Rigorous Proof and Exact Science
The so-called forensic reports of Leuchter,  Lüftl  and Rudolf  have for some time been used by Holocaust deniers as their chief argument - their ace in the hole - against the veracity of mass murder by gassing at Auschwitz-Birkenau.
The argument goes that what is physically impossible cannot be true, no matter what testimonial evidence, documentary evidence, or physical evidence is amassed to demonstrate it. No number of witnesses suffices to prove that water can run uphill, and likewise all evidence regarding the Holocaust could be wiped away, if deniers could only prove that gassings were physically impossible.
Thus it is, in the literature of denial, that we see great weight given to these three reports. In the late 1980s Leuchter was cited with great frequency, and now that distinction is gradually being passed to the work of Germar Rudolf. 
Possibly the pivotal moment in this line of argumentation came in 1988 when David Irving was so influenced by the Leuchter Report that he stepped into the deniers' camp - or at least, when the story is told, Leuchter gets credit for the conversion. The epiphany took place just before his testimony at the trial of Ernst Zundel. As described by Robert Lenski in The Holocaust on Trial, 1990, p. 400:
Ingrid Rimland of the Zundelsite describes the event in a 1998 Zundelgram:
Irving's own thoughts can be found in the introduction he wrote for his edition of the Leuchter Report, published in Great Britain:
In 1990, he elaborated on his editing decisions:
Note how heavily Irving relies on this so-called forensic report of Leuchter. Note also his mystical invocation of the "exact" science of this report that has the power to trump historical evidence.
Notwithstanding the fact that chemistry is an approximate science (as is for all intents and purposes the serious study of history), one of the present authors has taken up Irving's challenge. In the essays "The Chemistry of Auschwitz" and "Leuchter, Rudolf, and the Iron Blues", Dr. Richard J. Green has shown that the Leuchter Report and its cousins are not sufficient to cast doubt on the well-established facts of the matter.
In 1993, "to make a little extra money on the side,"  Germar Rudolf wrote his "Rudolf Report." He was working on a graduate degree in chemistry at the time, and indeed the heart of the Report is its claims about the chemistry of the gas chambers. At the time of its authorship and in subsequent revisions, Rudolf did nothing to discourage the overemphasis which chemical analysis has enjoyed in the denial movement. In the conclusion to that Report, he summarized as follows:
Specifically regarding the issue of Prussian blue and other cyanide compounds, his summary stated:
In the last year, Rudolf has attempted to address the articles on The Holocaust History Project website which refute the "forensic" reports. In January 1999, after we published a brief response to a brief criticism, he contacted us to make us aware of a more detailed criticism on his site. This article is, in part, a response to that criticism.
A New Approach
It is a criticism that contains a surprising admission. Germar Rudolf, who indeed knows chemistry better than any other professional Holocaust-denier, agrees with the central arguments of the papers he is criticizing (as we shall see below) and now claims that chemistry cannot be used to disprove the Holocaust.
To be precise, Rudolf writes:
It is precisely the chemical argument which deniers use to insinuate that historians have lied. Yet, now we have an admission by the foremost chemist of Holocaust denial - in direct contradiction to the conclusion of his famed report - that chemistry is not sufficient.
Let us be clear what Rudolf is now saying. It is true that he slides back into the old style of argumentation for just one sentence, where he writes:
Although this sentence might indicate a touch of confusion or indecision, there is a clear pattern of retreat away from the stronger, earlier position. Here is a fuller quotation from his new criticism:
(Emphasis Rudolf's.) Chemistry produces findings, but those findings cannot be proven rigorously. If it produces certainty, it will not be absolute certainty (an oxymoronic qualification, to be sure).
And his report is not a rigorous proof: his findings on chemistry are now merely a "well founded expert opinion" supported by "circumstantial evidences" which partly depend on "other evidence."
What he calls his summary of his own report no longer even includes mention of his work in chemistry, except to downplay it. After five years, Rudolf has demoted his work in his own area of expertise toward the irrelevance it deserves. At the same time, the arguments made famous by his colleague Robert Faurisson - arguments on which Rudolf has no special expertise or authority - have risen in his mind to the same level of objective certainty which he previously reserved for his own work.
One may fairly ask why this change took place, and why it took five years. We believe Rudolf has been backed into a corner by the systematic, scientific tearing-down of his assumptions and reasoning on the internet over the past few years. When his assertions in the field of chemistry went unchallenged, their claims to trump the historical process made it an important doctrine in the denial community, but now their author must take a few steps back to preserve what he can.
It remains to be seen whether Rudolf will stick to his guns in the long run. Perhaps he will revert to the old claim that his original work proves that gassing "could not have taken place," or perhaps he will find it more rewarding to begin working with other deniers and repeating their arguments. Time will tell.
A Road Map
Along the way to agreeing with the central points of the articles he criticizes, Rudolf offers some arguments that bear examining.
There are three major components to Rudolf's criticism, each of which we will address in turn. The first element is his discussion of rhetorical issues not directly related to the science of the matter. These rhetorical issues are worth discussing because they expose Rudolf's misrepresentations as well as some rather poor logic.
The second is a discussion of a topic on the periphery of these papers, the air photo evidence. Here we merely aim to show that Rudolf has not addressed the points made. Later this year, The Holocaust History Project will publish works which examine the aerial photographs in some detail.
It is the third element of Rudolf's criticism which actually addresses the central points of these papers: the issues related to chemistry. We examine the rate of evaporation of cyanide from Zyklon and consider its toxicity, and find that these physical properties are consistent with the testimony concerning the use of Zyklon B. We address the question of ventilation of the poison gas, and show how Rudolf's estimates are wrong and deliberately misleading. We then examine the five-step formation of Prussian blue and show where his model is faulty, namely we show that the probability of the formation of Prussian blue in the gas chambers was exceedingly small, and suggest that this low probability may explain the discrepancy in blue staining between the homicidal chambers and the delousing chambers. Finally, we examine the findings of the Institute for Forensic Research in Cracow and discuss Rudolf's dismissal of them.
Rudolf begins his criticism with five rhetorical points, the first four of which contain substantive misrepresentations of argument.
His first point:
The careful reader will note that the text in question makes an argument that is quite a bit subtler than Rudolf represents it. The argument in the "The Chemistry of Auschwitz" is that one ought to respond to the arguments of the deniers with accurate information. Whereas the argument agrees with Lipstadt,  in principle, that there cannot be a real debate between those who seek to understand history and those who seek to obfuscate it, it states that accurate information must be presented so that the gullible will not be taken in by those who wish to whitewash the Nazi regime. Rudolf twists this argument on its head and tries to make it into an argument for not addressing the scientific claims that he and others have put forward. The best that can be said for such an approach is that it is faulty reasoning.
