Before and after: to the top is the image of the study as it appeared in Sotheby’s catalogue for the Chatsworth sale in October last year, while to the bottom is an image of the restored picture as it appears in the exhibition, with the varnish removed.

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Originally catalogued as 'Circle of Peter Paul Rubens', the oil sketch Study of the Head of a Woman sold for £10,000 at the auction, but research and restoration undertaken by the buyer, Phillip Mould Fine Paintings, has led to it being reattributed to Van Dyck. It forms part of Mr Mould's Finding Van Dyck exhibition which opened earlier this month and runs at his Dover Street gallery in London until July 13.

Sotheby's, however, have taken the unusual step of releasing a statement rejecting the reattribution, saying "the overwhelming weight of scholarly opinion - consistent with Sotheby's original cataloguing - is that the painting is by an anonymous Flemish artist working in the 17th century, ultimately inspired by Peter Paul Rubens".

The 18 x 13in (46 x 33cm) oil on canvas laid on panel, now priced at £85,000, was part of the 1416-lot Chatsworth sale which Sotheby's held on the Derbyshire estate in October last year. It came in an 18th century giltwood frame, although the panel was branded with an earlier Antwerp maker's mark.

Sotheby's statement reads: "Six out of seven of the world's leading specialists in this field whom Sotheby's has consulted also categorically reject the attribution to Van Dyck (the only one supporting the Van Dyck attribution being the same specialist Philip Mould consulted)."

The gallery, however, maintain that the work is too animated and well painted to be by a copyist or studio assistant, and the research undertaken has uncovered two larger known Van Dycks which feature heads that closely follow this study.

They also point out that Sotheby's experts had not had the chance to examine the picture in the flesh since the removal of the varnish, nor had they seen the results of their forensic examination, which showed how the 'bun' in the woman's hair and the brown drapery were not part of the original picture. This, they say, confirmed their initial belief that the study was an original Van Dyck that had been 'finished' by a later hand, and the analysis shows that this was done at about the time of Van Dyck's death.

Meanwhile Sotheby's International Head of Old Masters Alex Bell told ATG that he felt the picture was "short on quality and uncharacteristic for a Van Dyck".

The Finding Van Dyck exhibition itself focuses on how paintings lose or gain their attribution or identities, and features a number of studio works themselves to illustrate the point.

It also features works which were bought from Christie's, a portrait of young girl catalogued as 'Flemish School' in a Paris sale which made €1m and a study for the head of St Joseph catalogued as 'Manner of Van Dyck' which sold in South Kensington for £121,250. Again, these works have been cleaned and researched and are now attributed to the Flemish master at the exhibition.

Christie's said that they had considered the possibility of attributions directly to Van Dyck, stating in The Guardian that the pictures "were shown to internationally renowned experts and museum curators before they were offered at auction".

However, the statement also left the question open as to the auction house's current position on their status. While Christie's accepted that "attributions can and do change over time", they also stated that "both works attracted determined bidding nonetheless and both sold for full prices".

The exhibition also features the Van Dyck self portrait, which sold for £7.4m hammer at Sotheby's in December as well as loans from public and private collections.

It will form part of London's Master Paintings Week from July 1-8, which is previewed in this week's Dealers' Dossier in ATG's weekly printed newspaper.

By Alex Capon