A new method for removing iron oxides from ancient glass: The restoration of the stained-glass windows in the Basilica of San Petronio, Bologna

The removal of rust is always a demanding challenge, whatever the material on which it formed. Many methods have been proposed and used when dealing with stone, paper and so on. For what concerns glass, in the past, the removal of rust stains has always involved the use of rather aggressive substances, such as EDTA or oxalic acid, which, if not applied in an extremely controlled manner, could damage the glass itself or deposit material that was difficult to remove. In this paper we present a new method, de- rived from a study for the removal of rust from stone and based on the use of sodium dithionite, that is demonstrated to be highly effective without any risk of damaging the glass. At first, the use of com- presses soaked with sodium dithionite buffered solution was tested in laboratory on original medieval glass samples. The good results and the absence of any risk for the integrity of the glass led to the use of this new method for the first time in an important restoration, the medieval stained-glass windows of the Santa Croce (Holy Cross) or Notari chapel in the Basilica of San Petronio in Bologna, Italy.