While the four crematoria of Auschwitz-Birkenau were still under construction, the involved engineers at Topf & Sons realized that the technology developed for Auschwitz would not be adequate. Kurt Prüfer and Fritz Sander reacted with differing suggestions for new high-performance ovens. They attempted to apply existing methods of industrial firing to achieve the even more efficient incineration of human corpses.
Fritz Sander designed his four-storey oven in the manner of an enormous refuse incineration plant. The corpses would be introduced sideways on a kind of conveyor belt (cf. sketch, top right). They would then slide down along the zigzag array of slanted grates, catching fire from the burning corpses already below them. After a preheating period of two days, the oven would thus be capable of remaining in operation "continually," without the introduction of further combustion material. The burning corpses alone would keep the fire going.
"The high demand for incineration ovens for concentration camps – which has recently become particularly apparent in the case of Auschwitz ... – has prompted me to examine the issue of whether the present oven system with muffles is suitable for locations such as the abovementioned ... In my opinion, with regard to the design of an incineration oven for the purposes of a concentration camp, the ideal solution would be an oven that could be continuously loaded and likewise operated ..., i.e. the corpses for burning would be loaded on at respective intervals – without interrupting the incineration process – and on their way through the oven would catch fire, burn, burn out and reduce to ash, and then land in the ash-chamber beneath the grate in the form of burnt-out ash. Here I am quite aware that such an oven must be regarded purely as a facility for extermination, so that concepts of reverence, the separation of ashes and emotions of any kind must be dispensed with entirely. All of this is probably the case already, however, with the operation of numerous muffle furnaces. Special war-related circumstances prevail in the concentration camps, making such methods indispensable. ... Bearing in mind the remarks I made above, it must be assumed that the authorities in question are also approaching other oven construction companies with regard to the supply of efficient, fast-functioning cremation ovens. In these companies, the question of the cheapest method of constructing such ovens for the abovementioned purposes will be examined as well. ... For this reason I consider it urgently necessary to have my suggestion patented, so that we can secure priority." (Excerpt of a letter from Fritz Sander to the Topf brothers,
14 September, 1942)