Radiocarbon dating for contemporary art: hardware developments and measurements

The research developed during my PhD can be inserted within the activities of the Archaeometry Laboratory of the Physics Department of the University of Ferrara, where works of art are studied using non invasive and non destructive techniques for diagnostic purposes. In particular, in the most recent years the attention has been focused on contemporary artworks. Concerning contemporary art, it is well known that one of the most important aspects is authentication and, as a consequence, identification of fakes. Sometimes a simple artistic style is not sufficient to solve this issue; therefore the support of scientific analyses is widely requested. In this framework, during my PhD, the capability to solve authentication issues by radiocarbon (14C) dating in an univocal way has been tested. The possibility to use the radioactive isotope 14C in contemporary art is given by the so called Bomb Peak, which is the huge increase of the radiocarbon concentration happened in atmosphere after the end of the Second World War. Actually, in the late 1950s and early 1960s, as a consequence of hundreds of nuclear weapon tests, large amounts of neutrons were produced in the atmosphere, thus significantly increasing the radiocarbon production rate. This artificial 14C input caused a global increase of the 14C/12C ratio in the atmospheric CO2 and in all the other reservoirs, i.e. in the oceans and in the biosphere. The increase was so large that the atmospheric radiocarbon concentration was almost doubled in less than 10 years. Then, after 1963, when the Nuclear Non-Proliferation Treat was signed, the 14C concentration started to decrease due to the rapid exchanges with the carbon reservoirs. This is just the peculiar trend of the 14C concentration that is exploited in forensics to date samples with a very high precision. In order to verify the possibility to use radiocarbon dating for contemporary artworks, several measurements were performed on paper and canvas samples collected from artistic objects of the XX century. The radiocarbon measurements were performed by Accelerator Mass Spectrometry (AMS) at INFN-LABEC (Laboratorio di Tecniche Nucleari per i Beni Culturali) in Florence, where a 3 MV Tandem accelerator and a sample preparation laboratory are installed. In these three years, several hardware developments concerning the experimental set-up were also performed. One of the hardware developments concerned the installation of a new graphitization line; thanks to this upgrade, the radiocarbon concentrations measured in blank samples were satisfactory and of the same order of magnitude of the pMC values measured in the first years of the operation of the AMS facility at LABEC. In addition, thanks to the new silicon photodiode installed as rare isotope detector on the AMS beam line, a remarkable improvement of the energy resolution was achieved, paving the way also to the measurement of rare isotopes other than 14C. Moreover, two new chemical pre-treatments were tested: these procedures were especially dedicated to deteriorate cellulose-based materials (alpha cellulose extraction protocol) and to restore artworks (chloroform-based procedure). Besides radiocarbon measurements on several test samples, the effectiveness of these new pre-treatments was also confirmed by using other techniques, i.e. Fourier Transform Infra-Red Spectroscopy, FTIR, and Attenuated Total Reflectance, ATR. Since these techniques are non destructive (ATR) or require just a very small quantity of sample (FTIR), their use during the sample preparation steps to verify the applied protocols can be proposed as a routine practice. The improvements regarding the experimental set up, especially the new chemical pre-treatments, and the radiocarbon measurements performed on paper and canvas samples collected from objects of the XX century were done in order to verify advantages and limits of radiocarbon dating for contemporary artworks. As far as paper is concerned, we analyzed samples collected from Italian newspapers published since 1930s until present and from fine papers used in limited edition drawing prints realized in the second half of the 20th century. The canvas samples were collected from contemporary art paintings. The measured 14C concentrations were compared to the expected concentrations estimated on the basis of the year indicated on each sample (i.e. the year of publishing/printing or the year signed by the painter), allowing us to highlight advantages and limits of the use of this dating method. The experimental data have clearly pointed out that we can easily discriminate between samples of the first half and samples of the second half of the 20th century, giving us the possibility to use radiocarbon to solve authentication issues. Important differences between materials derived from long-living plants (dailies paper from wood) and from short-living plants (fine paper from cotton and canvas from cotton or flax) have been shown. In particular, as far as newspaper samples are concerned, comparing the experimental results with the reference Bomb04NH1 curve, lower 14C concentrations than the atmospheric values were generally measured (in fact, in this case, the raw material is wood). Thus, even though we were able to perform radiocarbon measurements with a high precision, we have verified that, due to the characteristic manufacturing processes of paper, it is not possible to match each measured radiocarbon concentration to the corresponding calendar year in which the paper was produced. In the case of fine paper and canvas samples, we have noticed a general agreement between the measured radiocarbon concentrations and the expected values, although an offset of few years with respect to the expected date has been found. This discrepancy can be related to a reasonable time gap between the year when the plant, from which paper or canvas were manufactured, was cut and when these materials were really used. All these radiocarbon measurements have been supported by a characterization of the samples by optical and electron microscopy (SEM), to study the state of preservation and the raw materials. This is very important to choose the better chemical protocol to apply for the sample preparation and to better interpret the radiocarbon results. The results achieved with this work have demonstrated to be very important also for some real cases. The 14C measurements performed on a canvas sample collected from a restored painting have confirmed the effectiveness of the new chloroform-based sample preparation protocol. The dating of a painting attributed to F. Léger (1881-1955), that has been measured to be a fake, has shown how powerful radiocarbon can be for contemporary art authentication issues, i.e. for the identification of forgeries made in the last decades of the 20th century of artworks that are supposed to be of the first half of the same century. The research developed in these three years has shown advantages and limits of radiocarbon dating in studying of artworks of the 20th century. In particular, we have proved that it is possible to solve authentication issues especially for samples obtained from annual plants. This important result can be very useful for the art critics, the curators of the museum and the art merchants. It is obvious that the radiocarbon measurements and the other scientific techniques have always to be supported by the knowledge of other independent information concerning for example the story of the sample to be dated.