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The crematoria at Auschwitz and Birkenau were not typical cremation ovens, but heavy-duty industrial ovens designed to run continuously, using the heat energy produced by the burning of previous bodies to keep the oven hot for the next bodies. After they were fired with coke to their proper operating temperature, they required little or no extra fuel to operate. A considerable but well-documented technical achievement. The cremation unit that one muffle was supposed to handle in a given time was a weight unit, which means that one or several persons adding up to that weight unit could be put into each muffle simultaneously without increasing the cremation time. Unlike in crematoria ovens used for civilian purposes, there was no need to wait for one body to have cremated completely. The practice actually was to put the next body or bodies in the muffle before the cremation process of the previous was complete.