Rudolf seems to indicate that he does not like being called a Holocaust denier, but the careful reader will note that in his response Rudolf explicitly denies the Holocaust by echoing Faurisson's "no holes, no 'Holocaust.'" Rudolf is one who denies the Holocaust, thus he is correctly referred to as a Holocaust denier.
His second point:
Interestingly, Rudolf essentially acknowledges that the argument is true. Leuchter admitted in court that he was not an engineer and nevertheless represented himself as such.  The argument in question can be found in "The Chemistry of Auschwitz" in the section discussing the history of the forensic reports. An exposé of the chemistry of the deniers is in a separate section of the article.
The argument in question was not presented as a scientific argument. If Rudolf's concern is scientific argument why does Rudolf argue the point rather than defending Leuchter's work? Rudolf is careful not to defend the arguments of Leuchter because he knows that most of them are ludicrous. No doubt he is even more embarrassed by Lüftl's report.
His third point:
Here again Rudolf misrepresents arguments. The point is that Rudolf quotes his own pen names as authorities to bolster his own argument. Certainly, no one should criticize Ann Landers for the use of a pen name, but Ann Landers uses that pen name in an honest fashion. Certainly, if Rudolf felt safe enough to pen arguments under his own name, he could have identified that the source he used to support his argument was Germar Rudolf. In fact, Rudolf continues the practice in this very article. Note 14 reads:
Why does he not explicitly state that Ernst Gauss is none other than himself?
If Rudolf's use of pen names is merely for self-protection, why has he falsely used the pen name of Anton Maegerle on his VHO website? He altered the name slightly, to "Anton Mägerle." Maegerle is the pen name of a German reporter who writes exposés of the extreme right. The material on Rudolf's website that uses his name was not written by him.  Such appropriation of pen names is clearly dishonest.
Just as dishonest is the use of pen names with invented credentials. Rudolf has also gone by "Dr. Werner Kretschmer," "Dr. Christian Konrad," and even "Dr. Dr. Rainer Scholz."  Perhaps he can explain why being persecuted entitles him to a fistful of phony doctorates.
His fourth point:
Rudolf here implies that such linkage is unjustified. It is worth examining whether Rudolf's motives are as apolitical as he implies.
The original Anton Maegerle has exposed Rudolf's financial connections with Hans-Joachim Dill, a backer of Remer.  Dill testified in court that he first met Rudolf with (self-described National Socialist) Ernst Zundel, and that he provided financial support for the Rudolf Report. Rudolf indeed has contacts with the inner circle of Remer supporters. For example, Karl Philipp (aka Ernst Strack aka Rüdiger Kammerer aka Paul Gross) was Remer's secretary and press officer, and also the co-author of the "Rudolf Report."  In fact, it was stated by the court that the relationship between Rudolf and Philipp is "almost a symbiotic one". 
Sarah Rembiszewski points out in a footnote: 
Margret Chatwin has confirmed: 
Additionally, Rudolf was associated with "Burschenschaften" and Landsmannschaft Schlesien ("Schlesische Jugend"), and an author in the far right journals Staatsbriefe, Sleipnir, Deutschland in Geschichte und Gegenwart. 
Rudolf's credentials as a non-dogmatic and objective seeker of truth must be called into question.
His fifth and last rhetorical point:
Here Rudolf stamps his feet at the fact that reputable scholars do not take him and his associates seriously. The right of freedom of speech is not a right to have one's point of view taken seriously. One has to earn such a right by demonstrating that one's point of view is worthy of serious discussion. As to whether Rudolf has been deceptive in his arguments, the reader may judge.
Other Rhetorical Issues
Some of Rudolf's reasoning is so tortured as to be downright bizarre. Two examples are his accusation of hate speech and his comments regarding "exact science."
Rudolf accuses one of the present authors (Dr. Green) of hate speech. He writes:
And what exactly is "real hate speech?" It seems it is hate speech to label someone's speech "hate speech." If that is the case, he engages in hate speech by his own definition. If labeling speech "hate speech" is the equivalent of censorship, then Rudolf is a censor.
And if that absurdity is not enough, Rudolf next obfuscates, most likely intentionally, the argument to which he is replying. He writes:
The argument to which Rudolf refers explicitly states that hate speech ought to be protected speech. It is in "The Chemistry of Auschwitz" and reads as follows:
Rudolf's invocation of mental asylums, prisons, and pyres is unwarranted melodrama. The present authors desire nothing more than Rudolf's freedom to disseminate his shameful lies and half-truths without government interference.
There are, in fact, people (among whom Rudolf does not number) who deny the Final Solution out of mental illness. Intense emotional issues have an attraction to some people who are a bit on the edge. They could have latched onto any issue and picked the Final Solution by chance. We have nothing but compassion for such people, but that does not mean they deserve to have their viewpoint taken seriously. One must earn the right to be taken seriously. One must demonstrate seriousness in the way that one treats evidence and the context of that evidence.
When encountering apparent inconsistencies in documentary evidence or testimonies, honest researchers look for the most consistent explanation. Deniers on the other hand are content to find an occasional apparent inconsistency and assert that such shows that the Final Solution never happened. Legitimate researchers go the extra step to see how the inconsistency fits into the big picture. On occasion they find that pieces of the big picture need to be revised and such revision is the normal progress of legitimate history. To conclude on the basis of an individual testimony's inconsistency or an inexpert interpretation of an air photo that there were no gas chambers in Auschwitz-Birkenau is not a kind of reasoning that deserves respect.
Again: we agree that Rudolf deserves the right to engage in his hate speech unencumbered by law, but he does not deserve the respect that he appears to desire.
Rudolf quotes out of context the following remark from the The Chemistry of Auschwitz:
Again, chemistry is an inexact science, approximate at best. Before showing how Rudolf contradicts himself, we restore the context that Rudolf removed:
The argument quoted by Rudolf is a disclaimer prior to addressing the chemical arguments that a lack of Prussian blue in the gas chambers somehow shows that homicidal gassings did not take place there. Bad science cannot trump good history (and for a miniscule amount of the vast array of historical evidence that shows that gassing took place, the reader may peruse some of the documents elsewhere on this website).
The argument that Rudolf seems to dispute is the argument that chemistry has not refuted the historians. It is a curious claim in light of Rudolf's later admission:
Here he is correct, and refutes his own argument.
Rudolf is now arguing that it is the air photos that should undermine the arguments of historians. It is worth noting that Rudolf can claim no expertise in the realm of photo interpretation. Like the present authors he is at best an amateur on the topic. How the mighty have fallen: the chief chemist of the Holocaust deniers has reduced his "expert opinion" to the air photo claims of other deniers.
Only a few paragraphs of "The Chemistry of Auschwitz" were devoted to the air photos as incidental to the central points of that work. In the future, The Holocaust History Project will publish analyses of John Ball's arguments and aerial photographic evidence, and Rudolf is welcome to submit his comments at that time. In the meantime, we note a few facts regarding his claims.
