My name is Joyce Townsend and I am a senior conservation scientist at Tate. This Turner sketch known as Study of Sky (it features in the Watercolour exhibition) has a rectangle of reddish colour surrounded by blue. Why?
The picture once had a picture mount - which was originally placed skew. Why was this done? It’s been suggested that it was to make the hastily-sketched horizon appear level. From Turner’s viewpoint, a level horizon would not have been vital for a colour study such as this. Over time the work has been exposed to far too much light.
The colour alteration here is extreme. There has been a combination of blue lost from the indigo, and vermilion red mixture used for the grey sky, the same blue lost from the indigo and brown ochre mixtures used for the foreground landscape, and the white paper having yellowed severely where it was exposed to light.
It is still possible to see some of the intended effects in the sky: greyer and darker clouds to the left side, which had black mixed in as well. On the right, there was less red used generally, and more local variation, so the clouds were more bluish purple on this side, and paler towards the horizon. If both components of the mixture have faded it’s naturally impossible to work out what may have been the intended appearance: here it takes some effort, but it is possible.
Originally, the sketch illustrated the contrast between a purplish grey cloudy sky, and a brownish green foreground landscape. Turner often used ‘optical’ or mixed greys and greens. All his contemporaries did the same for greens, for the good reason that there were no useful, strongly-coloured green pigments to use directly, until the early to mid nineteenth century. Artists mostly used indigo for the blue component, and any of the earth colours to mix in: brown ochre, raw or burnt sienna, raw or burnt umber. All these earth colours are familiar today, and still feature in watercolour boxes. For grass greens they mixed indigo with yellow ochre, or a yellow lake, Indian yellow or gamboge. The last three faded even more readily than indigo does, which is why many landscape areas in watercolour tend to look bluish, having lost some of the yellow.
Meanwhile, Indigo was commonly used for the sky as well, mixed with Indian red or in Turner’s case, the brighter red vermilion, and darkened with black. For really intense landscapes or skies, Turner substituted Prussian blue for the indigo.