A painting that is four hundred years old rarely looks the way the artist left it. Over time, dust settles on the surface, smoke from candles darkens it, and the varnish on top turns yellow. A blue sky can slowly become a brown sky. The colours are still there, but they are hidden under a thin layer of dirt and old varnish.

A restorer is the person who cleans the painting. She works with a cotton swab and a very weak liquid that removes the varnish but not the paint below. The work is slow, because she cleans only a few centimetres each day. She often stops to check the surface under a strong lamp.

Before cleaning, museums take special photographs. An X-ray shows the wood or canvas under the paint, and it can show an older picture that the artist painted over. Infrared light shows the drawing that the artist made first. These pictures tell the restorer where the original paint ends and where later repairs begin.

Cleaning can go wrong. In some famous cases, a restorer removed too much and took away thin shadows that the artist had painted on purpose. However, the biggest rule today is simple: every new material must be reversible. If a future expert wants to remove the new paint, it must come off without hurting the old surface.

So a good restorer does not try to make an old painting look new. She only removes what does not belong and fills small holes with soft colour. The painting keeps its age, and visitors can finally see the sky the artist painted.