Even when strengthened by gilding, the image surface was still very easily marred and air would tarnish the silver, so the finished plate was bound up with a protective cover glass and sealed with strips of paper soaked in gum arabic. In the US and UK, a gilt brass mat called a preserver in the US and a pinchbeck in Britain, was normally used to separate the image surface from the glass. In continental Europe, a thin cardboard mat or "passepartout" usually served that purpose.