We note emphatically that the photos that Rudolf reproduces are not the enhancements reproduced in Shermer's book.  Dr. Bryant who did the analysis is a photo expert.
Rudolf completely ignores the issue of the John Ball Challenge.  We wonder why that is? Does Rudolf deny that John Ball is a fraud? Why does he not state a position publicly?
One of the present authors (Mr. McCarthy) has addressed John Ball's lack of expertise in a separate essay, "John Ball: Air Photo Expert?".
Evaporation and Cyanide Toxicity
Rudolf discusses the issue of evaporation in the context of guesswork trying to demonstrate that the wrong gas phase concentrations were assumed in the demonstration that Prussian blue formation was unlikely under the conditions used in the gas chambers. The point of the discussion of evaporation in "The Chemistry of Auschwitz" is that several deniers have claimed that Zyklon-B was either not deadly enough to kill the victims in a short period of time or, alternatively, that it was so deadly that the SS could not have used it safely. Rudolf does not state clearly whether he currently believes that either of these approaches is valid. Nevertheless it is worth reviewing this topic both to examine Rudolf's claims and the claims of less sophisticated deniers.
300 ppmv  of HCN is rapidly lethal to humans (see the discussion on toxicity below), whereas Degesch recommends a concentration of 8-16 g/m3 (7240-14,480 ppmv) for delousing.  300 ppmv is 0.33 g/m3. Some denier critics have assumed that this difference means that much less cyanide was used in the homicidal gas chambers as compared to the delousing chambers. For example, Mark Singer writes in The New Yorker:
The assumption that much less Zyklon B was used in the gas chambers than the delousing chambers is most likely erroneous and was not made in "Leuchter, Rudolf, and the Iron Blues". It should be noted, however, that delousing in the standard delousing chambers took far longer than homicidal gassing did. This longer period of time may be of considerable importance in terms of the ability of water to absorb HCN as well as in its effect on the ability of any alkaline materials to dissolve in ambient water (issues that we discuss later in this paper).
There is not a clear consensus on how much Zyklon B was actually used in the gas chambers, but most sources seem to put that number in the same order of magnitude as was used for delousing. For example, Mark Van Alstine estimates based upon information in Pressac that concentrations were on the order of 3-4 g/m3.  Pressac himself suggests a concentration of 12-20 g/m3. 
Note that here, for convenience, we are naming the concentration that would result from a full outgassing of the Zyklon used. These figures would be achieved in a delousing operation but almost certainly not during a homicidal gassing. As we shall see, the actual concentration reached is significantly lower.
In this article, we assume that enough Zyklon was used so that if all of the HCN were released at once, it would produce a gas phase concentration between 5 and 20 g/m3. This assumption is consistent with the treatment in "Leuchter, Rudolf, and the Iron Blues." These numbers are reasonable based on the properties of Zyklon B, as, again, the concentrations recommended by Degesch for delousing range from 8-16 g/m3. 
Deniers less sophisticated than Rudolf have argued that hydrogen cyanide cannot kill at temperatures below its boiling point.  They have not understood that liquids have vapor pressure. There is an equilibrium between gas and liquid at a given temperature. This fact can be illustrated by considering humidity. It is not necessary for the temperature to be 100°C in order for there to be a significant amount of water vapor in the air. This amount of vapor can be conveniently expressed as a partial pressure, and at equilibrium this pressure is called the vapor pressure of the substance.
Hydrogen cyanide is a liquid at room temperature with an exceedingly high vapor pressure. In fact, its vapor pressure is more than high enough to kill. Some deniers have suggested at temperatures cold enough to freeze HCN that there would be no vapor or very little; they are wrong. The vapor pressure of HCN was measured for the first time in 1926 by Perry and Porter.  Their results are in close agreement with the accepted values as reported by DuPont.  They measured the vapor pressure of the liquid and the solid as a function of temperature. A plot of their results converted to ppmv is displayed below.
Note that even at the lowest temperatures HCN has a vapor pressure far in excess of the rapidly lethal 300-ppmv level. On this scale, 300 ppmv is far too small to show: its height above the zero line would be one-fifth the width of a human hair.
This plot shows thermodynamically that HCN can kill, but it does not address the issue of kinetics. How fast can Zyklon B kill? Is it deadly enough to kill the victims in the time periods cited by witnesses (see Rudolf's note 29)? These questions boil down to two questions: 1) what is the lethal concentration of HCN, and 2) how quickly can such a concentration be reached?
Since 300 ppmv is rapidly fatal, the first question is already answered, but let us examine this question in more depth. Rudolf implies that 300 ppmv is a limit imposed for safety reasons. Rudolf is correct that limits imposed for safety reasons are much lower than lethal concentrations. We examine here what the safe exposure limit is. DuPont's literature on the topic is enlightening:
OSHA is the Occupational Safety and Health Administration, which sets workplace safety guidelines for the U.S. Department of Labor.
DuPont lists furthermore the following safety thresholds:
One does not provide treatment to someone that one intends to kill, so for our purposes 300 ppmv is "rapidly fatal." Rudolf's implication is incorrect. Additionally, we estimate conservatively below that the victims were exposed to 450-1810 ppmv within 5 to 15 minutes. In all probability the exposure was greater than that (see below).
Rudolf is correct that these values are estimates. Du Pont makes this point explicitly:
(Emphasis Du Pont's.) How quickly is quickly?
We note that conditions (e.g., temperature, quantity of Zyklon, etc.) undoubtedly varied from gassing to gassing so it is not surprising if there is some variation in the amount of time that witnesses report for a gassing to occur. Auschwitz commandant Rudolf Höss described such variation in his memoirs. 
Rudolf claims that Dr. Green is unaware of the fact that toxicological data cannot be applied to the strongest people in a group. Rudolf's mind-reading abilities are not very good - of course toxicological data is based on averages - but Rudolf's analysis misses an obvious point. In a given gassing the strongest undoubtedly survived longer than the weakest. This problem could be dealt with by the simple expedient of waiting until all victims are apparently dead. This time might vary from gassing to gassing. If one gassing took 10 minutes and another 15, a witness to both who reported that it took about 10 minutes would not be lying.
Rudolf states further that
He leaves it as an exercise for the reader to determine what his source is or what concentration he considers lethal. Du Pont's MSDS, however, disagrees with his claim: 
(Emphasis ours.) Du Pont is, of course, discussing a situation in which one wishes to give medical attention to the person. We cannot on this basis rule out the possibility that a person might receive an "insignificant" amount of poisoning by this standard and still die later without medical attention. Nevertheless, to solve such a possible problem, it is sufficient to adjust the dose to make sure that such does not occur routinely.
