The 12'' Satin Stainless 40-80mm All Stainless Steel Letterplate is lined with special intumescent material which expands to 50 times its size, blocking the hole in the cavity and preventing smoke coming through the cavity. The All Stainless Steel letterplates are made from 316 grade stainless steel, and are specifically designed for areas which are within a 25 mile radius of the coast, or within 5 miles of highly populated industrial areas.