Spit, polish, paint, glue, X-rays - all should add up to paintings as good as new for the Hugh Lane centenary exhibition later this year

A TOUCH OF spit and polish is a concept we're all familiar with, especially at this spring-cleaning time of year - but we don't necessarily associate the phrase with the cleaning of priceless masterpieces by Monet, Manet and Degas. Such, however, has been the activity at the Dublin City Gallery The Hugh Lane over the past few months as many of its most famous artworks get spruced up ahead of a major centenary exhibition planned for this summer.

As well as reuniting all 39 core paintings of the Lane bequest for the first time since they were removed from Hugh Lane's house on Harcourt Street in 1913, the show aims to re-create the original exhibition of 300 paintings, sculpture and works on paper with which the Hugh Lane gallery opened its doors 100 years ago. And the process begins in the gallery's conservation studio - often, quite literally, with a bit of spit and polish.

"We do use saliva," says the Hugh Lane's head of conservation, Joanna Shepard. "Honestly. It has particular chemical properties, and for a pre-clean on an even painting surface it brings things up beautifully. But not if you've eaten curry. Even if you've just eaten something quite bland, it takes about an hour for your saliva to revert to neutral. If you've had a curry the night before, forget it - you'll be out of action for days."

With a lengthy list of paintings to be checked out between now and the end of June, lost days are something the gallery's conservation team simply can't afford. Along with frame conservators Cathy Day Carrigan and her daughter, Georgina, Shepard and conservation intern Sarah Maisey have been working flat out since last autumn.

"We've done about 40, at the last count," Shepard declares, counting off some of the finished pieces on her fingers. They include a fistful of Orpen portraits, a couple of paintings by Jack B Yeats and three Fantin-Latours. "That's a lot of work. And it's a complicated team effort, which has to be co-ordinated both in-house and out-of-house, because some work - all the paper, for instance - is being done off-site."

Why do paintings need attention in the first place? "You should see the air-con filters from this gallery," says Shepard. "They're changed every few weeks. But after three months they'd be completely black."

Busy city-centre environments, such as that of Parnell Square, are, it seems, partly to blame. "You can't filter everything out. Diesel particles are so tiny that they can go through practically anything," Shepard says.

And then there are human beings. Visitors are the lifeblood of any art gallery - but also, at another level, its nemesis.

"People shed horrible things," says Shepard frankly. "Hairs, dust, you name it."

Just like humans, paintings are also subject to a natural ageing process. Shepard pulls out a drawer to reveal a tiny, exquisite canvas called The Shower, by Anton Mauve.

"The varnish had gone all honey-coloured," she says. "With age, many varnishes just get more and more yellow. So it was wonderful to clean this and find that all these cool greys and greens were underneath."

Nearby, on a surface which looks unnervingly like an operating table, a painting by George Russell is undergoing keyhole surgery. "It had a big hole in it," Shepard declares cheerily. When she and her colleagues have finished working their magic, the repair will, to all intents and purposes, be invisible.

We tend to imagine that when paintings get damaged, they've been set upon by a knife-wielding lunatic. The truth is rather more mundane: paintings mostly get damaged either in storage or when they're in transit around the world. Major punctures and slashes are, thankfully, rare. Most conservation work is of a less dramatic nature, as Shepard explains.

"It's not about creating a whole bright new work, or messing with the integrity of the piece," she says. "It's about relatively subtle things which add up to a much greater whole. When you restore both the painting and the frame, the effect is cumulative - and it looks terrific."

THE RESTORATION OF frames requires patience, skill and specialist knowledge. Some are made of plaster, so they're particularly vulnerable to being knocked about if the paintings are stacked during storage - and in the past, as Cathy Day Carrigan explains, repairs to frames were often ad hoc or non-existent. She produces a number of frames at different stages of the restoration process; one, a magnificent carved 18th-century frame which was adapted for a portrait of Hugh Lane, has been cleaned many times in its 200-year existence, its surface a web of tiny fillings and daubs and additions.

