Abstract: The invention relates to a process and apparatus for preparation of treatment gas used in heat treatments, whereby the treatment gas is produced in a furnace disposed catalyst retort at a temperature of that of the furnace in which the retort is positioned.With such processes, when using lower furnace temperature ranges, problems occur with respect to the reaction of the feed gas in the catalyst retort. To provide an improvement, the catalyst retort in the furnace is surrounded by a shield, and the treatment gas generated in and leaving the catalyst retort is first fed into a space defined by this shield and thereafter released into furnace space. By this, an isolation of the catalyst retort from the interior of the furnace is provided, and relatively higher retort temperatures are available.