THE RUDOLF REPORT

6. FORMATION AND STABILITY OF IRON BLUEment itself now becomes stable, so that increased quantities ofiron(II)-cyanide will be converted into Iron Blue, intimately mixedwith the lime which is now also forming in this area, with the nowsomewhat more easily soluble iron(III)-ion at the carbonationlimit. 4073. In the pH-neutral, carbonated part of the masonry, the formation isconsiderably dependent on the available cyanide concentration,which is strongly reduced there. Already formed iron(II)-cyanideis gradually converted into Iron Blue in the presence of humidity.Table 7 shows the adsorption values of hydrogen cyanide in variousbuilding materials. 409 They confirm the assumption of considerablyhigher reactivity of cements compared to brick, as well as the greatertendency of fresh cement compared to older and generally more humidbuilding materials toward accumulation of hydrogen cyanide. The hydrogencyanide accumulation in concrete masonry, the age of which isunfortunately not indicated, is astonishingly high. Since by definitionthere is no considerabledifferencebetween the compositionof cementmortar and concrete,it is furthermorenot clearhow the differinganalytical resultsare to be interpreted.These dataare therefore notwithout their difficulties.410 But atleast the tendencyof humid masonryto absorb higherTable 7: Absorption of hydrogen cyanide byvarious building materials under the effect of 2%HCN by volume over 24 hours. 409MATERIAL HCN [mg m –2 ]Terracotta........................................................................ 55.2Brick ............................................................................... 73.0Lime sandstone, naturally humid..............................22,740.0Lime sandstone, briefly dried .....................................4,360.0Lime sandstone, dried approx. ½ year at 20°C ..........2,941.0Concrete block, dried for 3 days.................................8,148.0Lime mortar blocks, a few days old*..........................4,800.0Cement mortar blocks, a few days old*.........................540.0Cement mortar blocks, a month old*.............................140.0Cement blocks, pure, a few days old* ........................1,550.0* 2.5 to 3.3% HCN by volume. 408 The vol. % data, according to the authors,represent theoretical nominal values, which, in practice, however, are onlyreached up to 50% or less, through adsorption onto walls and fumigationmaterials.407 From the CO 2 in the air and the Ca(OH) 2 in the mortar.408 F. Puntigam, et al., op. cit. (note 122), pp. 35ff.409 L. Schwarz, W. Deckert, Z. Hygiene und Infektionskrankheiten, 107 (1927), pp. 798-813;ibid., 109 (1929), pp. 201-212.410 The method cannot establish any possible chemical binding of hydrogen cyanide, since onlythat fraction of hydrogen cyanide was measured which evaporated from the samples.187