Céline Daher

Advanced discriminating criteria for natural organic substances of cultural heritage interest: spectral decomposition and multivariate analyses of FT-Raman and FT-IR signatures.

Natural organic substances are involved in many aspects of the cultural heritage field. Their presence in different forms (raw, heated, mixed), with various conservation states, constitutes a real challenge regarding their recognition and discrimination. Their characterization usually involves the use of separative techniques which imply destructive sampling and specific analytical preparations. Here we propose a non destructive approach using FT-Raman and infrared spectroscopies for the identification and differentiation of natural organic substances. Because of their related functional groups, they usually present similar vibrational signatures. Nevertheless the use of appropriate signal treatment and statistical analysis was successfully carried out to overcome this limitation, then proposing new objective discriminating methodology to identify these substances. Spectral decomposition calculations were performed on the CH stretching region of a large set of reference materials such as resins, oils, animal glues, and gums. Multivariate analyses (Principal Component Analyses) were then performed on the fitting parameters, and new discriminating criteria were established. A set of previously characterized archeological resins, with different surface aspects or alteration states, was analyzed using the same methodology. These testing samples validate the efficiency of our discriminating criteria established on the reference corpus. Moreover, we proved that some alteration or ageing of organic materials is not an issue to their recognition.

Raman Spectroscopy of cultural heritage Materials: Overview of Applications and New Frontiers in Instrumentation, Sampling Modalities, and Data Processing

Rooted in the long tradition of Raman spectroscopy of cultural heritage materials, in this work we provide a personal perspective on recent applications and new frontiers in sampling modalities, data processing, and instrumentation.

Non-destructive characterization of archaeological resins: seeking alteration criteria through vibrational signatures

Natural resins are prone, like all natural organic products, to ageing and degradation. Characterizing this alteration provides better knowledge about their chemistry, and when they are involved in cultural heritage artefacts, it helps to define relevant conservation protocols. In this study, a set of archaeological African copals that present specific surface degradations and colors were analyzed by FT-Raman and ATR-IR spectroscopies. A methodology based on the modifications of the vibrational features between the bulk and the resin surface was defined. A relationship between the evolution of the molecular structure of the copal samples and some vibrational bands could be established; and possible ageing reactions occurring at the surface of the samples could be proposed. This specific surface aspect is actually related to an alteration, and the surface states and the color variability are linked to different alteration degrees. This could refer to different conservation contexts (presence of air, water, etc), possible preparation or different origins: for example the collection of fresh or partly fossilized resins.

Alteration of Asian lacquer: in-depth insight using a physico-chemical multiscale approach.

Oriental lacquer has been used in Asian countries for thousands of years as a durable and aesthetic coating material for its adhesive, consolidating, protective and decorative properties. Although these objects are made from an unusual material in Occident, Western museum collections host many lacquerwares. Curators, restorers and scientists are daily confronted with questions of their conservation and their alteration. The characterization of their conservation state is usually assessed through visual observations. However deterioration often starts at the microscopic level and cannot be detected by a simple visual inspection. Often, ageing and deterioration of artworks are connected to physical, mechanical and chemical transformations. Thus new insight into alteration of lacquer involves the monitoring of macro-, microscopic and molecular modifications, and this can be assessed from physico-chemical measurements. Non-invasive (microtopography and Scanning Electron Microscopy - SEM) and micro-invasive (infrared micro-spectroscopy using a synchrotron source - SR-μFTIR) investigations were performed to study the degradation processes of lacquers and evaluate their level of alteration. In particular, spectral decomposition and fitting procedure were performed in the 1820-1520 cm(-1) region to follow the shift of the C=O and C=C band positions during lacquer ageing. The present work proves the potential of this physico-chemical approach in conservation studies of lacquers and in the quantification of the state of alteration. It evidences chemical phenomena of alteration such as oxidation and decomposition of a lacquer polymeric network. It also demonstrates for the first time the degradation front of artificially aged lacquer and the chemical imaging of a more than 2000 years old archaeological lacquer by using SR-μFTIR.

Preserving process: conservation of a Helen Frankenthaler maquette for Card

ABSTRACT Contemporary American print publishers, such as Universal Limited Art Editions (ULAE), collaborate with artists to produce prints using high quality materials for their editions. However, for developing their imagery, artists often use more affordable materials that may be prone to degradation, discoloration, and structural instability. This article discusses the treatment of a maquette by Helen Frankenthaler, a multi-layered assemblage of paper and transparent film joined together with pressure-sensitive tape that is one of the several preliminary works for her 1971 lithograph Card, now in the collection of The Art Institute of Chicago. Treatment of the maquette was undertaken to reduce degraded tape adhesive and replace it with a stable pressure-sensitive alternative while preserving the integrity of the work and its original materials. FTIR analysis in reflectance and transmittance modes identified the transparent films as cellulose acetate and the tape carriers as both cellulose acetate and regenerated cellulose (cellophane); these findings informed treatment and storage decisions. An analytical survey of several works in the ULAE collection also confirmed the use of cellulose acetate in works by other artists from the 1960s and 1970s. Therefore, concerns for the stability of this material would ultimately factor into the broader preservation plan for the archive.