Both McCrone and the STURP team studied the samples from the sticky tapes. However, there was one crucial difference. McCrone only studied the samples while they were on the tape. STURP was able to study the samples by removing them from the tape and the adhesive.
In sharp contrast, Heller and Adler removed all the specimens from the tapes before their
studies. Then, they removed the adhesive with toluene and verified the final removal of the
adhesive under UV irradiation (the adhesive gives a bluish-white fluorescence).
Observing the samples while on the tape would’ve affected the refractive indices.
Heller noticed that the Mylar tape was optically active so that any red particle looks
birefringent when the light has to pass through the tape and particle 28 . Certainly, the tape also
changes the measured refractive indices of the particles.
Visual analysis of particles through a microscope is also subjective.
His identification of iron-oxide was based on neither chemical testing nor physics-based testing, but consisted simply of looking through his microscope and seeing particles that seemed to have the appearance and crystalline characteristics of ironoxide.
So, though iron oxide and vermillian were detected by both McCrone and STURP, there were false positives that were likely observed by McCrone. This is evidenced by his claims differing over time.
From McCrone himself, it is clear that most of his observations were performed on the
anisotropic Mylar tapes that change the optical properties.
It is important to notice that McCrone’s claims about the red particles have changed at various
times 72 : the paint was composed of 1) simply iron oxide particles, then 2) jeweller’s rouge, an
iron oxide available only after about 1800, identical to hematite, then 3) earth iron oxide
pigment: the red ochre in a proteinous binder and finally, after the discovering of Vermilion
by McCrone associates, 4) his final claim of liquid earthy iron-oxide paint with liquid
vermilion paint to enhance the red color of the “blood”.
And polarized light microscopy is considered an antiquated technique, even during the time when McCrone tested the samples. From his own paper:
Very few chemistry students, undergraduate or
graduate, have ever looked through a microscope except,
perhaps, in high school biology. The direct micro-
scopical approach to the solution of chemical problems
has been largely lost during the past three decades.
This project on the authenticity of the Shroud of Turin
has been an excellent opportunity to gain wide publicity
for the unique capabilities of polarized light microscopy,
providing a real impetus to its deserved renascence.