Nelson Firestop Pillow (PLW™) is a compressible firestop device that is well suited for installations where frequent changes may be required. PLW™ consists of a mineral fiber treated with a highly intumescent material enclosed in a strong polyethylene bag. When exposed to fire, the pillows expand to lock the seal in place. Even though PLW™ is classified as a permanent firestop system for several applications including steel and aluminum cable trays, PLW™ also is an excellent temporary seal.