At the Mauritshuis, the conservation studio is in the attic. In this attic, a team of experts pores over ancient masterpieces, fixing small cracks with layers of paint. Sometimes they use wax resin to replace a painting’s stretcher, sometimes they need to revarnish the work, or to even out the paint by removing thin layers. This exhibition reveals the secrets of conservation. How did the museum restore its Rembrandts, Vermeers, and Rubens? —E.C.