This sounded like a dubious suggestion to me but, as I'm not a chemist, I sought some authoritative opinions. As I suspected, it's not recommended if you're an artist who cares about the longevity of your paintings.
Sarah Sands, Technical Services Supervisor at Golden (which owns Williamsburg Oils), said: "We would certainly not recommend trying it! The suggestion probably comes from the fact that many of the water-miscible oil paints achieve that property through the use of surfactants, which are basically within the family of soaps and detergents. The danger with using something like laundry detergent is that, while it might seemingly work in the short run, the surfactants are ultimately trapped inside the paint film once it dries. However, in all likelihood, they are still mobile and able to migrate up to the surface or be pulled down towards the substrate, causing problems with curing, intercoat adhesion, etc. To make matters worse, most of these problems would likely not show up for decades, so there can be a false sense of everything being fine for quite some time.
"Keep in mind that beyond the surfactants, laundry detergents include a wide array of other chemicals and perfumes, many of them proprietary in nature, which only compounds the unknowns. If an artist does decide to work with these types of household materials, they need to fully realize just how risky and experimental their processes are."
Painting and Technical Advisor for Winsor & Newton, Paul Robinson, said: "Yes you can make conventional oil colour water miscible, to a degree, by adding washing-up liquid to it. As you may imagine, I would strongly not recommend this as it causes all sorts of issues with stability and performance. In doing so it is very likely that you will end up with a complete mess that will be unstable, may never dry, be full of bubbles and will more than likely fall apart."
Paul explained that W&N's brand of water-soluble oil paint, Artisan, is made with "modified linseed or safflower oil. The modification allows a temporary bond between the water molecules and the oils molecules; it can therefore be thinned and cleaned using water. As the painting dries the components used to modify the oil and any water used both evaporate out of the paint film leaving only oxidized oil and pigment, that is exactly what you are left with once traditional oils dry."
Always know what you're using to create your paintings. Decide how experimental you wish to be, how much you care about how long an artwork lasts, and make an informed decision.
