Tenderness and tension captured on canvas

'Face painting," as British portraitist Thomas Gainsborough named his trade, is a public art. Certainly patrons in the 18th century wanted something more than merely to see themselves painted. They wanted status. They wanted to be remembered. The portraitist's skill was to translate a private face into a public image. Gainsborough was an outstanding master of this art.

It is true he made a living painting portraits, and did it with feeling and real interest in his sitters. But like a musician who can't stop practicing all hours, he also made "private" pictures, including portraits of his own family - brother, wife, two daughters.

The privacy of these paintings allowed his own sensibility freer scope. This affectionate painting of his daughters, Mary and Margaret, aged between 11 and 9, is one of a series he painted of them growing up. Each portrait was experimentally different from the others.

This one is perhaps more conventionally posed than an earlier one in which, holding hands, they are chasing a butterfly. Nevertheless, here the two girls, not without a subtle suggestion of tension between them as older and younger siblings, sit closely and informally in relation to each other. Apart from sisterly love, there is another, not immediately apparent, reason for this: their cat.

Gainsborough didn't finish this picture. The cat (cradled, and possibly annoyed by the sisters as they try to prevent it from leaping down) is indicated by quick, incomplete outlines. Gainsborough often included dogs in his portraits, suggesting a civilized bond between sitters and their amenable canine pets. But cats are more difficult as "sitters." And it's just possible that small daughters also sometimes object to hours of sitting still.

• This painting is part of an exhibition called 'Gainsborough' at the National Gallery of Art, Washington, D.C., through May 11. It will be at the Museum of Fine Arts, Boston, from June 9 to Sept. 14.