Effects upon the Formation of Iron Blue

The first step in the formation of Iron Blue in masonry is the absorption of gaseous hydrogen cyanide. A cool (10°C ) wall in a cellar with atmospheric humidity near the saturation point, due to its higher water content (by a factor of at least 10), has an increased ability (by a factor of 10) to absorb hydrogen cyanide compared to warm walls in a heated room built above ground with lower atmospheric humidity (20°C, 50% rel ). 

The second step in the formation of Iron Blue is the ionic split (disassociation) of the hydrogen cyanide, that is, its conversion into simple cyanide.405 This procedure requires an alkaline environment, 

404 The reason no formation of very finely crystalline alumosilicate with higher surface area. 

405 In masonry, this largely corresponds to the neutralization of the hydrogen cyanide by calcium hydroxide Ca(OH)2 into calcium cyanide Ca(CN)2. 

Larger quantities of cyanide ions can accumulate in the non-carbonated portion, due to the alkaline medium, further favored by the increased absorption of hydrogen cyanide by the still-humid material. The cyanide is only bound as iron(III)-cyanide to a slight extent. This is converted quite rapidly into the more stable iron(II)-cyanide due to its strong oxidation behavior in the alkaline medium. An accumulation of iron(III)-cyanide will therefore take place over a longer time period. 

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