Abstract: In the preferred embodiment depicted, the invention comprises a limb, having a depending lug, fixed to a door. The lug makes a close clearance, near engagement with a socket-retainer fixed to a door-interfacing mullion. The upper end of the mullion is foreshortened, and addressed to a free space; it is slidably engaged with an internal socket. Accordingly, in the event of fire, the mullion may freely extend vertically, due to the heat. Such free extension causes the socket-retainer, and the depending lug, mutually to engage. By this arrangement, the door is secured closed, on the exposure to severe heat, even if the normal latching and/or locking device therefor comes loose or disengaged. The method comprehends fixing the mullion at one end, securing it against angular movement, exposing the other end to a free space, and mounting to the closure and mullion locking components which come into engagement only in response to thermal extension of the mullion.