The intumescent seals possess the property of expanding significantly their volume when exposed to elevated temperatures. They must be incombustible or flame-retardant. As well as heat resistance, they must also offer a good mechanical resistance. They must therefore simplify installation on frames at normal atmospheric and operating temperatures, but, at the same time, remain securely in place during the expansionnphase of the same, due to effect of the heat and(or the fire at high temperatures. In other words, following expansion the seal must remain in position to seal off the rooms while withstanding the mechanical stresses to which they are subjected as a result of the fire. At 450°C with fire load, the product forms a stable, slightly elastic and porous foam. The expansion pressure and the onset of foaming commence at a temperature of approximately 180°C but, above this temperature, it starts hardening once more, and becomes substantially harder than the original material.