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Reinforced concrete gas chamber (1)

Reinforced concrete gas chamber in the Nazi-operated Majdanek concentration camp, designed for killing prisoners by carbon monoxide, which was introduced through a tube, or Zyklon B (hydrogen cyanide), as evidenced by the residual blue coloring on the back wall and portions of the ceiling and side walls. The blue coloring is Prussian blue--iron(III) hexacyanoferrate(II), which has the chemical formula Fe4[Fe(CN)6]3 --a dark blue pigment produced by oxidation of ferrous ferrocyanide salts. (Source: Photograph by Jolanta Dyr, 2013. Creative Commons Attribution 3.0 Unported [CC BY 3.0] license, creativecommons.org/licenses/by/3.0.)