 The last major outcry against the use of radical conservation techniques at the National Gallery was sixty @-@ six years ago in the immediate post @-@ war years , following a restoration campaign by Chief Restorer Helmut Ruhemann while the paintings were in Manod Quarry . When the cleaned pictures were exhibited to the public in 1946 there followed a furore with parallels to that of a century earlier . The principal criticism was that the extensive removal of varnish , which was used in the 19th century to protect the surface of paintings but which darkened and discoloured them with time , may have resulted in the loss of " harmonising " glazes added to the paintings by the artists themselves . The opposition to Ruhemann 's techniques was led by Ernst Gombrich , a professor at the Warburg Institute who in later correspondence with a restorer described being treated with " offensive superciliousness " by the National Gallery . A 1947 commission concluded that no damage had been done in the recent cleanings . 
