily for their value. Copyright (C) 2004 John Wiley Sons, Ltd.