A survivor of a gassing who was unconscious could be cremated along with the dead. In the extremely unlikely case that the murderers opened the doors before all were unconscious, there is no reason a bullet would not have sufficed. 
How fast could a lethal concentration be reached? We first review some relevant literature on the topic. The Holocaust History Project has three technical papers that address this question. All three were written in the context of the use of Zyklon B for delousing. The first work is a 70 page monograph by Gerhard Peters of the Degesch company that was published in 1933.  This document has not yet been transcribed or translated. The second work is a 1941 paper by Peters and Rasch.  This document has been transcribed and translated. The third paper is a 1942 paper by Irmscher that has also been transcribed and translated. 
The Chemistry of Auschwitz quotes portions of Peters' first paper that were translated by Dr. Ulrich Roessler. These portions indicate that the poison began to evaporate "with great vehemence" as soon as the tins were poured out, and that "the greatest part, nearly all" of the Zyklon B evaporated within 30 minutes.
In the second paper Rasch and Peters investigated the speed of evaporation of Zyklon at colder temperatures as well as its efficacy on insects at colder temperatures. The latter topic concerns us only in that it is worthwhile to note that delousing took much longer than homicide. Rasch and Peters found that:
They found furthermore that:
An inspection of their data shows that concentrations begin to drop after 2 hours, which confirms their claim.
In the 1942 paper Irmscher continued the study of evaporation rate as a function of temperature. Irmscher specifies which solid supports were used for his studies (cardboard and Erco, a gypsum product) and provides a higher time resolution on the evaporation process. Irmscher studied evaporation at temperatures ranging from -18°C or -19°C up to 15°C. Excluding the lowest temperature results, Irmscher's results are in the same ballpark as those of Rasch and Peters for the most part. At -6°C, 0°C, and +15°C, Irmscher finds that within two hours 84.1%, 90.7%, 96.8% of the HCN, respectively, evaporates for the Erco support. The corresponding values for the cardboard support are 73.0%, 85.7%, 96.4% respectively.  The small discrepancies between these latter two papers most likely owe to differences in the support material and/or differences in humidity. Irmscher's results for -18°C show that evaporation slows substantially at this temperature.
We now examine the question of how fast a lethal concentration can be built up. In the discussion above we cited sources that put the amount of Zyklon used in the gas chambers at between 5 and 20 g/m3. These values correspond to 4500 and 18,100 ppmv respectively. Inspection of illustration 1 of the Irmscher paper shows that about 10% of the Zyklon evaporates within a period of about 5 to 15 minutes even at the coldest temperatures he studied. Irmscher did his studies at temperatures ranging from -18°C and 15°C. The gas chambers are likely to have been much warmer than the warmest temperature he studied. Human body temperature, for example, is 37°C. Even at the cold temperatures studied by Irmscher, lethal concentrations would have been reached in a few minutes (450-1810 ppmv)! Besides showing that it is not unreasonable to expect Zyklon B to kill quickly, these results also demonstrate the assumptions made about concentration in the article "Leuchter, Rudolf, and the Iron Blues" were adequate. (See Appendix 1 of that article.)
It is worth mentioning at this point an argument made by Holocaust denier Wolfgang Fröhlich.
Rudolf is far too clever to make such a ludicrous argument, but it is worth examining the type of denier argument advanced by those who claim that they have trumped the historians with "exact science."
What stands out in Fröhlich's argument is the large "estimate" of Zyklon necessary to kill in a short period of time. Our examination of numbers between 5 and 20 g/m3 shows that from 2.5 to 10 kg would suffice. 
But even Fröhlich is more accurate than the denier Carlos Whitlock Porter. After echoing the notion of poisoning the entire camp,  Porter outdoes Fröhlich by a factor of twenty. The estimated 40-50 kg has become "a ton of pure cyanide." 
These arguments are clearly ludicrous, but here we may segue into the discussion of whether the conditions employed were safe for the Sonderkommando.
The Sonderkommando were slave laborers: to their SS slavemasters they were expendable. The SS certainly did not have to obey OSHA regulations. They would not be averse to exposing the Sonderkommando to concentrations of approximately 40 ppmv ("slight symptoms after several hours"). Even if the full concentration of 4500 to 18,100 ppmv had released from the Zyklon into the gas chambers, it would only be necessary to reduce that concentration by a factor of 100-500 times to reach this tolerable level.
It is not the case that the full concentration of Zyklon was present. At the largest Auschwitz crematoria (II and III), the Zyklon was removed after a lethal quantity of gas was given off, using the same devices which inserted it. At these buildings, where the large majority of gassings took place, essentially any absolute rate of outgassing could be achieved, at any temperature and humidity, by pouring in sufficient Zyklon. Once the victims were dead, the remaining carrier material could be lifted out by SS men wearing gas masks, to continue outgassing harmlessly into the open air until spent.
Inspection of Irmscher's paper shows (assuming the Erco carrier) that the concentration that would be present after 30 minutes, for example, would have been 20 to 40% of the total, i.e., 900-7200 ppmv. So it was only necessary to reduce the concentration in the gas chambers by a factor of 20-200 times in order for the Sonderkommando to enter even without gas masks. The remainder of the Zyklon could outgas safely in the outside atmosphere - without, needless to say, "poisoning the entire camp."
The gas chambers were 30 m long by 7 m wide: 210 sq m. They were 2.4 m high, for a volume of 504 cu m.  Those same chambers had a ventilation system with both intake and exhaust fans, capable of cycling 8000 cu m through the room each hour.  This is commonly referred to as 8000 ÷ 504 = 15.8 "air exchanges per hour."
Note that the Holocaust-denier Carlo Mattogno has misrepresented these figures in his essay, "Auschwitz: The End of a Legend." 
It is impossible, of course, to get an exact figure for how long it actually took to clear the air in the gas chamber. But we can obtain approximations through mathematical modeling. The equation used is a simple one: the concentration in the gas chamber is cut to 1/e, or about 37%, for each room replacement of air. Where C(t) is the concentration of HCN at time t in hours,
This equation supposes that the fresh air mixes with the air in the chamber immediately and completely. In reality it does not do so. Ventilation systems are designed to have an air flow such that the expelled air has a higher concentration of poison, so this equation might seem conservative. In addition, the victims' corpses take up space which has not been figured into any of the below calculations; this would reduce the volume and increase the replacement rate, again indicating that this figure is conservative. But blockages caused by the same corpses, and the possibility of laminar airflow, might work in the other direction. All in all, this estimate will suffice.
Using this equation, if C(0) = 900 ppmv, the concentration is less than 20 ppmv after just 15 minutes.
The American Conference of Governmental Industrial Hygienists produces an Industrial Hygiene Calculator program for the Windows operating system.  When the size and ventilation rates of the gas chamber are converted to cubic feet and minutes, it returns identical results to the above equation.