Day Carrigan points to two shiny gilded frames hanging at a crazy angle on the studio wall. "That's what frames look like when they've just been re-gilded," she says. "We leave them there for a while, then we coat them with glue."

That's when impurities start to stick to the surface, and the brassy look doesn't last for long. On another table sits the kind of plaster frame you might see on a Rembrandt, all curlicues and finely detailed cross-hatching. "But," says Day Carrigan sadly, "it has been sealed with what I think is shellac."

A substance made from the wings of beetles, shellac is dried, smashed up and mixed with alcohol to make a kind of natural varnish.

"The Victorians used it a lot, and the trouble is, it's always slightly tacky - although it doesn't feel like that when you touch it. But it's very difficult to renovate because it's quite corrosive. We've cleaned this frame three times, and as you can see, we haven't made much headway with it yet. I may have to settle for tip-gilding it," she says, running her fingers gently over the exuberant garlands of flowers.

Outside in the anteroom, meanwhile, a frame from a portrait of Douglas Hyde is propped against some shelving. A 19th-century Irish moulded composition frame, it was completely black before restoration, but its curvy, leafy surfaces are now glowing softly with good frame health. A little further along, another elegant figure is leaning casually against the wooden slats: Lady Lavery as painted by her husband, Sir John.

"She had a very dark varnish, so first we did a varnish removal," says Sarah Maisey. "She'd had several restorations from more than 50 years ago, some of which were not done - shall we say - very sympathetically. They put a very distorting paint on her face, some of which we can't get off without digging deeper into the surface. And her hair is a problem. Because of the way the artist has applied the paint, it has cracked and looks like a hairnet - so we'll just be knocking that back a little."

The painting will be X-rayed to find out what's original and what isn't. "Lavery had a habit of changing his portraits after they'd been on display," says Maisey. "If you look closely, you can see how he has changed the lines of the pearls around her neck, and removed what may have been a pearl drop from the bottom of the choker around her neck." Why? "Perhaps he decided the composition was unbalanced in some way. Or maybe she protested that it made her neck look too fat." Trust me; fat she ain't. But X-rays and other high-tech approaches such as ultraviolet scanning provide conservationists with a valuable new arsenal of weapons in their fight against the ravages of time and entropy - as do developments such as the Hugh Lane's new purpose-built store, designed by Shepard to the exact requirements of the collection. Each painting is stored in a moveable rack and can be viewed easily without touching it or bashing it around.

AT THE END of the day, however, conservation isn't about expensive equipment or elaborate machinery. "It's about complex issues and delicate decisions," says Shepard.

She switches on a UV light, transforming a portrait of Hugh Lane into an eerie, skeleton-like pattern of light and dark patches. This shows conservators which parts of the painting have been restored already - and, perhaps more to the point, which haven't. His face and cravat have been done. The dark background - which takes up more than 50 per cent of the canvas - hasn't.

"It's notoriously difficult to clean dark areas," says Shepard. "So, in the past, people often cleaned the bits they could, and left the really dark parts. It could take a week just to clean the varnish off this painting, and another three or four to put it back on again - a whole month of work on just one painting. So it's a matter of trying to balance out what we can get done properly in the time - and also what's best for the painting. Part of the job is knowing when to stop, and when not to even go there."

For the moment, however, stopping isn't the problem. Conservators must keep a meticulous record of the work they do, taking photographs and writing reports before, during and after the restoration process.

"We eat, sleep and breathe conservation at the best of times, but this project is particularly exciting," says Shepard. "Artists who were the crème de la crème of their day, and who were collected by Hugh Lane, but have gone out of fashion for one reason or another, are getting some proper attention. Works which might have languished a little over the years are coming into the limelight."

• The Hugh Lane Centenary Exhibition opens on June 26 and runs until Sept 14