A safety curtain (or fire curtain in America) is a fire safety precaution used in large proscenium 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 widely known. The safety curtain is sometimes referred to as an iron in British theatres, regardless of the actual construction material. In the UK, it is a requirement that a safety curtain must be fully down within the proscenium opening within 30 seconds of being released.