It should also be pointed out that, halfway through their period of use, the size of these gas chambers was cut in half: 
If one makes the logical assumption that the intake and exhaust vents were also blocked off in the unused portion of the gas chambers, this modification doubled the ventilation rate of the remaining portion. However, we will continue to use the figures from 1943; if a gassing from 1944 is referenced, ventilation times would be cut in half.
We return to the question of how long it would take to ventilate the gas chamber from the level used in killing to a level which the Sonderkommando could safely tolerate without a gas mask. We have seen that this took place in less than 15 minutes from an initial concentration of 900 ppmv.
If the initial concentration were more than seven times higher (7200 ppmv), owing to the nature of exponential math, the same concentration of under 20 ppmv would be reached in less than 23 minutes. Even if the residual Zyklon had not been removed and the chambers had the full concentration of 18,100 ppmv, the concentration would be less than 20 ppmv in 26 minutes.
In fact, since OSHA guidelines (above) give specifications not for maximum exposure but for mean exposure over fifteen minutes, we can use these values to understand what the Sonderkommando would experience. In the graph below, an initial concentration of 900 ppmv is assumed for the solid lines. The concentration is plotted in red. In blue is plotted the mean exposure over fifteen minutes for someone entering the gas chamber at the specified time. The dashed lines show the same information assuming an initial concentration of 7,200 ppmv: 
After ten minutes, in the former case, the ambient concentration was about 65 ppmv, and someone who entered the room at that point would receive a mean exposure to HCN, from t=10 minutes to t=25 minutes, of about 17 ppmv. Recall that 20 ppmv is the low end of Du Pont's symptom category: "slight symptoms after several hours."
It is thus safe to say that, with these assumptions, the Sonderkommando could enter the gas chamber ten minutes after ventilation began, wearing no gas masks, and experience no significant effects from the HCN.
If we instead assume the highest estimated initial concentration of 7,200 ppmv, the dashed lines would apply. Thus, the Sonderkommando could enter after eighteen minutes with no serious effects.
This conservative estimate fits with Pressac's conclusion that the doors were typically opened after twenty minutes of ventilation. 
Rudolf, however, has come to the opposite conclusion, albeit without any calculations or evidence. In his essay "The 'Gas Chambers' of Auschwitz and Majdanek," section 18.104.22.168, he cites a figure of 6 to 8 air exchanges per hour. This figure is based partially on an erroneously interpreted document from Pressac. Compounding the error are his other references: an unpublished denier source, and an unreferenced denier source both apparently claiming the ventilator would not work efficiently - no details are given.
He then writes:
His next sentence reveals the purpose of this line of argumentation: to try to prove that the eyewitness testimony is contradictory.
He gives no citation for his claim that the inlets are "right above" the outlets. If we understand his English, this is not exactly true: fresh air entered at ceiling level, toxic air exited at floor level.  Perhaps he means to consider the placement of the outside vents. If so, he would need to show that they were separated by less than 1.8 m and that such placement was quantitatively incompatible with reported ventilation times. It should be noted that a separation of only 1.8 m would suffice to prevent cross contamination. 
Rudolf points out that the bodies must be taken into account. His point about bodies crowding the room is not obviously wrong, but without quantification it is a meaningless argument. In fact, such crowding might produce mixing through turbulence, making the ventilation efficiency tend toward the limits that we have calculated. He claims (for crematory I) the difference will be "a factor of 10 or more" yielding a ventilation time minimum of "2 hours" - but provides no basis for these numbers. He appears to have pulled them out of thin air.
We show above that testimonies "claiming adequate ventilation after 20 to 30 minutes" are extremely credible - using a mathematical model known to be conservative, that is, to have room for error. Rudolf, on the other hand, produces only unsupported assertions that take no account of how large a reduction in the HCN concentration is necessary.
We should not let drop the subject of Rudolf's analysis without mentioning a turn of phrase which is suspiciously misleading. Rudolf gives a probabilistic explanation of the same mathematical model we are using to estimate dilution. In his description, balls take the place of air molecules:
This model is the same as have described in our formula above. In Rudolf's example a single room exchange of air would be equivalent to 100 "exchanges."
Rudolf's next sentence seems designed to seriously misinform the reader:
By using the same word "exchange" in two contexts he can give the impression that ventilation occurs very slowly. What he calls an "air exchange" in the second paragraph corresponds to 100 of his "exchanges" from the first, though he nowhere makes this clear. It seems the reader is expected to believe that it would take ten hours before the level of poison was cut in half; certainly Rudolf does nothing to dispel this notion. Rudolf is not unintelligent nor sloppy: his dual use of the word "exchange" is surely not a mistake. We presume it is an intentional deception.
The math is easy to do. Why does Rudolf not demonstrate it for his reader? We suspect, first, because even at his inaccurate figure of only 6-8 exchanges per hour the results do not support his conclusion. Below, we replot the data assuming 8 exchanges per hour:
The time before it was safe for the Sonderkommando to enter the gas chamber without a mask and with no ill effects ranges from 20 to 40 minutes, again within a range that does not contradict testimony.
But Rudolf is not done. Because even the incorrect figure of 6 to 8 exchanges per hour would produce times within a reasonable range, he must reduce that figure further. In the Rudolf Report, there are two deliberate and mendacious distortions which accomplish this goal.
Firstly, Rudolf cites Pressac in support of the figure of four air exchanges per hour. In section 22.214.171.124 of his Report, he writes: 
Nowhere in section 1.3.1 does Rudolf provide the source for this figure of 15 minutes. Section 3.4.1 is equally unhelpful, being a look at eyewitness testimony. Footnote 60 refers to a chapter of Pressac's book Technique and Operation. Only in footnote 256 do we finally get the source of this all-important number: Pressac's page 16 is cited.
On that page, Pressac tells us:
(Emphasis in original.)
Pressac gave 15 minutes as the duration before the air was renewed so thoroughly that the doors could be opened. As we have seen above, this fits with our calculations, because almost four air exchanges would have taken place after that duration. Rudolf has fraudulently taken this to be the time period for one air exchange.
Secondly, Rudolf then further slows the ventilation process, arbitrarily, until he arrives at a number he likes. He writes in fractured German:
The number "2 to 4," which in his Table 10 becomes simply "4," has been pulled out of a hat. Since his already-fraudulent figures do not support his claims, he calls them "ideal," and simply multiplies the numbers by four to get the "real."
Thus it is that, by following one dishonest maneuver with another, Rudolf converts a respectable air-exchange rate of 15.8 per hour into precisely one per hour. This multiplies the supposed ventilation times by the same amount, so that a reasonable calculation of 15 to 20 minutes turns into:
To discover the real values, one must simply divide Rudolf's durations by 15.8.
