Wyeth executed the portrait in egg tempera, which was not only archaic (it was used up until the 16th century until the invention of other mediums), but, most importantly, distinct from his father’s preference for oils. Wyeth pushed the medium to its very limits. Tempera is notoriously difficult to work with as it must be mixed by hand, and the ratio of water, pigment and binder must be constantly adjusted to maintain balance and consistency. Yet, Wyeth persevered and continued to work in tempera for his entire career. (https://blog.lofty.com/featured/mondays-muse-helga-testorf/)