chamber formed by the enclosure 10 is at a pressure above atmospheric gas leakage from the chamber to the outside of the enclosure through the slots through which the foil strip enters and leaves the chamber is acceptable there are relatively large temperature gradients within the medial and end sub chambers and between the medial sub chamber and the end sub chambers therefore the foil strip s which is thin and thus transfers heat readily in the thickness direction is subject to gradients of temperatures as it passes into through and out of the sub chambers in the enclosure 10 the coolest regions of the chamber are the lowest portions of the end sub chambers 18 and 20 as the inert gas flows from the openings 13 and 15 it gives up heat to the top and end walls of the enclosure heat is also given up to the relatively cool incoming foil strip s the highest temperatures in the chamber formed by the enclosure 10 are in the center region of the medial sub chamber 16 which is remote from the cooled walls of the enclosure and in proximity to the heating elements 30 the guides 34 and the tool set 36 within the medial sub chamber 16 which contact the foil strip are kept heated by the inert gas so heat exchange between those components and the foil strip is minimal the guides 34 may be of a ceramic or other material with a low heat conductivity the tortuous path for the foil strip s enables the strip to reside in the hot medial sub chamber 16 for a sufficient time to be highly heated before it is worked by the tool set 36 the inert gas prevents.