During World War II (1939–45) Nazi Germany used specially built furnaces in at least six extermination camps throughout occupied Poland including at Auschwitz-Birkenau, Chełmno, Belzec, Majdanek, Sobibor and Treblinka, where the bodies of those murdered by gassing were disposed of using incineration. The efficiency of industrialised killing of Operation Reinhard during the most deadly phase of the Holocaust produced too many corpses, therefore the crematoria manufactured to SS specifications were put into use in all of them to handle the disposals around the clock, day and night. The Vrba–Wetzler report offers the following description.
At present there are four crematoria in operation at BIRKENAU, two large ones, I and II, and two smaller ones, III and IV. Those of type I and II consist of 3 parts, i.e.,: (A) the furnace room; (B) the large halls; and (C) the gas chamber. A huge chimney rises from the furnace room around which are grouped nine furnaces, each having four openings. Each opening can take three normal corpses at once and after an hour and a half the bodies are completely burned. This corresponds to a daily capacity of about 2,000 bodies... Crematoria III and IV work on nearly the same principle, but their capacity is only half as large. Thus the total capacity of the four cremating and gassing plants at BIRKENAU amounts to about 6,000 daily.
The Holocaust furnaces were supplied by a number of manufacturers, with the best known and most common being Topf and Sons as well as Kori Company of Berlin, whose ovens were elongated to accommodate two bodies, slid inside from the back side. The ashes were taken out from the front side. The furnaces were also unique, in that they were of a "stand alone" type. Meaning that there was no visible duct work for the exhaust gases. These furnaces, based around a design commonly used for hospital incinerators, instead vented the gasses down through a series of ducts embedded in the floor, with the help of a draft fan located at the far end of the structure. Once outside, the gasses then rose through a free standing chimney, most notable for the fact that it was not directly attached to the structure of the building itself, nor had a visible duct leading into it.