Andrew O'Connor, the senior conservator at the National Gallery of Ireland, has a hunch that a picture classified as a 19th-century fake is in fact a genuine Rembrandt. Head of an Old Man has been in the gallery's collection since 1871 when it was purchased by the then director, Henry Doyle. Doyle was anything but gullible. "He was a remarkable connoisseur," O'Connor says, "who rarely made a mistake."
However, in the late 1960s, Dutch scholars set about the daunting task of sorting the real Rembrandts from the fakes and lookalikes. In 1971 the standard work on the painter dismissed the National Gallery picture out of hand, claiming it wasn't even Dutch and hadn't been painted in the 17th century. This judgment was accepted by Homan Potterton when he wrote the catalogue of the gallery's Dutch collection in 1986. He described it as being by an "imitator of Rembrandt, 19th century".
O'Connor remained unconvinced. "I always thought the work was a typical Dutch 17th-century picture." Furthermore, like Doyle, he thought it was a genuinely good picture. So he took the decision to clean and examine it more closely, a process which only strengthened his opinion. When the chairman of the Rembrandt Research Project, Prof Ernst van der Wetering, was attending a seminar in Dublin, O'Connor took the opportunity to show it to him. Van der Wetering quickly agreed it was a 17th-century Dutch painting.
At this stage another expert, Peter Klein of Hamburg University, was drafted in to provide some hard scientific evidence. Dendrochronological analysis - putting an age on tree rings - of the wooden panel produced a felling date of about 1642. The net effect of this is that the painting is inching its way back from the 19th century towards Rembrandt's studio.
While the research project, the final authority, has yet to give a definitive opinion, that is exactly where O'Connor's instinct places it. In fact, he reckons Rembrandt painted it himself. Van der Wetering, meanwhile, is keen to have a closer look at another painting in the gallery, Playing La Main Chaude, currently ascribed to the School of Rembrandt. So it too may be in line for promotion.
Head of an Old Man is one of the pictures highlighted in an exhibition opening at the National Gallery of Ireland on Wednesday next. The Deeper Picture is devoted to the work of the gallery's conservation department. It coincides with the arrival in Dublin of a veritable army of conservators for their 17th biennial congress. We need this army to do battle with the ravages of time and environment that would, given half a chance, reduce the world's art treasures to dust.
They are still popularly thought of as picture restorers but they describe themselves, more accurately, as conservators. The emphasis is important. Whereas restoration has a distinctly pro-active ring to it, conservation sounds like a more judicious enterprise - but also a much more wide-ranging one.
In fact, as O'Connor points out, by comparison with museums in other countries, "conservation came rather recently to the National Gallery of Ireland". The department has only been in existence since the late 1960s. After some preparatory moves it was established at the instigation of the then director James White, under the guidance of Italian experts in the field. Since then it has handled a huge volume of work.
Occasionally the department finds itself in the news. As the Beit paintings trickled back after the infamous 1986 robbery at Russborough, they passed through O'Connor's hands. One of the best, Goya's stunning Portrait of Dona Antonia Zarate, is included in the show. Hacked off her stretcher, roughly rolled up and left mouldering in damp conditions, as photographic exhibits demonstrate, the hapless Dona was in a terrible state when she was recovered.
The removal of overpainting can produce surprises. A missing fragment of Breughel II and Rubens's Christ in the House of Martha and Mary had been extensively overpainted to make sense as an independent composition. Minus these layers it re-integrated perfectly with the main part of the picture. Elsewhere, Uccello's Madonna lost her veil, revealing some exquisitely detailed paint-work. Manetti's Cupid shed his modesty with a strategically placed, overpainted piece of drapery.
Oil painting conservation tends to steal the limelight, but oil paintings are vastly outnumbered by the huge volume of works of art on paper. And from the conservator's point of view, paper is trouble. Paper conservator Niamh McGuinne itemises a daunting list of potential threats: inherent flaws in the manufacturing process, oxidation, too much light, dampness, dryness, environmental pollutants, fungal attack, insects, rodents, not to mention, she adds rather severely, "careless or even well intentioned humans". She also takes a dim view of sellotape, blue-tack and paper clips.
Given all this, it's hardly surprising the business of conservation can be incredibly painstaking. "And sometimes," as McGuinne observes, "having gone through it, there's very little visual difference." This has to do with professional ethics as much as anything. "We don't try to disguise things. We try to remove the causes of damage and place the work in a suitable, stable environment. Really we want people to be able to look at a work without thinking: My God, it's in bits. So that they see the work and not the damage."
This happens with Margaret Stokes's print, which was torn, buckled and discoloured but now looks remarkably well. Before and after images of a drawing copied by Cheron from a Raphael fresco first show it disfigured by irregular dark blotches. In the restored version, the blotches have disappeared. With the air of a conjuror explaining a trick McGuinne explains: "The artist used lead white for the highlights. But sulphur dioxide in the atmosphere reacts with the lead and turns it black. As it happens, exposure to hydrogen peroxide reverses the process and the blackness simply disappears."
Unlike your average magician, she has no compunction about revealing the tricks of the trade. "Most damage we see arises from bad framing and storage practice," she remarks, and her text in the publication accompanying the show is a comprehensive, thoroughly useful guide to keeping paper healthy. It will more than earn its keep in the home of any art lover.
The Deeper Picture: Conservation at the National Gallery of Ireland is open from Wednesday September 2nd to December 14th. The accompanying catalogue costs £9.95.