In the second half of the 90s, some of the damage phenomena produced in oil paintings were associated, for the first time, with the formation of metal soaps. Since then, the harmful effect of these compounds on the structural integrity and aesthetical appearance of works of art is well known [1, 2].
Metal soaps are metal carboxylates containing both fatty acids and metal cations. In traditional oil paintings, metal soaps are formed by the chemical reaction between the fatty acids of the drying oil with the metal ions (Zn, Pb, Cu, etc) of some pigments or additives. In modern paintings, these compounds are sometimes added on purpose as dispersion agents, stabilizers, and extenders . Metal soaps show up in the painting in a heterogeneous way, causing different kinds of damages: protrusions, efflorescence, increased transparency, etc. [4–7].
The damage phenomena due to metal soaps have been pointed out in paintings of different age, since Rembrandt (S. XVII) to Salvador Dalí (S. XX), evidencing the extension of the problem and alarming the conservation departments of many museums all over the world. It is, indeed, a problem intrinsic to the painting components, unavoidable though, in some kinds of artworks. Although some metal soap aggregations have been successfully reduced using cleaning treatments , great caution is needed in the choice and use of cleaning agents. Furthermore, when the damage compromises the structural integrity of the paint layers, no restoration treatment exists that can return the paintings to their original condition. Preventive conservation is, therefore, the only strategy to adopt.
Although the formation mechanism of metal soaps has been extensively studied, the correlation with the environmental conditions of conservation still needs a deeper investigation. Some studies have demonstrated that certain conditions of temperature and relative humidity favour the formation of metal soaps and their subsequent crystallization [9–15]. However, in most of the published works, extreme conditions of ageing were used, which are far away from the real conservation environments. Besides, the normal fluctuations of the environmental parameters with time, were not taken into account.
Hence, this research was born to study the formation of metal soaps in environmental conditions that are more similar to the real ones, trying to simulate both control and uncontrol storage rooms, with the aim of establishing the best values of temperature and relative humidity to avoid or delay the formation of metal soaps in oil paintings.
Lead white, Pb(CO3)2·Pb(OH)2, zinc white, ZnO2, and titanium white, TiO2, are the most used white pigments in oil paintings. Both lead and zinc cations are known to be prone to react with the free fatty acids of the oil. Lead and zinc are indeed the most common metals involved in the formation of metal soaps, although metal soaps of potassium, calcium, copper, and aluminium have been also identified [5, 13, 16–21]. On the other side, titanium white does not present such reactivity since it is a photocatalytic pigment that does not behave as Lewis acid or alkali, so not able to react with the drying oil [22, 23].
Besides the pigments, the nature of the binder agent has been proved to influence the metal soaps formation. Although oil paintings are the works of art where the metal soap formation has been mostly studied, a few publications also report this phenomenon with beeswax, egg yolk and resins [24, 25]. In this work, two different linseed oils were used: cold-pressed linseed oil and alkali-refined linseed oil, whose concentration of free fatty acids could be different as a consequence of the extraction/refining processes. Alkali refining, indeed, reduces the amounts of free fatty acids, unlike cold pressing that does not alter the oil .
Different oil paintings based on lead, zinc and titanium white pigments, formulated following the traditional preparation methods, have been subjected to cyclic conditions of relative humidity (RH) and temperature. The formation of metal soaps was followed in time by ATR-FTIR, focusing on the spectral region 1650 − 1380 cm− 1 where the stretching bands of the carboxylate groups (νCOO−) appear . The results were compared with the behaviour of paintings conserved in standard environmental conditions (50%±5 RH, 21 ± 2ºC).