[George O'Hanlon]

In regards to water-miscible oils (WMOs), this statement by Brynn Sundberg and Anthony Lagalante provides an important reason why we should examine the properties of WMO paints:

Water-mixable oil (WMO) paints are a relatively new addition to the artists’ palette that have increased in usage and popularity over the past few decades. Understanding the composition and properties of WMO paint is essential for the artists who use them and for the people who work with WMO paintings—particularly art conservators. However, the formulations that make WMO paints possible remain undisclosed by paint manufacturers.

Winsor & Newton released their Artisan Water-Mixable oil paints and mediums in 1997. Today, WMOs are available from more brands, including Royal Talens (Cobra), Grumbacher (Max), Holbein (Aqua Duo), and Lukas (Berlin). Each brand has its proprietary means to create water-miscible products.

Due to the rising popularity of WMOs, there is a need to uncover and understand their chemistry. In addition, many artists are combining traditional oil (TO) with WMO paints in ways most likely not anticipated by manufacturers. It is essential to learn more about their use in contemporary painting. Artists can make informed decisions about using WMO paints in their works, and conservators can improve their treatment of paintings.

In their paper, the authors analyzed Winsor & Newton Artisan WMO paints and found the following:

An exudate that formed from the cured paints was also analyzed. More-saturated fatty acid esters, POE oleate and POE stearate, were detected; however, more-reactive POE fatty acid esters were found absent, providing insight relating to the stability of surfactants in the Artisan WMO paints as well as potential curing reactions.

Surfactants can provide positive benefits to paint, but their use does not always positively affect all properties. The addition of surfactants can decrease the water resistance of the coating since surfactants can be very water-soluble and will easily wash out of the paint. Moisture resistance is a particularly prevalent problem for art conservation and issues with adhesion, loss of optical clarity, and dirt pickup caused by surfactants in contemporary acrylic emulsions used in artworks with acrylic paints.

The use of surfactants in paint is a concern because of its potential for phase separation. When a solution undergoes phase separation, it becomes cloudy; hence, the process in surfactant solutions is often referred to as the clouding phenomenon. It has been well-established that phase separation is primarily due to the dehydration of the surfactant’s hydrophilic groups when the temperature is increased.

HPLC-MS studies were contextualized with physical observations and surface measurements that were performed on the cured samples. Tacky surfaces observed on the cured paint films showed significantly higher surface gloss and surface hydrophilicity; these observations were quantified using glossimetry and droplet contact angle measurements. These results provide insight into the unique chemistry present in WMO paints and provide a basis for future studies for art conservators and conservation scientists working with this new media.

Based on their study, the authors made several conclusions regarding the crucial physical properties of WMO paints:

The clear variation in surface gloss and the persistent tackiness on the surfaces of the film likely result from surfactants that have migrated to the boundaries of the paint films.

Evidence of surfactant migration was found in the samples studied, which supports the general observation that surfactants tend to separate from dried paint films and migrate to the surface, leading to glossier and tackier paint films.

More conclusive evidence of surfactant migration was found in the fatty acid ethoxylates and other POE surfactants exuding from the WMO paint-outs. While the exudation of polyethylene glycol, fatty acid ethoxylates, and other POE surfactants is of concern, it has not yet been observed in other studies. Again, more complex, layered paint models will change whether such outcomes are observed, but the tendency for medium separation seems to be inherent to the less-cured WMO paints. Initial solubility results indicated that if such an exudate was to be removed from a painted surface, polar solvents should be avoided.

The exudate also makes cleaning more complex when compared to traditional oil paints, and polar solvents are to be avoided.

The paper recommends further studies to verify the findings and to understand the long-term mechanical behavior of dried WMO paintings.

Source

Brynn N Sundberg, Anthony Lagalante (2021). Characterization of non-ionic surfactants in Winsor & Newton’s water-mixable oil paints. Journal of Cultural Heritage. January 2021. DOI: 10.1016/j.culher.2020.12.008.