March 1940 to September 1941
One of Höss' deputies at Auschwitz developed an efficient method that featured crystallized prussic acid, mass produced under the trade name Zyklon B, and widely used as a pesticide. At Auschwitz it was being used to fumigate barracks and disinfect prisoners' clothes. When the crystals dissolved in air, they created a lethal gas. Block 11, the most feared location in Auschwitz, was chosen for the first Zyklon B experiments.
On a day in late August or early September 1941, the doors and windows in the cellar of Block 11 were sealed.
Block 11, Auschwitz
August Kowalczyk, a Polish political prisoner at Auschwitz, watched what happened the day Zyklon B was first used on Block 11.
Polish political prisoner, Auschwitz
Our attention was drawn by SS men running around with gas masks. The windows of the bunker had been covered up with sand, and in the cellar Soviet prisoners of war were assembled. And it turned out the following day that the SS—actually, it was [Gerhard] Palitzsch in particular who attracted attention because he was running around like crazy. It turned out that the gas hadn't worked properly and that many of the prisoners, the people, were still alive. So they increased the dosage—added more crystals—and finished the job.
Rudolf Höss later wrote that the experiment with Zyklon B had a calming effect on him: "I was always horrified of executions by firing squads. Now I was relieved to think that we would be spared all these bloodbaths."
But the bloodbaths would continue and grow even larger when a new camp was built a mile and a half from Auschwitz, at a place the Poles called Brzezinka, and the Germans Birkenau. It also became known as Auschwitz II.