That's a blatant lie, typical of the 'holocau$t' Industry shysters. The real reason is that there were no homicidal gassings and consequently no residue can be found to support the absurd claim.
See here as Germar Rudolf sends the 'exposure to the elements' lie packing.
'Critique of Claims Made by Robert Jan Van Pelt'
Prof. van Pelt claims that Iron Blue wouldn't survive 45 years of exposure to the environment, especially when acid rain is involved [p. 306]:"
"Then he [Leuchter] took no account of the fact that […] their [the crematoria's] remains had been exposed to the elements for 45 years, and that the walls had been washed with acid rain—a fact of some importance because, contrary to Leuchter's belief, ferro-ferri-cyanide is not stable under all conditions, but tends to slowly dissolve in an acidic environment."
Prof. van Pelt's claim is totally unfounded and has been disproved: The relevant literature consistently describes Iron Blue as an extremely stable pigment. It is insoluble in water, resistant to acid rain and also surprisingly resistant to sunlight. When exposed to weathering, other compounds of hydrocyanic acid will even convert preferentially into Iron Blue. Three examples will demonstrate the environmentally resistant nature of Iron Blue:
The outside walls of the delousing building in Birkenau, which are stained blue by Iron Blue, have lost none of their color despite 50 years of exposure to the adverse environmental conditions of the industrial region of Upper Silesia.
A long-term test begun in the 1950s to ascertain the environmental resistance of paints has clarified this matter. In this test, many pigments including Iron Blue and iron oxide (i.e. ›rust‹) were tested by applying them only superficially and without protective coating onto a piece of aluminum. After more than 20 years’ exposure to the air of a western industrial suburb of London, two pigments exhibited the least (barely noticeable) changes: Iron Blue and iron oxide (rust).
Even scattered on the ground, Iron Blue remains stable and fixed for decades, as tests in gas works shut down decades ago have shown. In this case the Iron Blue obtained in the city gas works was used as a herbicide, and is still present today in virtually undiminished quantities.
Therefore, if Iron Blue has formed on and in a wall, one may expect to find a long-term stability similar to that of the iron oxide from which it formed. Thus, once noticeable quantities of hydrocyanic acid salts have accumulated in brickwork, plaster or mortar, and once damp conditions have allowed these to convert into Iron Blue, then no appreciable reduction in content of hydrocyanic compounds is to be expected after 50 years.
A typical example of the way the media deal with these facts is the report that was issued by the German press agency dpa and carried on March 29, 1993 in almost all major German daily newspapers and even in some radio news broadcasts. In it dpa claimed that, according to unnamed expert, the hydrocyanic acid salts at issue here have a life of only a few months. Inquiries at the Stuttgart dpa office responsible for this press release revealed that the editor in charge, Albert Meinecke, had invented this 'expert opinion' out of thin air. Evidently even the dpa press agency does not shy back from issuing false reports.
 The literature frequently gives only the rather unsatisfactory term ›insoluble‹. For more details see the Rudolf Gutachten, op. cit., note 3, pp. 45ff.
 Iron blue is considered an acid-resistant pigment; cf. e.g. B. J. A. Sistino, in: T. C. Patton(ed.), Pigment Handbook, v. 1, Wiley, New York 1973, pp. 401-407; no appreciable decomposition occurs until the pH drops below 1. The pH of Iron Blue suspensions, for ex., is around 4-5; H. Ferch, H. Schäfer, Schriftenreihe Pigmente, 77, Degussa AG, Frankfurt 1990.
 Ullmanns Encyklopädie der technischen Chemie, v. 13, Urban & Schwarzenberg, Munich 1962, p. 794; ibid., v. 18, Verlag Chemie, Weinheim 1979, pp. 623ff.; L. Müller-Focken, Farbe und Lack 84 (1987), pp. 489-492.
 Illustration of this can be found in Rudolf Gutachten, op.cit., note 3. Similar are the cases of the delousing facilities in the Majdanek and Stutthof concentration camps, see Jürgen Graf, Carlo Mattogno, KL Majdanek. Eine historische und technische Studie, Castle Hill Publishers, Hastings 1998 (www.vho.org/D/Majdanek/MR.html); Jürgen Graf, Carlo Mattogno, Das Konzentrationslager Stutthof und seine Funktion in der nationalsozialistischen Judenpolitik, (with Jürgen Graf), Castle Hill Publishers, Hastings 1999 (www.vho.org/D/Stutthof/index.html).
 J. M. Kape, E. C. Mills, Transactions of the Institute of Metal Finishing 35 (1958), pp. 353-384; ibid., 59 (1981), pp. 35-39.
 D. Maier, K. Czurda, G. Gudehus, Das Gas- und Wasserfach, Gas × Erdgas 130 (1989), pp. 474-484.
 For a detailed discussion of this and more see the acc. sections in my report (Chapter 2.4.ff & chapter 2.5.6.).
 cf. W. Schlesiger, Der Fall Rudolf, Cromwell, 20 Madeira Place, Brighton/Sussex BN2 1TN, England, 1994, pp. 21-24 (www.vho.org/D/dfr/Fall.html); G. Rudolf, DGG 42(2) (1994), pp. 25f. (www.vho.org/D/DGG/Rudolf42_2.html; English: www.vho.org/GB/Books/cq/media.html).