Returning to Turner’s use of oil paint:
As you will note on the first page Cooke’s description of the bas-relief resulting from pure lead white paint buildup. Turner would also use a palette knife to just apply light paint across the skies in a wide swath.
Here’s a description from Townsend:
By the 1830s and 1840s Turner was again using stiff, pure oil impasto, applied with a palette knife, which has retained its original buttery texture and tone. The ‘limp impasto’ of linseed drying oil and wax (unrefined beeswax in some case and spermaceti wax in others) which Turner began to use in the later 1820s was used very frequently too. It could be mixed to form softer paint than pure oil paint, and applied with a brush or a palette knife. It sagged slightly before it dried, giving a contrast with the pure oil paint, and has not yellowed significantly. Once the formulation had been worked out, Turner used it for the rest of his life.
I also read that
Turner bought ready-mixed paint medium (megilp and asphaltum) from about 1835, the earliest known date… when such mixtures were first sold in pots. Later he bought them in tubes too.
Many of the glazed shadow areas in his paintings have darkened considerably, even during this lifetime. This might have been caused by the use of megilped paint.