FT-Raman analyses of dyes and lac pigments in folk arts and crafts in the interiors of Decorative Farmhouses of Hälsingland, Sweden, UNESCO World Heritage
Konferensbidrag (offentliggjort, men ej förlagsutgivet), 2015
This pilot study is about method development using FT-Raman supplemented with computational methods for analyzing dyes and lac pigments used in folk art and craft from Hälsingland, Sweden, during the 18th and 19th century. The study is part of an interdisciplinary four years long project funded by the Swedish Research Council (Vetenskapsrådet). Conservation scientists, chemists, physicists, conservators, art historians and ethnologists from different Swedish universities are collaborating to make a holistic approach and material characterization of wall paintings, painted wall-hangings, decorated wooden furniture and patterned textiles in Decorated Farmhouses of Hälsingland, UNESCO World Heritage since 2012. Spectroscopic methods combined with historic sources are used to understand the materials and the context and time when these artefacts were made. The interdisciplinary collaboration is essential and in this way, new and deeper knowledge of the paint, coatings, painting techniques and dyeing methods in the 18th and 19th century Swedish folk art and handicrafts can be obtained. The relevant research questions span over a wide range, for example about the existence and time frame of woad and turkish red dye. Suitable materials for characterization are for example clothing, which are often possible to date historically due embroidered monograms. Therefore, the material characterization of clothing dyes may shed light on the usage and distribution of various folk art techniques.
In the pilot study we present different historic dyestuff such as safflower, cochineals, madder, turkish red, brazilwood, fustic, birch leaves, weld, turmeric, woad, indigo and indigo carmine - all common in Sweden during this period. The dye references were made according to old Swedish recipes. Different textile fibres - cotton, wool and linen/flax - were dyed. In addition various colouring matters - for example purpurin, alizarin, atranorin, orcein, myricetin, luteolin, quercetin, rhamnetin, emodin, crocin, curcummin, kaempferol, indigotin and indirubin - were tested to identify specific fingerprint regions for each dye. In order to assist the interpretation of the vibrational spectra of the samples, computational methods were performed.
A typical case study is presented below: threads from an authentic textile, a blue skirt from early 19th century Delsbo parish, were analyzed using FT-Raman spectroscopy. The spectra were then compared with reference spectra. The Raman spectrum of a blue colored linen warp thread is shown together with the spectrum of pure indigotin as a reference (Figure 1). Preliminary results indicate that we are able to identify the main peaks as originating from indigotin in addition background from the linen fibre, suggesting indigo as the dye. Woad was also a common dye during the 18th and 19th century in Sweden . Different from indigo, woad may also contain kaempferol. The comparison with reference spectra however did not yield a match in the fingerprint regions of kaempferol at f.ex. 1188 cm-1 and 1606 cm-1 [2, 3]. Therefore we may exclude woad as an ingredient in the colouring dye of this specific piece of clothing. Historic sources from 18th century usually mention woad as an additional ingredient in the indigo vat to catalyze and help the bacterial fermentation process [4, 5]. During 19th century though, woad is more seldom mentioned as an ingredient and perhaps this is an example of a more modern indigo vat.
Other preliminary results shows that almost all pure colouring matters and some of the dyed textile fibres are easy to analyze with FT-Raman. However, it is difficult to analyse red dyes with this method. Furthermore, wool sometimes complicates the analyses. Therefore, we will continue using FT-Raman analyses and the computational models, supplemented with other analytical methods such as SERS and field-free APCI-TOF MS.
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This work is financially supported by the Swedish Research Council, Vetenskapsrådet.