Technique and condition
This sketch was made on an off-white paper with a bluish grey wash applied first. The initial graphite pencil sketching for the architecture in this composition has been reinforced with iron gall ink over all the main elements. Highlights have been added in an under-bound lead white gouache. This gives a very crisp and opaque gouache, and Turner frequently used lead white for the purpose, a material he used in every oil painting, instead of the less opaque chalk used by many of his contemporaries in watercolour.
The sheet had been covered with a window mount and exposed to light, which has made the grey wash lose colour and has the caused the paper to turn perceptibly brown. The iron gall ink survives in fairly good condition here. It is normal for its initially black colour to turn brown as it ages, and this is its state today in most works where Turner used it. Extensive exposure to light would alter it chemically and eventually cause it to attack the paper surrounding each pen stroke, leading in the worse cases to extensively inked areas cracking, then falling out of the sheet altogether. Extensive exposure to the polluted nineteenth-century atmosphere of London tended to cause patchy or complete discolouration of lead white gouache to a dark brown. Neither has happened here to the slightest observable degree, which implies that the sheet has not been displayed for very long periods. This in turn implies that the grey wash must have faded quite rapidly, and suggests that indigo was used in the wash, because it fades readily in light.