The National Fireproofing Company manufactured the interior terracotta fireproofing. Other sources of asbestos-containing materials include fireproofing and acoustic materials. The steel structure was protected by structural clay tile and concrete fireproofing. Various cement-based plasters are also used as proprietary spray fireproofing products. Spray fireproofing of structural steel is best accomplished before interior partitions are built. Therefore, old spray-applied fireproofing, pre-1991, may contain small quantities of asbestos. Fireproofing plasters are closely related to firestop mortars. MgCO is also used in flooring, fireproofing, fire extinguishing compositions, cosmetics, dusting powder, and toothpaste. There were mills whose architectural innovations included fireproofing by use of iron and reinforced concrete. Plasters have been in use in passive fire protection, as fireproofing products, for many decades. Astaneh-Asl also suggested that the fireproofing became dislodged during the initial aircraft impacts. Intumescents that produce hard chars are unsuitable for interior spray fireproofing. However, the plywood may be treated with various chemicals to improve the plywood's fireproofing. It is a principal fireproofing ingredient in construction materials. Metal-framed buildings were also vulnerable to fire and required special fireproofing.