A safety curtain is a fire safety precaution used in large proscenium arch theatres. It is usually a heavy fibreglass or iron curtain located immediately behind the proscenium arch. Asbestos-based materials were originally used to manufacture the curtain, before the dangers of asbestos were discovered. The safety curtain is sometimes referred to as an iron in British theatres, regardless of the actual construction material. Showing the curtain in the lowered position.