Blind testing was not done

Rigorous scientific tests should involve blind testing.

In a blind or blinded experiment, information which may influence the participants of the experiment is withheld until after the experiment is complete. Good blinding can reduce or eliminate experimental biases that arise from a participants’ expectations, observer’s effect on the participants, observer bias, confirmation bias, and other sources.

Blinding is an important tool of the scientific method, and is used in many fields of research. In some fields, such as medicine, it is considered essential.

In addition to testing the shroud, control samples with known dating to the overseer, but unknown to the labs, would test the accuracy of the C-14 results. All of the samples should not be known by the labs whether it is the actual shroud or control samples. By doing a blind testing, it would remove all bias.

However, blind testing was not done. All labs knew which sample was from the shroud. Further, all labs knew ahead of time the dates of the control samples.

The blind-test method was abandoned, because the distinctive three-to-one herringbone twill weave of the shroud could not be matched in the controls, and it was therefore still possible for a laboratory to identify the shroud sample. … d_of_Turin

The laboratories were not told which container held the shroud sample. Because the distinctive three-to-one herringbone twill weave of the shroud could not be matched in the controls, however, it was possible for a laboratory to identify the shroud sample.

The three control samples, the approximate ages of which were made known to the laboratories, are listed below.

All was proceeding well until, one by one, the protocols established by Tite to
ensure an accurate dating were, for various reasons, set aside. This included the
“Blind Test” provision. Tite had failed to find suitable medieval linens with the same weave as the Shroud as control samples.

Regarding the control samples, the labs were inexplicably told beforehand the dates of the
samples, negating any significant value to that aspect.

The labs were told the age of the historical
known-age control pieces, a fact that rather diminished their value as controls.
Paradoxically, the pretense of “blind testing” was maintained for the whole dating
exercise, despite the fact that everyone knew that the Shroud weave was easily recognizable. … UD_PT2.pdf

Prior to the dating results, Harry Gove criticized discarding blind testing. But after the results were announced, he backtracked on his own criticism.

In a later letter to the editor in Nature, a reader asked about the
procedures. Tite would answer that it happened to follow the blind procedure,
even if this aspect was “quite illogical, because in that moment we knew that
because of the unusual weaving of the Shroud, the blind test was not feasible
without unraveling the samples. … UD_PT2.pdf

Harry Gove, director of Rochester’s laboratory (one of the four not selected by the Vatican), argued in an open letter published in Nature[28] that discarding the blind-test method would expose the results – whatever they may be – to suspicion of unreliability. However, in a 1990 paper Gove conceded that the “arguments often raised, … that radiocarbon measurements on the shroud should be performed blind seem to the author to be lacking in merit; … lack of blindness in the measurements is a rather insubstantial reason for disbelieving the result.” … d_of_Turin

Seriously? “lack of blindness in the measurements is a rather insubstantial reason for disbelieving the result”? Instead, this is another breach of the scientific process.

On the following day the Vatican Press Office issued a bulletin, published by the
Osservatore Romano 92 , where, among other things, it is written: “The samples, of the total
mass of about 150 mg, were obtained by cutting a strip of about 1 cm x 7 cm”. It is also
specified that the control samples “come from a cloth dating from the first century A.D. and a
cloth from the eleventh century A.D.; a fourth sample, dating from about 1300 A.D., was
provided as an additional control. There is also a specification on the sampling area: “The
sampling site was chosen so as to ensure that the sample belonged to the main body of the
Holy Shroud and that its removal could cause the least possible damage to the fabric”.

The fact is that having inopportunely renounced the double-blind
procedure, they calmly told the laboratories the ages of the witness samples!
The results were invalidated by procedural defects.

Dr. Jerome LeJeune of the Pontifical Academy of Sciences,
September 1989. … Dating_S25