When a painting is this large, there is a lot of real estate to cover.
For sky, I have found the best way to make it feel atmospheric is to start by mixing several colors. Then I paint them on my board and blend them into each other. Three brushes were used to put the paint on and blending took 5-6 flat sable brushes. Once my brush was filled with paint, it was no longer effective for subtle blending and would only leave distracting streaks. Time to grab a clean brush. The clouds were added near the end.
It is important to note that after I paint my turpentine underpainting (see previous blog), I don't use turpentine again on the painting. All my paints are mixed directly from the tube. Rembrandt oil paints are juicy enough that they move well without any additives or mediums. The color stays fresh and is only ground pigment in linseed or safflower oil.
My favorite part of this section was the water. It adds depth to the background and brightens up the scene. Next up the extinct volcano.