Study of the chemical composition and the mechanical behaviour of 20th century commercial artists' oil paints containing manganese-based pigments
Introduction
Modern and contemporary paintings are facing complexities not only due to the nature of materials and their ageing and response to environment conditions, but also derived from the interactions which different materials may experience in the context of a painted structure.
From this perspective and considering the variety of industrial formulations produced since the late 19th century, it is evident that the identification of specific damages in modern and contemporary painted surfaces is not easily ascribable to a single cause but to the synergic situations (even under strictly controlled environmental conditions). This suggests the need of combining the chemical–physical characterisation and the study of mechanical and dimensional properties of painting materials to understand failure mechanisms and to design the appropriate conservation strategies.
The topic of the current research is the investigation of 20th century manufactured oil paints containing manganese-based pigments, such as raw and burnt umbers.
The growing interest around the study of the nature of 20th century artists' oil paints arose in the last decade to increase the knowledge related to modern and contemporary art materials and to respond to practical conservation issues. In particular, the investigation of manufactured oil paints was essential to understand the vulnerability observed in several well-bound oil paints from unvarnished artworks [1], [2], [3], [4], [5]. Recent studies highlighted the possible correlations between the degradation phenomena exhibited by modern oil paintings and 20th century oil paint formulations, focusing in particular on the variety of vegetable oils used and the various organic additives typically introduced by paint manufacturers [6], [7].
A fundamental role in the drying mechanism of commercial oil films is played by the presence of pigments. Umbers (both the natural/raw and the calcinated/burnt forms) are natural earth pigments composed of mixtures of iron and manganese oxides [8], [9], [10], [11]. Previous studies showed that iron oxides have an inhibiting effect on the oxidative polymerisation of the drying oils [12] and accelerate the formation of scission products by photo-oxidation [13].
Generally, paint films containing earth pigments are extremely flexible, weak [14] and sensitive to moisture [15], [16], [17]. The clay content in earth-based oil paint films is the reason for their ability to absorb water and undergo severe swelling, even with moderate oscillations of relative humidity. It has been proved that paint films containing umbers experience a significant dimensional response with high levels of relative humidity (between 75% and 90% RH), with both linseed and safflower oil media [15]. Under these conditions, umbers are flexible and elongable at break and present a very low ultimate tensile strength [16].
However, manganese is also a primary drier and therefore manganese-based pigments are known to have a siccative effect on oil paint films [18], [19], [20], [21]. During the oxidation reaction in film forming, the ion Mn2 + exerts through-drying properties which promote surface drying in the curing of oil paints. During hydrolysis of an oil film, free fatty acids are released and can migrate in the oil, reacting with metal ions present in the pigment and leading to the formation of metal soaps. As reported in [22], the greater amount of manganese oxides is present, the higher percentages of manganese ions dissociate from these oxides and dissolve in the oil accelerating its drying. Finally, an increase in the amount of manganese ions result in an increased stiffness of the paint film. The intrinsic characteristics of umbers and their interactions with the binding media govern paints performance and stability in mid-to-long terms, being also responsible for paints specific ageing and vulnerability.
Based on the above mentioned previous researches, this study addresses the issues related to the chemistry and the mechanical behaviour of 20th century raw and burnt umber oil paint films. The present research, in fact, correlates the chemical composition and the mechanical properties observed for selected manufactured umber oil paint films after being naturally aged for several decades, aiming to understand the influence of manganese in the formation of oil films, the role of manganese in paint films' mechanical behaviour and the way in which paint films containing manganese may perform in the mid-to-long term in the context of a painted structure.
Section snippets
Paint samples and sample preparation
All the paints tested in this study belong to the Mecklenburg's Paint Reference Collection at the Smithsonian Center for Materials Research and Education (USA). The selected raw and burnt umbers paints were manufactured by Winsor&Newton® (W&N), Gamblin®, Speedball® and Grumbacher®. The oil films tested and discussed in this paper were prepared in the 1980s and 1990s on polyester film and allowed to dry in controlled environmental conditions during a period ranging from 15 to 35 years.
Table 1a
Chemical characterisation
This section presents the results from the study of the organic and inorganic fractions of the selected paint samples. Some considerations regarding the oxidation state of the paint samples are also reported.
Conclusions and future perspectives
This research was built up on previous studies dealing with the chemistry of 20th century oils and the study of the mechanical and dimensional properties of painting materials, and aimed to provide an insight into the chemistry of selected commercial raw and burnt umber oil paint films, their drying and degradation as well their implications in the changes observed in their mechanical properties during ageing.
The results obtained evidenced:
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The addition of iron and manganese oxides to natural
Acknowledgements
The authors are deeply indebted to Dr. M.F. Mecklenburg (Museum Conservation Institute-Smithsonian Institution, USA) for providing the samples tested in this study and for the equipment donated. Gamblin Artists Colors and Kremer Pigmente GmbH & Co. KG are equally acknowledged for assisting with technical information regarding the materials tested. The authors also want to thank M. Planes and Dr. Moya (Microscopy Service, UPV) as well as Dra. Mestre (XRD Service at S.C.S.I.E, Universidad de
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