The first clue to John Bettes' new-found significance was uncovered by Tate curator Karen Hearn in the British Museum. In William Musgrave's celebrated account of his visits to English country houses during the 1780s, Hearn found a description of a painting by Bettes at Brome Hall in Suffolk which perfectly matched the Tate's work. Musgrave refers to a "portrait of Dr Buttes, physician to Henry VIII", whose portrait by Holbein is well-known. Holbein's portrait, however, is of a much older man and we know that Dr Buttes died in 1545, the year the Tate portrait was painted. So who then is the rheumy-eyed gent who stares from the Tate's walls? Buttes' eldest son William is the subject of a painting attributed to Bettes, now in Boston, Mass, and it seems likely that the Tate picture is of none other than Edmund Buttes, the doctor's third son, who, on inheriting his legacy, commissioned this icon to commemorate his new position.
A painted portrait, particularly in the founding days of the Protestant landed Establishment, was the ultimate piece of self-promotion. For the son of the late Court Physician to have chosen Bettes confirms the artist's reputation as a pupil of Hans Holbein. In 1545, Holbein represented art's cutting edge. Celebrated for the gigantic mural of Henry VIII and his family at Whitehall Palace, completed in 1537 (and destroyed in 1698), Holbein had also produced the notable series of portraits of Henry's court seen at the NPG last year. Courtiers were queueing up to sit for the overworked painter and a portrait by his talented pupil must have seemed a good second-best.
Of course, Bettes' work could not be anything quite so brilliant as the portrait of King Henry - a sparkling mass of lapis lazuli, emerald, scarlet and gold. To have topped that would have been the ultimate faux pas. It would, however, have to be sufficiently impressive to convey to the viewer the social status of the sitter, capturing Buttes in the style of the moment and setting him against an appropriately rich background.
It might seem strange then that the Tate portrait should be a grey, somewhat lacklustre image in which only the face appears to have anything to offer. Here Bettes has gone to town, taking pains to model the strands of the beard and portraying the eyes with a delicacy that seems extraordinary for the period. Such virtuosity, though, seems strangely jarring against a dully painted background all too typical of those images of the English gentry of the time which hang in the long galleries of Tudor houses. For an explanation we must look to Bettes' technique.
Although, on account of its inscription, the Tate's portrait has long been attributed to Bettes, for Rica Jones, Paintings Conservator at the Tate, the new exhibition represented an opportunity for further investigation, to "reduce style to technical features" as she puts it. Jones's most significant piece of research involved a cross-section of the painting, which now, magnified 250 times, shows quite clearly the paint layers of white ground, red and grey underpainting. On top of these, though, lies a thick layer of translucent pigment. This is smalt, a quaintly named medium made from cobalt glass, the blue used in stained-glass windows. It was used extensively by painters including Brueghel and Holbein himself. Although it has a vibrant blue tone when first applied, with exposure to light and the passage of time, smalt fades to no more than a dirty neutral.
According to received wisdom, for this reason smalt was despised by Tudor painters. The best blue of the period, ultramarine, was mined in Afghanistan and shipped via Venice. In 1600, Nicholas Hilliard wrote that for "best Ultramarine of Venice" he had paid "eleven pounds ten shillings the ounce". Consider that the annual salary of a farm bailiff in the same period was pounds 2 and you have an idea of its worth. No wonder Henry VIII insisted on ultramarine for the background to his portrait by Holbein.
It is significant, though, that when Holbein presented Henry with a new year's gift of a portrait of his son Edward Prince of Wales in 1539, he used not ultramarine but smalt to create an intensely blue ground, which now appears to us a dull purplish grey. That Bettes inherited Holbein's liking for smalt is evident from further analysis of the backgrounds of Bettes' other attributed portraits: of William Cavendish (at Hardwick Hall) and William Buttes (in Boston). In cross-sections of both Jones also discovered a significant presence of smalt.
Taken with its use in the Tate portrait, it becomes clear that, although smalt was cheaper than ultramarine, it wasn't thought inferior and it seems unlikely that Holbein and his followers were aware of its fugitive nature.
Here then is a core of works which when first painted would have shone with the brilliance of miniatures. The paintings' original appearance was further confirmed by Angela Geary of the Tate, who, at the suggestion of the show's sponsors, Pearson, painstakingly recreated the original colour of freshly applied smalt on a seasoned oak panel. Transposed onto this colour, the Buttes portrait would leap from the frame with an extraordinary 3-D quality which proposes for Holbein and his followers an even greater innovatory importance than is generally acknowledged; and, it being not unreasonable to presume that the same might apply to other portraits of the period, redefines the appearance of Tudor court painting. With this one technical discovery, the Tate's conservators and curators have transformed not only our knowledge of a painter's art and the self-aggrandising intention of his sitter, but our entire picture of the Tudor age.
! 'Dynasties: Painting in Tudor and Jacobean England 1530-1630': Tate, SW1 (0171 887 8000) Thurs to 7 Jan.