Non-invasive analysis of natural textile dyes using fluorescence excitation-emission matrices

Elsa Vanker
Sigrid Selberg

Our group recently published a new article Non-invasive analysis of natural textile dyes using fluorescence excitation-emission matrices, Talanta, 2022, 123805 led by Sigrid Selberg and Elsa Vanker.

In this study, multidimensional front-face fluorescence spectroscopy measured from surfaces using a fiber optic probe was assessed as a non-invasive and non-destructive method for the analysis of components in natural textile dyes. Multidimensional fluorescence data was acquired for a collection of wool yarns dyed with natural dyes (31 dyed wool yarn samples that were self-dyed with 18 different natural dyes) that were used as references in a case study of two historical textiles for which liquid chromatography-mass spectrometry was used as a confirmatory technique.

Self-dyed reference yarns
Selection of characteristic EEMs of self-dyed reference samples

Natural dyes are multicomponent mixtures and can originate from different sources (e.g., plants, insects, and fungi). Due to their complex chemical composition and the inherent lability (photooxidative fading and bleaching), the analysis of natural dyes can be quite challenging and in order to analyze dyes on textiles, it is often best to combine different analytical methods.  However, for the analysis of dyes, common and often the most informative methods, like chromatographic separations coupled with different detectors, are all invasive/destructive. The aim of this work was to explore the potential and limitations of fluorescence spectroscopy in analyzing natural dyes from dyed wool yarns using EEMs (excitation-emission matrices), measured directly from the surface of the objects, non-destructively and without any sample preparation.

To demonstrate the utility of the fluorescence method, analysis was conducted on two case study samples – fibers from historical artifacts. Comparing the EEMs of the reference yarns with our unknown case study samples, we were able to identify that dye from a plant of the Rubiaceae family (bedstraws and madders) was used for dyeing the case study samples. 

Here you can find the 50-day free access to the article.

One of the case study objects; Tapestry “Solomon is receiving a bride. Solomon Court” (year of production 1547); Textile sample was obtained from the Conservation and Digitization Centre Kanut (Estonia).

Order of Merit of the White Star – Prof. Hilkka Hiiop

Prof. Hilkka Hiiop

Today it was announced that the president of Estonia, Alar Karis, has honored 148 Estonians or our friends from abroad with national awards, also known as decorations.

It is a great pleasure to announce that the Order of Merit of the White Star was awarded to Prof. Hilkka Hiiop, a valued member of our workgroup! She has been acknowledged for bringing the conservation of cultural heritage into the focus of Estonian society.

The press release can be found here (in Estonian). But to translate some of the words of our president: “On the eve of Independence Day, the Republic of Estonia thanks people with its decorations whose perseverance, selflessness, dedication, ingenuity, or creation are an example to many others and have made Estonia better, more caring, more noticed, and more entrepreneurial.”

Congratulations from all of us to you, Hilkka!

Dr. Matti Laan – Laser cleaning in cultural heritage

Yesterday, on the 7th of May, Associate Professor emeritus of physics Matti Laan gave a highly interdisciplinary lecture about laser cleaning in the field of cultural heritage.

On this project, Dr. Laan (presenting his lecture in the picture on the right) worked with the late Associate Professor emeritus of chemistry Tullio Ilomets. Dr. Laan gave an exciting lecture about different lasers (e.g., Nd:YAG, XeCl, Er:YAG) and which of them is most suitable for laser cleaning of various artefacts (such as paintings or sculptures). For this laser ablation is used, which removes any undesired material (including ageing products and materials from previous conservation works) layer by layer.

Most of the listeners participated via the Zoom platform – over 70 physics, chemists, conservators, material scientists, and people from other disciplines joined in this interdisciplinarity lecture. The lecture was organised by our Cultural Heritage workgroup, Institute of Physics, and The Estonian Academy of Arts in the framework of Dr. Signe Vahur’s PRG1198. The recording (in Estonian) can be found here.

The secrets of two mummies have been unravelled

PLOS ONE has recently published a multidisciplinary paper on two Egyptian child mummies lead and participated by the members of our group. The team of 19 experts unravelled the secrets of two Graeco-Roman child mummies by applying most recent analytical techniques from archaeology, forensic sciences, analytical chemistry, medicine, entomology, and genetics. This is one of the most extensive multidisciplinary study of ancient mummies and the paper is free to download here.

New publication – Instrumental techniques in the analysis of natural red textile dyes

A wide variety of different dyes, complex composition of natural dye sources and low concentrations in samples make the identification of textile dyes challenging. In our cultural heritage group, work has been done for years to overcome some of these problems. Using seven different red dye sources and five instrumental approaches, a method for the analysis of textile dyes has now been developed in our lab to study different archaeological findings, museum artifacts, and other textile pieces. More about the results can be found in the article published in the Journal of Cultural Heritage by P. Peets, S. Vahur, A. Kruve, T. Haljasorg, K. Herodes, T. Pagano and I. Leito.