Finally - this question of ventilation is purely academic anyway. The Sonderkommando had gas masks available and wore them at least some of the time, as numerous witnesses have attested. The duration of ventilation before the doors were opened is only of interest to those Sonderkommando who wished to remove their gas masks after, say, fifteen minutes instead of twenty. The prisoner Dr. Nyiszli describes the scene: 
(The cough was surely caused by the Zyklon warning indicator, a lachrymatory irritant. For safety reasons, the warning was designed to be noticeable even at low levels of cyanide. Eyewitnesses untrained in handling of Zyklon would probably not know this. Although there were some shipments of Zyklon without the warning agent, the use of such Zyklon was not universal.)
Gas masks are also referenced by Szlama Dragon at a gas chamber which lacked ventilation: 
So why is ventilation time a concern at all? Deniers usually reference Höss' memoirs, which mention that the Sonderkommando ate or smoked while they worked (thus, without gas masks).  But Höss did not specify that this took place inside near the gas chamber, nor that this took place shortly after ventilation began, so there is no contradiction.
Although gassing and ventilation together lasted somewhat under an hour, it is the act of burning the corpses which took the most time. One thousand people could be killed in five to fifteen minutes. To cremate their bodies, the Sonderkommando would work the better part of a day. Höss was probably referring to activity he saw well after the gassing operation was complete and the chamber thoroughly ventilated, when gas masks were no longer necessary.
Rudolf has researched the characteristics of gas masks.  Assuming for the sake of argument that he is not misrepresenting his sources, they indicate that the German HCN-type removable filter would provide protection for 25 minutes at 50,000 ppmv. Rudolf tries to find reasons to reduce that time period, but as we have shown, even a full outgassing (in, for example, the unventilated gas chambers) needed only release 4500 to 18,100 ppmv. And if the filter ran out after 25 minutes, the Sonderkommando could step outside and change it (Rudolf fails to mention this obvious possibility).
Another point he raises is that of poisoning through the skin. It seems this would not have been a serious danger for the Sonderkommando. Until Rudolf is able to show any quantification for this, it can be ignored. 
To summarize, these are the worst-case ventilation assumptions for the main Auschwitz gas chambers, in crematoria II and III:
Using those assumptions, the Sonderkommando could remove their gas masks after just 18 minutes and suffer no significant effects (beyond a cough caused by the nontoxic Zyklon irritant). If they were to wait just 24 minutes, their workplace would conform even to OSHA regulations.
But OSHA, of course, does not publish regulations that apply to slave labor incineration of hundreds of freshly murdered corpses.
If Rudolf wishes to show some sort of logical impossibility, he will first have to find some testimony, considered reliable by historians, dated in 1943 before the halving of the gas chamber's size, which states that gas masks were removed in the gas chamber after only a short time. To our knowledge, there is no testimony which addresses how long the Sonderkommando waited before taking off their gas masks to work in the still-ventilating gas chamber.
Next, if Rudolf wishes to contest the mathematical model, he will have to present some hard evidence for his reasoning regarding ventilation efficiency, and he will have to work out the numbers.
Rudolf will have to meet all these conditions before he can show a contradiction. But he has met none of them. Without evidence indicating why the model is flawed, without having worked out the numbers, and without a single eyewitness giving different numbers regarding the duration before gas-mask removal, his "expert opinion" remains of no value. We have not seen even the ghost of a contradiction.
Prussian Blue in Five Steps
Perhaps the central issue in understanding the significance of the various attempts to measure cyanide traces in the remnants of the gas chambers is the issue of the formation of Prussian blue. Yet many deniers completely ignore this issue in their defense of the Leuchter Report. For example, CODOH has just published a document that ignores the mechanism by which Prussian blue forms, at http://www.codoh.com/newrevoices/nddd/ndddstern.html.
Leuchter and Rudolf claim to have detected far more cyanides in the delousing chambers than in the homicidal gas chambers. This finding, according to some, should trump all historical evidence and show that the Final Solution is a made-up tale. The bulk of the cyanides detected by Leuchter and Rudolf were in the form of Prussian blue and/or related compounds. That there is a discrepancy between the amount of Prussian blue between some of the delousing facilities and some of the homicidal facilities is clear from inspection of the prominent blue staining on some of the delousing chambers (and the chemical work of Leuchter and Rudolf, even if honestly conducted, shows no more than is evident from inspection). The important question is whether such staining is an accurate marker for exposure to HCN. Must it always be present in buildings exposed to HCN?
The essay "Leuchter, Rudolf and the Iron Blues" shows that such compounds in the gas chambers were exceedingly unlikely to form. Factors such as the shorter exposure time and the greatly reduced concentration of aqueous cyanide ions (in part, because of the washing of the chambers with water) distinguish the gas chambers from the delousing chambers. Here we extend these results, make them more quantitative and address Rudolf's criticisms.
Rudolf agrees with Dr. Green on the most plausible way to answer this question. Rudolf quotes "The Chemistry of Auschwitz":
The disagreement is perhaps centered on understanding what the probability is for Prussian blue formation in the gas chambers. Rudolf wrote his criticism in August 1998, so he could not have been aware of the revised version of "Leuchter, Rudolf, and the Iron Blues," in which it is argued based upon the findings of Alich et al. that such formation by Rudolf's proposed mechanism is extremely sensitive to conditions, especially pH, moisture, and concentration of Fe(CN)63-, and that significant amounts of Prussian blue were unlikely to form under the conditions of the homicidal gas chambers.
Rudolf proposes five steps in the development of the blue staining.
We examine each step in turn.
Step one. The first step relies on the ability of water to absorb HCN.
Appendix 1 of "Leuchter, Rudolf, and the Iron Blues" addresses this issue. Rudolf asserts that the gas phase concentrations assumed are likely to be wrong but presents no evidence. Most likely he was not familiar with Appendix 1 when making that claim. At any rate, we show above that those concentrations are indeed reasonable. Even assuming a gas phase concentration as high as 16 g/m3 the maximum amount of HCN that could be absorbed by water would be less than 0.3 M at 10°C and 0.4 M at 0°C. More reasonable assumptions lead to a maximum concentration of about 0.1 or 0.2 M, and that is assuming that all of the Zyklon was allowed to evaporate inside the chamber. Above, we point out that only 20-40% of the HCN would have evaporated within the chamber. Additionally, these are equilibrium values! They assume that the HCN in the gas phase had time to equilibrate with the HCN in the liquid phase. This assumption is in fact unlikely and the maximum liquid phase concentrations are likely to be much lower (delousing on the other hand took much longer than a homicidal gassing and it is thus likely that the liquid phase concentrations would be higher). After the gassing the chambers were washed down with water drastically reducing this concentration. (See footnote 15 in "Leuchter, Rudolf and the Iron Blues").
