Technique and condition
The canvas is primed with an ochre-brown ground layer, which is visible with the naked eye and under the microscope through worn paint layers. Light microscopy of the cross-section suggests that it consists of lead white, orange/brown ochre and ground glass mixed into it. The medium was not analysed but is thought to be oil. This layer is painted thinly and uniformly over the whole of the remaining surface.
The portrait appears to be very thinly painted, and the cross-section from the black line on the left ear of Tristram (dark Spaniel) shows that there are four distinct layers of paint. Directly above the ground are two layers of light yellow/green pigment, consisting of lead white, yellow ochre, bone black and ground glass. Both layers contain the same pigments but appear to have been laid on consecutively. Above these lighter layers there is dark brown layer. Although the pigments are too dark to resolve in the cross-section, the dispersion, made from fragments of this sample, revealed a large amount of bone black, dark brown particles, probably brown ochres, some lead white, some ground glass and the odd particle of vermilion. The upper-most layer above the dark brown was a thin layer of pale pink consisting of lead white and a very few particles of vermilion or Mars red.
In the mid-brown background just above Tristram's ear, revealed a similar layer structure with slightly different colouring. Above the light brown priming is a pale pink layer consisting of lead white and vermilion with either ground glass or chalk. On top of the pink is a very dark brown layer, which in dispersion appears to consist of brown ochres, bone black and vermilion. Above this is a fragmentary grey layer made up of bone black particles in lead white.
The sample taken from a light area of Fox's tail was made into a dispersion and the pigments visible in light microscopy were lead white, brown ochre, red ochre, bone black, probably naples yellow and ground glass.
The technique used by Gainsborough in this painting seems to comply with what is known of his approach to other works, although the range of pigments here is minimal. Apparently Gainsborough liked roughness which was 'of use in giving force to the effect at a proper distance' ('The Great Artists: Their Lives, Works and Inspiration' 48 Gainsborough, Marshall Cavendish Weekly Collection, p.1507).
Gainsborough is said to have used long brushes to achieve this effect and diluted his paints with turpentine to such an extent that they almost spilled from his palette. He is also known to have been fond of feathery brushwork. All of these elements appear to be present in Tristram and Fox, particularly in the figure of Fox. The lively texture of his fur is made up of slightly rough, feathery brushstrokes in dilute paint over the lower layers of darker colours. This rough brushwork is off-set by the precision with which Gainsborough has executed the smooth, shiny surface of his eyes, nose and mouth. The unfinished figure of Tristram is laid in with rough brushwork, of various brown and black tones, but does not have the finish of Fox's fur, created by the feathery strokes.
The background varies from area to area on the painting. The left hand side consists of a warm brown tone painted roughly over the ochre ground, giving it a reddish glow and allowing the ground to play a role in the final effect. On the right hand side this is transformed into a cooler, greyer tone, presumably by scumbling an opaque grey layer over the dark brown. The foreground also consists of opaque scumbles over a dark brown layer, but these vary slightly in tone, creating the step upon which both dogs rest and the darker shadow beneath.
The inclusion of glass amongst the pigments used by Gainsborough was possibly a device to lend luminosity to a semi-transparent paint film. The pigments used by Gainsborough in this painting, as seen in the samples, tend to be black, white, semi- transparent ochres and vermilion with the odd particle of crimson lake. The glass would provide a degree of translucency to lift the final paint effect and liven up the layers. There is also a possibility that the glass was included as a drier for oil. As far as is known, Gainsborough used pure oil medium, often using poppy or walnut oil for the lights. These oils are known to be slow drying and the glass may have speeded up the process. The inclusion of glass in greater quantity has been noted in many of Gainsborough's early landscapes and conversation pieces.