Harder chars are produced with sodium silicates and graphite. These products are suitable for use in service penetration firestop seals, as well as exterior steel fireproofing. In those applications, it is necessary to produce a more substantial char capable of exerting quantifiable expansion pressure.
In the case of a service penetration firestop seal, a burning plastic pipe, for example, that is melting must be squeezed together and shut off, so that there is no opening for fire to propagate through the fire-resistance-rated wall or floor assembly.
The expanding intumescent will push against the softening plastic and close any void that forms, creating a protective char that prevents the fire from passing through the penetrated fire-separating element.