The sea and sky were both worked in first with unmodified oil paint. The sky was developed with thin broken washes of opaque paint, called scumbles. Turner worked up the buildings with paint applied with a palette knife, then smoothed it with a brush. Elsewhere he thinned the paint considerably, probably with oil of turpentine. This evaporates to give matte paint, necessarily in a thin layer, which contrasts with the naturally glossier unthinned paint used to make impasto (texture). The process took several days, since the impasted paint overlies partially-dried paint.
Years later, the sky was damaged in Turner’s studio, perhaps by water or even rain, and some paint flaked off. Turner reworked the sky with bodied paint – that is, paint that retains impasto, perhaps a megilp – using yellow barium chromate, not available before 1843. He used the end of his brush to enliven the surface, leaving characteristic straight marks in the paint.