Search 'painting restoration potato' you'll find a group of people who have been fooled by the idea of cleaning a painting with a freshly cut potato and some have used onions. While the initial affect is the desired one, the painting will darken substantially over time. The way it works is the carbohydrates in the potato will react with the linseed oil in the paint (linseed is the binder in most oil paints) practically removing the oils from the paint. The reason varnishes and paints change colour and darken is because they oxidize, sort of like rust forming on steel. Removing the varnish and even the oils in paint is the same as removing the clear coat on a car, you are inviting damage to the paint. Varnish and oil protect the pigments (essentially crushed minerals, dirt, or even bone) from oxidizing and destroying themselves. The potato trick might make the painting look nice for now but soon enough you wont have a painting left to save.
Now, I know more about oil paintings and the chemistry of paint, but this caught my attention this morning as I was perusing the daily assortment of art news. An article by Christina Ruiz discussed the unstable printing process modern photographers have been using, known as C-printing, high intensity colour prints. Materials used in c-print colour photography are complex webs of organic compounds, which unfortunately are highly unstable. These volatile ingredients continue to change or in most cases today, fade. Atmospheric conditions like light, heat, and water accelerate the process as well. This makes conserving these photographs a challenge, which the MoMA has gladly taken on since their collection holds a large number of these c-prints from various modern photogs. What's more many of these same prints have been mounted face down to acrylic glass, or Plexiglas, in a process called Diasec. This gives the print protection from UV rays because of the properties of the Plexiglas. However if the process were not done as per instructions, the print itself would have to be reprinted/replaced by the artist every ten to twenty short years.
What?! If the most expensive photo ever sold (Andreas Gursky's '99 Cent II Diptychon') will soon fade into oblivion, only to be replaced by a newly printed one several years later, the piece is really only worth the price of the paper it was printed on, not the reported $3.34 million it sold for at Sotheby's February 7, 2007 auction. As far as the headache of conservation is concerned, the silicone gel used to face-mount these c-prints to acrylic glass, while stable and highly protective, is irreversible! (Huge no-no!) Under these conditions a print will last longer, but once fading starts there's no stopping it. Only if the artist is still alive or the negatives/digital images are available and can be re-printed can a collector recoup the loss. Quel nightmare!
Pigments fade and change. It's what they do. Oil paints are not immune to the aging process, however they have the decency to age gracefully. I'm glad somebody's dealing with c-prints and it's not me.