Three wooden panels, painted in Italy at the dawn of the Renaissance, lie at the forefront of new efforts to use technology to restore old masterpieces.
Griselda (left) had clothes painted on her by a restorer
The panels - painted in around 1500, and attributed to an artist from Siena whose name is unknown - have been subject to 500 years of abuse and neglect, and have not been restored since the Gallery bought them in 1874.
Jill Dunkerton, who has been restoring the panels for the last 18 months, told BBC World Service's Discovery programme that computers and lab work are helping show how the panels - badly damaged by previous restoration attempts - once looked.
"A lot of scientific work is being done to show the way in which pigments are likely to change colour," she explained.
"You can even, by using computers, sometimes get an idea as to what the original colouring of the picture might have been."
Ms Dunkerton pointed out that restoration is as old as painting itself.
But she explained that often, restorers did far greater damage to the work than simple ageing. One famous example of this is Leonardo da Vinci's The Last Supper.
"The ideal painting, in many senses, is one that's been left alone," she said.
The Last Supper was extensively damaged by past restoration efforts
In the case of the Patient Griselda panels - which tell the last of the stories in Giovanni Boccaccio's Decameron, about a young woman humiliated by her rich husband - two of the panels had been greatly damaged by restorers, although the first was not.
"What's happened to these paintings is successions of cleanings over the years - but it's changed colour," Ms Dunkerton added.
"All paintings do that. The idea that a freshly-restored painting now looks exactly as it did when it left the artist's studio is a completely false one."
Work in the science lab is therefore essential in attempting to recreate more accurately the original colours.
Many paints - especially those based on plant or animal extracts - can fade dramatically over the years.
Pinpoint-sized samples of paint are taken from cracks or damaged areas and then analysed side-on under powerful microscopes, to gain a greater understanding of their composition.
"If you want to work out how these things age and change, you've got to get the real stuff," explained Catherine Higget, who tests the samples in the Gallery's Science Department.
"The problem is if you go out and buy what is now labelled as vermillion, it'll be the right colour, but it won't be the true mercury sulphide pigment anymore.
"So we do get involved in trying to find original recipes, and then trying to follow them in the lab as much as we can.
"Since a lot of them seem to involve manure and horrible things like that, we spend a lot of time trying to find a slightly more attractive way of doing it.
"But to work out how these things are really going to age, you really have to have the true material."
Some of the old ways of obtaining colour were relatively simple, such as mixing water with egg yolk.
But others are more complex, and much harder to imitate - even using modern methods.
Prussian Blue, for example, was obtained by burning bull's blood. But efforts to recreate this in the lab have resulted in a pigment with a different molecular structure.
Meanwhile, restoring the Patient Griselda panels has involved other uses of technology too.
X-rays and infra-red have been used to pick up where the work has been retouched.
This was particularly important on one panel, which detailed one of the humiliations Griselda faced - being stripped naked in front of the male courtiers.
"This was too much for the National Gallery in 1874, and so they got the restorer - a Mr Bentley - to paint some clothes on her," Ms Dunkerton explained.
"Luckily they'd recorded it - we could see in the X-ray that underneath she was naked as she should be, according to the Decameron."
Fortunately, more traditional methods of restoring have also worked on the panels.
Mild solvents were used to undo Mr Bentley's work, and Griselda is now as naked as the original artist intended her to be.