Why did fixing damaged art stop being a job just for artists?

Because science got involved. Until the early 20th century, artists were normally the ones called upon to repair damaged artworks. But during the 19th century, scientists began studying what actually harms art, and museums started hiring chemists to look after their collections, turning repair into a field of its own: conservation.

That started to change in the 19th century, when scientists such as Michael Faraday began studying how the environment damages works of art, and Louis Pasteur ran his own scientific analysis of paint. Museums took notice. In 1888, Friedrich Rathgen became the first chemist ever employed by a museum, Berlin's Royal Museums, where he developed a scientific approach to caring for collection objects and spread it through a Handbook of Conservation in 1898. The field that grew from this, conservation of cultural heritage, uses any methods that prove effective in keeping an object as close to its original condition as possible for as long as possible.

Britain's own turning point came from an unlikely source: wartime storage damage. The British Museum created a dedicated Research Laboratory specifically to address the deteriorating condition of objects that had been stored in London Underground tunnels during the First World War, and in 1924 the chemist Harold Plenderleith began working there. His appointment is often said to have given birth to the conservation profession in the UK, even though skilled craftsmen had already been repairing art in museums and workshops for generations. A similar shift happened in the United States, where Rutherford John Gettens became the first chemist permanently employed by an art museum, while on the practical side, Boston's George T. Oliver invented and patented a vacuum hot table for relining damaged paintings.

The field that emerged from all this treats restoration as something closer to a methodological act than an artistic one: the theorist Cesare Brandi described it as the moment when a work of art is appreciated in both its material form and its historical and aesthetic meaning, with an eye toward handing it on to the future. Modern conservators follow simple rules, such as minimal intervention and full documentation, and now emphasize reversible treatments, to avoid creating problems for future treatment and study; more recently, they've added a new ethical concern: making sure that caring for an artifact doesn't come at an undue cost to the environment.