Step one is one of the crucial points in attempting to understand the differences between the gas chambers and the delousing chambers. The delousing chambers were exposed to HCN for longer; the Zyklon evaporated completely, the possibility of reaching equilibrium with HCN in solution was much higher. Additionally, the gas chambers were washed with water after a gassing.
Step two is the dissociation of HCN into H+(aq) and CN-(aq). HCN is a weak acid, which means that in aqueous solutions it dissociates somewhat but not completely. In other words the concentration of cyanide ions in solution is even less than the concentration of HCN. The strength of an acid is measured by a quantity known as the pKa. The lower the pKa the stronger the acid. The pKa is defined as -log(Ka) where:
In equation (1) the square brackets represent the molar concentration (M) of the given species in aqueous solution. [H+] is related to the pH by the simple expression [H+] = 10-pH. The pKa of HCN is 9.31.  At neutral pH the cyanide ion concentration is only 1 percent of the HCN concentration. To calculate this value at other pH's we define the initial concentration of HCN as [HCN]0, and using the identity [HCN]=[HCN]0-[CN-] rewrite equation (1) as:
The following figure expresses the percent dissociation of HCN as a function of pH.
Rudolf would like to claim a pH of around 10 (a claim that we shall examine in further detail). Note that if Rudolf were correct that the concentration of cyanide ions would be about 80% of the initial HCN concentration. If the pH is 6-7 as measured by Markiewicz et al., it is about 1% of the initial hydrogen cyanide concentration. Appendix 1 of "Leuchter, Rudolf, and the Iron Blues" shows that the concentration of aqueous HCN before washing with water is on the order of 0.1 M: 1% of this concentration is on the order of 10-3 M.
Alich et al. found that concentrations of cyanide ions that were less than about 3.3 x 10-4 M did not form Prussian blue even though an excess of CN- was still present (dilution with 13% water by volume).  Considering that the gas chambers were washed with water, it is no wonder that very little if any Prussian blue formed there. Even if Rudolf is correct that the pH was about 10, that would lead to concentrations of cyanide ions on the order of 0.1 M. A mere thousandfold dilution by washing with water would reduce this concentration to the same level.
Rudolf's present view is that lime (a.k.a. calcium hydroxide, Ca(OH)2 [in its hydrated form]) is not responsible for the alkaline pH he wishes to claim:
We therefore discount without further ado the possibility that lime from plaster or whitewash is a reason for an alkaline pH. As for the other construction materials, we await Rudolf's analysis of their actual content. His claims about pH rest on pure speculation. The IFRC, on the other hand measured the pH to be between 6 and 7.
During the gassing process itself any water in the chamber was likely to be slightly acidic owing to the influence of carbon dioxide in the air from the exhalation of the murderers victims (see Appendix 2 of "Leuchter, Rudolf and the Iron Blues"). Here is another significant difference from the delousing chambers. The influence of CO2 inhibits dissociation, but the whitewashing which took place after the washing with water enhances it.
Step three. Without this step, Prussian blue will not form by Rudolf's mechanism. Alich et al.  exposed Fe(III) to CN- and observed no Prussian blue formation within the timescale of the experiment. This fact shows that step 3 is not a fast process by any means, and yet it is a necessary step. Cyanide ions in the presence of Fe3 do not reduce the iron. Rather, the iron must already be complexed to cyanide in the form of Fe(CN)63-.
Rudolf notes correctly that basic environments inhibit this process. An oversimplified way of considering this reaction is the following reaction:
in which we have omitted the complexing of water molecules for simplicity. Lechatelier's well-known principle predicts that a higher concentration of hydroxide ions (OH-) should drive this reaction to the left. The basicity that Rudolf needs to have even a snowball's chance in hell of producing Prussian blue by his proposed mechanism (under the conditions in the gas chambers) inhibits the formation of a necessary precursor! Rudolf claims that the pH must reach 11 to hinder the process, but offers no support for this assertion.
This graphic shows a schematic structure of Fe(CN)63-. (It is not an optimized structure, and not intended to reproduce bond-lengths and angles with any precision.) The yellow represents iron (Fe), the gray represents carbon (C) and the blue nitrogen (N). The point of this structure is to facilitate the intuition of those not used to thinking of chemistry in three dimensions as well as those not used to violations of the octet rule in atoms with energetically accessible d- orbitals.
Each CN- ligand must react with the iron individually. The formation of this molecule is a step-by-step process each step of which is inhibited by basicity.
Step four. Rudolf is correct to note that there is a photoreduction mechanism to convert Fe(III) to Fe(II), however, the mechanism is different than that suggested by Rudolf in his response. Ozeki et al.  studied the photochemical reaction on glass-fiber and cellulose filter paper and proposed the following mechanism:
(The *'s indicate molecules in an excited state).
These researchers have done some elegant work to exclude other possible mechanisms. L. Moggi et al.  have proposed a mechanism in aqueous solution as follows:
With absorption of subsequent photons the complex loses cyanide ligands which are replaced by waters until ultimately one is left with Fe2+(aq.). This ion reacts with Fe(CN)63- to form Prussian blue. They cannot completely exclude a direct photoreduction mechanism, but certainly do not argue for one. The other citation Rudolf gives concerns a similar reaction in methanol and is not truly relevant here although there is certainly some likeness. 
The bottom line is that Rudolf is correct about the existence of a photoreduction mechanism, but that mechanism is complex and not direct. Its sensitivity to cyanide concentration and photoflux is great. If such a mechanism is active (and it may well be relevant to some of the staining outsides of buildings), it is even more sensitive to conditions than the non-photochemical mechanism. The non-photochemical mechanism of reduction has been discussed in great detail in "Leuchter, Rudolf, and the Iron Blues," in which it is argued that the probability of this step occurring under the conditions of the gas chambers is quite small. Rudolf's response was written before the revised version of that study was presented, however.
Step five. This step is not controversial. If the concentrations of Fe(III) and Fe(CN)64- are adequate, "soluble" Prussian blue forms readily.
Although Rudolf's proposed mechanism for Prussian blue formation is not an implausible explanation for the staining on the delousing chambers, he has not truly demonstrated this fact. When asked to do so Rudolf and colleagues keep presenting the case of a Bavarian church, in which Prussian blue staining occurred, ad infinitum, but what is this case supposed to prove? The claims about the church were published by "Ernst Gauss" and are available at http://www.codoh.com/inter/intgrgauss.html. Again, Gauss is a pseudonym of Rudolf himself. 
The key element is pointed out by "Gauss" himself:
The staining in the church is an event that occurred, but it does not represent a phenomenon that always occurs. As "Gauss" notes, the specialists were surprised. The fact that blue staining occurred in this church is not sufficient to demonstrate that the same mechanism is responsible for the blue staining in the delousing chambers. Nevertheless, this church demonstrates some interesting chemistry that may be the topic of a future article on this site.
