Abstract: The herein-disclosed putty is for forming air-tight and smoke-tight seals under normal and fire exposure conditions, and bubbles up when heated with flame, producing a foamed carbonaceous layer having outstanding fire-retardancy. It is a mixture having a cone penetration value of 2-40 which includes (a) liquid or semi-solid organic material having a viscosity of 50-100,000 c.st. at 210.degree. F and (b) a phosphoric acid compound substituted with one or more nitrogen-containing groups. The putty may also contain (c) polyhydric alcohols and/or carbohydrates for increased fire retardancy, (d) blowing agents for increased insulating effect, (e) grease, for high temperature flow resistance, (f) heat resistant fiber, for reinforcement, (g) microballoons, for increased insulating effect and other additives. The enclosed drawing discloses particularly desirable proportions for the components (c) and (d) in relation to the amount of component (b).