Ceiling opening in gas chamber at Krematorium I through which Zylon B crystals were thrown. Zyclon B is the trade name of a cyanide-based insecticide. It was stored in airtight containers; when exposed to air, the substrates evolved gaseous hydrogen cyanide (HCN). Two German firms, Tesch/Stabenow and Degesch, produced Cyclone B gas after they acquired the patent from Farben. Tesch supplied two tons a month, and Degesch three quarters of a ton. The firms that produced the gas already had extensive experience in fumigation.
From the statement of Hans Stark registrar of new arrivals, Auschwitz:
".... They then started to cry out terribly for they now knew what was happening to them. I did not look through the opening because it had to be closed as soon as the Zyklon B had been poured in. After a few minutes there was silence. After some time had passed, it may have been ten to fifteen minutes, the gas chamber was opened. The dead lay higgledy-piggedly all over the place. It was a dreadful sight." See http://www.nizkor.org/faqs/auschwitz/auschwitz-faq-06.html