Multiple paint layers
Cross section of a paint sample from the left background, magnified 200 times
The most noticeable feature of all the paint samples from Gainsborough's 'Viscountess Folkestone' is the multiple paint layers. There are up to eight thin layers of varying colours, the lower layers having little relation to the finished painting.
This is not unexpected, as the visible alterations to the composition made by Gainsborough are typical of his technique.
The x-ray of the painting confirms this with vigorous and rapid brushstrokes clearly overlaying each other, particularly in the drapery. On closer inspection of the paint samples and x-ray, more significant alterations to the composition become apparent.
X-ray of the Gainsborough's 'Viscountess Folkestone' showing underlying brushstrokes that are not visible in the finished painting