Degradation phenomena

water sensitivity

Causes of Water Sensitivity in Oil Paint
Unvarnished twentieth-century oil paintings are often sensitive to aqueous cleaning, a method routinely employed by conservators for surface cleaning. Previous studies of modern oil paints and paintings have suggested a range of factors that can contribute to the sensitivity of surface cleaning of passages of oil paint media using aqueous solvents.

Studies have shown that manufacturer’s formulations are contributing to one of the known causes of water sensitivity: the formation of magnesium sulphate heptahydrate, or Epsomite crystals (MgSO₄.7H₂O). These crystalline entities are thought to be formed due to an interaction between the paint extender magnesium carbonate, an additive in some twentieth-century oil paints, and atmospheric sulphur dioxide (SO₂) at elevated humidity (Silvester et al. 2014, Tempest et al. 2013). The peaks in environmental sulphur dioxide levels during the industrial periods in Europe and North America of the 1950’s to the 1970’s correlate with the peak number of oil paintings known to exhibit water sensitivity to aqueous swabbing of this same period (Cooper et al. 2014). The formation of Epsomite is largely found to be a surface phenomenon (not in the bulk paint) and leaves indentations on the surface, seen with SEM-EDX. It is often found in what appears to be well-bound paint and seems to form a water sensitive skin. Beneath this surface skin the bulk paint has greatly reduced water sensitivity and solubility (Silvester et al 2014,Cooper et al. 2014).

On occasion, water sensitivity due to the formation of Epsomite is due to the paint composition, and not necessarily due to a reaction with atmospheric SO₂. Windsor and Newton cadmium yellow and cobalt blue are highly sensitive to aqueous solutions, due to an internal source of sulphur added during the manufacturing process. This has been observed in paint films directly from the tube, clearly indicating that it is a manufacturing issue (Cooper et al. 2014). Water sensitive iron oxide and ultramarine paints have also been found to be associated with a high degree of oxidation. Water sensitivity likely relates to the degree of crosslinking, saponification, and hydrolysis (Lee et al. 2018).

Films containing zinc oxide were also investigated (Silvester et al. 2014). These formed zinc and sulphur containing salts. Sensitivity to swabbing with water before and after exposure was evaluated. Films that developed salts, demonstrated increased sensitivity to aqueous swabbing after exposure to SO2. This suggests that increased water sensitivity may be due to a combination of the formation of hygroscopic degradation products and to weakening of the paint film due to salt-induced disruption of the surface.

Other problematic colours include synthetic ultramarine, the cadmium lemon yellows, chromium oxide green, cobalt violet and cobalt green (Tempest et al. 2013). These paints often appear to be well-bound and medium rich. These colours were found to be soft and tacky, indicating that they had never fully dried. Studies have shown that metal stearates (such as Aluminium and Zinc), added to paints during manufacturing to reduce the pigment volume concentration, also have an effect on the water sensitivity of oil paints. These can plasticise paints or retard the drying process by inhibiting the polymerisation process of paints, leading to an open polymer network that is susceptible to swelling by polar solvents. The hydrolysis of metal stearates increase water sensitivity during treatment with the formation of surfactant-like functionalities in the paint (Cooper et al.).

Studies have also identified the presence of water soluble medium skins (Burnstock et al. 2016). These skins are often soft and imbibe dirt. Evidence of the separation of medium from pigment and the formation of a skin at the surface of paint films provide evidence for a hypothesis that there is a difference between the surface and bulk paints that is related to water sensitivity on swabbing. Analysis of the surfaces indicate that water sensitive paints have a slightly thicker skin than non-sensitive paints, by 3 microns or more. Analysis with IR and ATR-FTIR indicate a concentration of polar carboxylic acids toward the surface of water sensitive paints. Medium rich skins indicate a lower pigment concentration in the surface skin and a higher concentration of organic material (Burnstock et al. 2016).

Treatments for water sensitive oil paint
Discussions regarding the ethical implications of cleaning water sensitive oil paints are still in full swing. What is regarded as ‘acceptable change’ has many implications, as many analytical techniques detecting change have become so sensitive they can detect changes that cannot be detected by the human eye. For example, if a trace of original material such as a medium skin with imbibed dirt is removed from a painting during cleaning, but this removal is completely invisible to a viewer and does not adversely affect the flexibility of the paint, then can that change be tolerated if the overall cleaning of the work results in significant aesthetic improvement (Ormsby et al. 2016).

The future stability of a paint layer also plays a role. It is hypothesised that Epsomite formation will likely stop when the sulphur dioxide is ‘used up’. In current clean museum conditions, with air filtration systems and temperature and humidity controls, the levels of sulphur dioxide are extremely low and the chances of Epsomite crystals forming in these conditions extremely rare (Cooper et al. 2014, Silvester et al. 2014). One potential treatment involves removing or thinning the water soluble skin which contains the imbibed dirt particles, leaving the bulk paint which is then not water sensitive, and less likely to imbibe dirt in the future as it is less soft and tacky (Cooper et al. 2014).This treatment has not yet been trialled, but could form the base of a potential long-term study on the effects of the stability of water-sensitive paints after the removal of the ‘skin’ with the Epsomite crystals.

Other treatments have so far focussed on removing surface and imbibed dirt without removing the water sensitive upper layer. These treatments have included various dry cleaning methods, the use of mineral spirit micro-emulsions with added surfactants, pH adjusted waters and gels, the use of silicone-polymer emulsifiers (such as Velvesil Plus and Shin-Etsu KSG 210) (Ormsby et al. 2017, Hartman et al. 2017, Finozzi et al. 2013, Cremonesi 2016, Steyn et al. 2017).