Here, from John Constable, “Cloud, Study,” 7 drops per inch
Metal and viscosified water
"Cloud Study" is part of a small series of paintings depicting clouds, drawn from drops of water - as clouds are. The patterns formed by the drops reproduce the skies of classic paintings by different painters who have carried out studies of clouds or whose cloudy skies are one of the characteristics. The representation corresponds to a low resolution digitized image, at 7 dots per inch – here 7 drops per inch. The water is sufficiently viscosified so as not to flow or evaporate during the exposure time. A specific choice of viscosity, however, makes it possible to control the speed of evaporation so that the image takes exactly the time of the exposure to fade away by evaporation. And leave us in front of a cloudless sky.