The usefulness of this developed method has been proved by analyzing several case-study samples from Estonian National Museum, KANUT, and private collections. Thanks to the multiple instrument combination and especially the usage of high-resolution mass-spectrometry, it was also possible to identify synthetic organic dyes without the use of any standard substances.

MaSC 2019 Workshop and Meeting in Ottawa, Canada

Eliise giving her talk

The 9th Workshop and Meeting of the Users’ Group for Mass Spectrometry and Chromatography (MaSC ) for the study of cultural heritage objects took place on June 3-7, 2019 in Ottawa, the capital of Canada. One of our group members – PhD fellow Eliise Tammekivi – also participated in the workshop and gave an oral presentation at the meeting.

Practical instructions for the analysis of dyes, resins and volatile organic compounds (VOCs)

The workshop of MaSC 2019 took place in the Canadian Conservation Institute (CCI) and Parks Canada, where the Thermal Separation Probe (TSP) was presented as a novel device for thermal desorption and „slow pyrolysis“ analyses in gas chromatography-mass spectrometry (GCMS) by the leading researchers in the study of cultural heritage materials. With the instructions from Jennifer Poulin (CCI) the participants were able to conduct the analysis of dyes and identify natural resins by the help of AMDIS  software. With instructor Dr. Gregory D. Smith (Indianapolis Museum of Art at Newfields) participants could perform evolved gas analysis of museum exhibition materials once again with TSP and GCMS.

Topics from fish oil in paintings to the interpretation of highly complex GCMS data

The Meeting of MaSC 2019 took place in the National Gallery of Canada. Presentations were given by the world’s leading analysts about the GCMS analysis of art and historical artifacts. For example, Kate Fulcher (British Museum) talked about the complications of identifying black liquids applied to ancient Egyptian coffins and Corina Rogge (The Museum of Fine Arts Houston) about the utilization of fish oil containing house paints to artists’ paintings in economically difficult times. Additionally, Michael Schilling and Henk van Keulen presented the wonderful advantages of using AMDIS software and ESCAPE for the identification of compounds in complex and small cultural heritage samples. Also, it was Eliise’s first full talk at an international conference, where she presented our groups’ work on the topic „Quantitative analysis of binders in cultural heritage objects“. Specifically, she presented the comparison of four common derivatization procedures in the field of cultural heritage for the quantitative GCMS analysis of oils. For the opportunity to attend the workshop and meeting she would like to thank the organizers of MaSC 2019 and Dora Pluss scholarship (Archimedes).

New publication – Comparison of derivatization methods for the quantitative gas chromatographic analysis of oils

Often paints contain oils that bind together the components (pigments, fillers, etc) of the paint. Analysis of these oils requires derivatization, but which derivatization method is the best for a quantitative approach?

The work on this topic began with the need of our cultural heritage group to have a routine method for the determination of fatty acid composition of oils with gas chromatography (GC). The analysis of oils with GC requires derivatization and for this a wide variety of procedures have been applied. It turned out there are only few comparisons of derivatization methods in the literature and all are quite limited by scope. For that reason, it was decided to explore this important topic comprehensively and, importantly, in the terms of absolute (not relative) quantification.

A member of our group – PhD student Eliise Tammekivi – implemented four common and well-known derivatization procedures to perform the absolute quantification of fatty acids with GC-MS and GC-FID. The four compared derivatization methods were: (1) methylation with m-(-trifluoromethyl)phenyltrimethylammonium hydroxide (TMTFTH), (2) two-step derivatization with sodium ethoxide (NaOEt) and N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA), (3) two-step derivatization with KOH and BSTFA and (4) acid-catalyzed methylation (ACM).

This study has now been completed and the results have been published in the journal Analytical Methods by E. Tammekivi, S. Vahur, O. Kekišev, I. D. van der Werf, L. Toom, K. Herodes and I. Leito. In the publication, a comprehensive and wide-scale quantitative comparison of the four derivatization methods is presented. The results demonstrate that methylation with TMTFTH is the least work-intensive and most accurate derivatization method – both in terms of reproducibility and derivatization efficiency (yield).  For further information see: Analytical Methods, 2019, 11, 3514 – 3522

TechnArt 2019 conference: Analytical techniques in cultural heritage

Anu, Signe, Pilleriin and Eliise

From 7th to 10th of May 2019 four members of our UT Analytical Chemistry group – Dr Signe Vahur, Dr Anu Teearu-Ojakäär, PhD students Pilleriin Peets and Eliise Tammekivi – attended the 7th international TechnArt conference in Bruges, Belgium.

