On his own initiative, he set up a process to produce soap from human fat in 1943-1944 and a limited quantity of the soap was produced on his order to clean autopsy rooms. In his book "Russia at War 1941 to 1945", Alexander Werth reported that while visiting Danzig in 1945 shortly after its liberation by the Red Army, he saw an experimental factory outside the city for making soap from human corpses. According to Werth it had been run by "a German professor called Spanner" and "was a nightmarish sight, with its vats full of human heads and torsoes pickled in some liquid, and its pails full of a flakey substance - human soap".
During the Nuremberg Trials, Sigmund Mazur, a laboratory assistant at the Danzig Anatomical Institute, testified that soap had been made from corpse fat, and claimed that 70 to 80 kg (154-176 lbs) of fat collected from 40 bodies could produce more than 25 kg (55 lbs) of soap, and that the finished soap was retained by Professor Rudolf Spanner.
Eyewitnesses included British POWs who were part of the forced labor that constructed the camp, and Doctor Stanislaw Byczkowski, head of the Department of Toxicology at the Danzig School of Medicine. Suggested sources for the fat include Stutthof concentration camp, Danzig Municipal Jail, and a Danzig psychiatric hospital.
Allegations of large scale human soap production is a myth with origins dating back to World War I. Holocaust survivor Thomas Blatt, who investigated the subject, found little concrete documentation and no evidence of mass production of soap from human fat, but concluded that there was indeed evidence of experimental soap making.