Despite the fact that he has not demonstrated the case, we agree that formation of Prussian blue by a mechanism similar to that which Rudolf proposes in his three steps is not an unlikely explanation for the presence of blue staining on the delousing chambers. It is, perhaps, not so important whether the mechanism is exactly as Rudolf states or takes place by a similar mechanism.
Supposing that Rudolf is correct or nearly correct regarding the formation of blue staining in the delousing chambers, it is exceedingly unlikely that the same process would have taken place to any great extent in the gas chambers used for mass murder.
We agree with Rudolf that "insoluble" Prussian blue is less susceptible to weathering than other cyanides. "The Chemistry of Auschwitz" pointed out that it is Rudolf's burden to show that such is the case for the compounds formed on the delousing chambers, but we are not making the contrary argument. The weathering is important, if one considers its implications for buildings that were exposed to HCN but did not form Prussian blue stains (as Gauss's fumigation experts attest is the normal state of affairs). A building in which Prussian blue formed would have much higher levels of detectable total cyanides than a building in which Prussian blue did not form. We must therefore conclude that Prussian blue is not a good marker for exposure to hydrogen cyanide. Because of the fact that Prussian blue is much less susceptible to weather, a building that has Prussian blue stains will have a total cyanide content much greater than one in which Prussian blue did not form. Because of these facts, we must conclude that judging exposure to cyanide by means of the total cyanide content is inappropriate. A fair marker for exposure to hydrogen cyanide is to measure the remnant cyanide content when iron compounds are excluded. Such an experiment was actually performed by the Institute for Forensic Research in Cracow.
The Institute for Forensic Research, Cracow
The Institute for Forensic Research in Cracow did the appropriate experiment. At best, the likelihood for formation of Prussian blue is uncertain in the gas chambers, and we have shown that such formation has a very small probability. Comparing the total cyanide concentrations of structures that actually do have Prussian-blue staining with those that do not is a comparison of apples and oranges, especially because Prussian blue is much less susceptible to weathering than other cyanides. A fair comparison, as stated above, is to measure remnant cyanides excluding iron-cyanide complexes. The IFRC did just that and they conclude that:
Rudolf's criticism of the work of Markiewicz et al.. of the Institute for Forensic Research, Cracow, makes no sense at all. Can he mean this argument to be serious? Recall that Rudolf agrees with Dr. Green that the most plausible explanation for the disparity in Prussian blue is:
Quite simply this argument means that Prussian blue is not a reliable marker for exposure to HCN. We agree with Rudolf that it would have been nice if Markiewicz et al. had tested for total cyanides as well as cyanides not complexed to iron. It would have shown among other things whether Leuchter and Rudolf treated their samples honestly. It would not, however, have shown much more than that. It is clear to the naked eye that the delousing chambers have visible blue staining. There is no question that there is a disparity in the concentration of Prussian blue between the delousing chambers and the gas chambers. The relevant question is whether such a disparity is inconsistent with the historical record.
Rudolf proceeds to make an argument that we can only view as disingenuous. Whereas as he acknowledges, in so many words, that Prussian blue is not a marker for HCN exposure, i.e., that exposure to HCN does not necessarily result in Prussian blue formation, he now wishes to claim that the ratio of non-iron cyanides to total cyanides should be such a marker. Such an argument can only be valid if the delousing chambers and the gas chambers were exposed to exactly the same processes and conditions and then only if it can be shown that the entire process for Prussian blue formation is linear with respect to HCN concentration. A plausible explanation for the difference of the ratios is that a significant amount of Prussian blue formed in the delousing chambers, whereas such did not occur in the chambers used for mass murder. Plausibly, the delousing chambers were exposed to more cyanide as is evidenced by the measurements of Markiewicz et al.
Rudolf is correct to point out that Prussian blue is less susceptible to weathering, but he has not shown that the exposure to HCN is proportional to the amount of iron cyanides formed. Markiewicz et al. were able to detect cyanides not bound to iron only in locations that were protected from the weather. Rudolf complains that they did not check the possibility of contamination. This argument ignores the fact that Markiewicz et al. investigated dwelling accommodations where they consistently found a level of 0 ug/kg of with their 1 ug/kg sensitivity.
After this enlightening criticism, Rudolf says of their work that it is "reeking like a fraud." Is this how "exact science" proceeds? Rudolf calls into question the honesty of Markiewicz et al. Anyone who questions the honesty of the deniers is accused of engaging in political polemics rather than science, but the deniers feel free to call any document a fraud and any researcher a liar if they do not like what they hear.
Rudolf complains that Markiewicz et al. have not responded to his queries. Why should they do so? What credibility does Rudolf have, that demands they answer his every objection no matter how ill-founded?
At any rate, Markiewicz died in 1997, so Rudolf will be waiting a long time for his response. 
There are a few points that may be learned from examining Rudolf's response. The first is Rudolf's admission that chemistry cannot offer the absolute disproof that the Final Solution that so many deniers would like to see. Additionally, the chemical arguments that he makes do not hold up under scrutiny.
Should we then conclude that Markiewicz et al. have proved that the Final Solution took place? No, these researchers showed rather that all of the installations they studied, in which the historical record shows that mass murder by gassing took place, contain traces of cyanide. Historians and (real) scientists share a concept called the convergence of evidence. Absolute proof exists only with the postulates of pure math or logic. In the physical world, the most that we can hope for is a convergence of evidence: to borrow a phrase from the legal world, we might seek proof "beyond a reasonable doubt."
Meanwhile, the field that Rudolf claims can offer such proof is a field in which he has no expertise. As if to confirm the worthlessness of his own work, he has been reduced to yet another parrot of Faurisson and Ball. The arguments regarding aerial photos and the supposed lack of holes in the gas chamber roofs will be addressed on this website at a future date. At the present time, it is sufficient to show that Rudolf's criticisms of "The Chemistry of Auschwitz" and "Leuchter, Rudolf, and the Iron Blues" have not successfully addressed those works. His opinion is just one man's opinion, and it happens to be wrong.
Thus we see that the deniers' chemical arguments are faulty and do not - cannot - demonstrate the invalidity of history.
About the Authors
Dr. Richard J. Green received his PhD. in physical chemistry from Stanford University in 1997 under the guidance of Professor Richard N. Zare. He is currently a postdoc and faculty intern at a major university in the western United States.
Jamie McCarthy has examined and researched the claims of Holocaust-deniers since 1992.
The authors would like to thank Mark Van Alstine, Stéphane Bruchfeld, Margret Chatwin, Daniel Keren, Gord McFee, Ulrich Roessler, and John Zimmerman for valuable assistance.
This essay is dedicated to the memory of Stig Hornshøj-Møller.
A postscript addresses Rudolf's inadequate response to this article.
Last modified: July 28, 2000