The first rumors that the Germans were making soap from human corpses appeared during the First World War. The Times reported in April 1917 that the Germans were making soap and other products from the corpses of their soldiers. In 1925, the British Prime Minister called this message a lie.
Rumors of making soap from human corpses in Germany reappeared with the outbreak of World War II. So, the abbreviation RJF on boxes of soap produced in Germany was deciphered as Reines Jüdisches Fett ("pure Jewish fat"). It actually meant Reichsstelle für industrielle Fettversorgung (State Office for the Supply of Industrial Fats).
Later, Professor Rudolf Spanner produced up to 100 kg of soap from the corpses of prisoners. The testimony that appeared at the Nuremberg trials was given by witnesses working at the institute. On May 28, 1945, Sigmund Mazur testified during interrogation that he worked as a preparator at the Anatomical Institute. The corpses were delivered from the prisons of Königsberg and Danzig. According to the recipe, 5 kg of human fat with 10 liters of water and 500-1000 g of caustic soda was cooked for 2-3 hours. After cooling down, the soap floated to the surface. Salt, soda, fresh water were added to the mixture and boiled again. The soap had an unpleasant odor, which was destroyed by benzaldehyde. Mazur said that he used this soap for his needs (for toilet and washing), taking 4 kg for himself.
According to Mazur, the institute was attended by the Minister of Education Bernhard Rust, the Minister of Health Leonardo Conti, the Danzig Gauleiter Albert Forster. Testimonies were also given by two British prisoners of war - John Henry Vuitton and William Anderson Neely, who were engaged in construction at the institute. They confirmed the fact that soap was made from corpses.
Nazi hunter Tuvia Friedman in his autobiographical book claimed that while working as an investigator in Danzig, just liberated from the Germans, he came across a room where similar experiments were being carried out.
We felt like we were in hell. One room was filled with naked corpses. Another room was lined with planks on which were stretched skins taken from many people. At the other end of the building, we found detailed notes and reports, medical and surgical instruments.
We left these rooms, unable to believe what we saw with our own eyes, and decided to put up security. As we sent sentries around this documentary evidence of Nazi barbarism, we discovered a much smaller structure, heavily padlocked. We burst in there too, and almost immediately found an oven in which the Germans were experimenting in making soap using human fat as raw materials. Several bars of this "soap" lay nearby.
A number of scholars studying the Holocaust consider the “legend of the soap made from the Jews” to be the folklore of the war years. In particular, historians Walter Lucker, Gitta Sereni, Deborah Lipstadt, professor at the Hebrew University Yehuda Bauer and director of the Yad Vashem archive Shmul Krakowski adhere to this point of view.
In 2006, the Warsaw Agricultural Institute conducted an official analysis of the soap used by the Nazis to clean laboratories during the war. It contains human fats. The analysis used the very soap that figured in the Nuremberg trials, but technology made it possible to prove this crime only in the 21st century.