The biggest conference of analytical techniques of cultural heritage

TechnArt is a place to present and discuss the newest results of the usage of analytical techniques in the field of cultural heritage. It is the biggest conference among its kind as it was also seen in TechnArt 2019, where the number of participants was about 400! The conference included three parallel oral presentation sessions, two poster sessions with 270 posters, a visit and dinner at the Halve Maan Brewery and an excursion with a boat trip in the historical city centre of Bruges, that has been listed as a UNESCO world heritage.

From Girl with a Pearl Earring to warship Mary Rose

Some of the most interesting talks included the presentation by Dr Abbie Vandivere from The Hague about the analysis of the painting Girl with a Pearl Earring (Johannes Vermeer, approx. 1665) and the difficult conservation of the remains of the warship Mary Rose (warship of the English navy under the command of King Henry VIII, sank in 1545) by Dr Eleanor Schofield from the Mary Rose Trust/Imperial College. Another interesting topic was addressed by Dr Lucia Toniolo who gave a talk on the conservation and monitoring issues of historical architecture, also addressing the hazard of climate change. However, with four days and three parallel oral sessions filled with presentations by the top scientists and conservators of the world, it is almost impossible to highlight all of the interesting and inspiring talks.

The highlights of our recent results

Also, TechnArt 2019 was the conference, where the attendance by our Cultural Heritage group members was the highest! Anu presented her poster „Analysis of resinous materials“, where ATR-FT-IR, SEM-EDS, GC-MS and ESI-FT-ICR-MS methods were combined for the analysis of the embalming materials obtained from two human mummies originating from Egypt and now exhibited at the University of Tartu Art Museum. Pilleriin presented her poster „Attenuated total reflectance and reflectance approaches for analysis of textile fibres with FT-IR spectroscopy“. This study showed, that both mentioned approaches are suitable and very useful methods for the identification of natural and synthetic fibres. Eliise presented her poster „Comparison of derivatization methods for the quantitative gas chromatographic analysis of oils“ where four widely used derivatization methods for the analysis of heritage samples were compared on the basis of absolute quantification.

Overall, TechnArt 2019 gave the members of our Cultural Heritage group the possibility to introduce their scientific work results, hear the inspiring lectures and have fruitful discussions in the magical historic city of Bruges.

IRUG 13 Conference on 5-7 December in Sydney, Australia

13th biennial conference of Infrared & Raman User Group, shortly IRUG, was for the first time ever held in Sydney, Australia in a beautiful Art Gallery of New South Wales. IRUG conferences are gathering people using IR and Raman spectroscopy for the analysis of cultural heritage, architecture, and forensic materials. This year 101 people were attending, including Dr Signe Vahur and PhD student Pilleriin Peets from our group. Pilleriin was also introducing her results in analyzing textile fibres with IR spectroscopy in an oral presentation. Development of methods for the analysis of various textiles using ATR- and reflectance FT-IR spectroscopy turned out to be very relevant: throughout all three days, many people approached to discuss the topic, ask questions and advice or to make contacts for later cooperation. We were also very glad that conference participants were interested in our ATR-FT-IR spectral database of cultural heritage and conservation materials, available here.
Three days were filled with a wide range of interesting presentations from analysis of Aboriginal Australian pigments and identification of contents from the coffin of a 2500-year-old Egyptian mummy Mer-Neith-it-es to investigations of cultural heritage materials using neutron techniques. Besides the analysis of traditional pigments, organic pigments, especially synthetic organic pigments and their use in art were discussed (oral presentation by Dr Suzanne Lomax and Dr Steven Saverwyns). Another very interesting and thought-provoking presentation was done by Dr Gregory Smith from Indianapolis Museum of Art. He discussed the difficulties in getting accurate standard reference materials in the field of cultural heritage analysis. His results showing how many false materials can be found on the market (and not only vendors like Kremer Pigmente, but also providers like Sigma Aldrich) were very surprising.
All in all, IRUG 13 conference offered three full days of interesting presentations, a lot of knowledge and useful tips for further research and a pleasant atmosphere with excellent company.

Ancient men and women had different menus!

A new paper on ancient dietary practices was recently published by our group (led by Dr. Ester Oras) in the Journal of Archaeological Science: “Social food here and hereafter: Multiproxy analysis of gender-specific food consumption in conversion period inhumation cemetery at Kukruse, NE-Estonia”.

We demonstrated the fruitfulness of multiproxy dietary analysis combining plant microfossil, human bone stable isotope and pottery related organic residue analysis. The results reveal that even 800 years ago men and women had different dietary habits: men preferred fish and higher trophic level terrestrial animals (e.g. pork), whilst women declined towards ruminant carcass (a nice steak!) and dairy products.

The paper is one of the few of its kind illustrating ancient food consumption as a highly social phenomenon, and setting an example for microscale dietary analysis in the future.