Bleaching

[Since I published the first version of this post, César Barta has pointed out that the comparison spectrum I used then was actually from a modern French textile rather than the Ancient Egyptian one. I have rectified the error below, and it makes no difference to the conclusion. When reading the comments, bear in mind that Barta was referring, quite correctly, to the earlier version.]

Joe Marino’s invaluable weekly round-up of Shroud news draws our attention to a new paper by César Barta and Elisa María Ruiz-Navas, called ‘Bleaching of Historical Old Linen Fabrics: the case of the Shroud of Turin.’ The premise, I think, was to determine if spectrographic techniques could verify how any particular fabric had been bleached, and, with reference to historical bleaching techniques, suggest the age of the fabric. Spectra from the Shroud could be compared with any findings. The conclusion is “To apply the findings of this study to the Shroud, the spectra of the Shroud provided in [the paper by Roger and Marion Gilbert] and digitized here in the selected band 1500–1700 cm⁻¹ reveal that the results are more in line with those obtained for the ancient sample than for the modern sample.” This is somewhat disingenuous, as the “ancient” sample referred to was from 16th century Spain, and does not appear to have been bleached at all. Undaunted, the authors appeal to other authorities: “It is possible to gain insight into the time of manufacturing of the Shroud if the different sequences of bleaching throughout history are analyzed. According to Rogers [see below, 1], “bands” of different white intensity can be observed along the Shroud, which could be due to the possibility that the linen was bleached by hanks of thread rather than whole cloth. Such bleaching of hanks is documented in Egypt during the time of the Roman Empire [see below, 2]. Medieval linens, however, were bleached as the whole cloth. This points to a bleaching method previous to the appearance of the Shroud in France in the 14th century.”

1). I discussed Rogers’s comments on bleaching in his ‘Chemist’s Perspective’ in an article for the British Society for the Turin Shroud, and the paper in general here at medievalshroud. He appeals to Pliny the Elder’s Natural History, saying, “According to Pliny the Elder, who wrote his Natural History in about AD 77, ancient linen yarn was spun by hand on a spindle whorl. When the spindle was full, the spinner prepared a hank of yarn for bleaching by the fuller. Each hank of yarn was bleached separately and each was a little different…” There is nothing in any of Pliny’s Natural History to suggest that this is true. I wonder where he got it from?
2). The reference for this is ‘L’Industrie Textile dans l’Egypte Romaine,’ by Ewa Wipszycka. It says nothing about “such bleaching of hanks” being documented in Egypt during the time of the Roman Empire or any other time.

The whole ‘hank-bleaching’ thing is an invention to try to place the Shroud further back in time than the Late Medieval, and is not justified by any contempory literature or archaeology. Visually the Shroud appears to have been bleached, making it much lighter than the 16th century cloth examined by the authors. The other samples were modern, some of which the authors seems to have bleached themselves.

2 is the 16th century Spanish sample.
10 and 7 are the same modern Portuguese sample which the authors bleached themselves using oak ashes.
Both these had a dark yellowy colour unlike ordinary raw linen, which was not very much whitened (in the case of 7) by the bleaching process. Oak ashes do not seem to be very effectual.

1 is a sample of raw linen from France. The colour is typical.
3 is a sample of Italian linen, which I think was bought raw and bleached by the authors using sodium hypochlorite.

4, 5 and 6 are from America. 4 is raw, 5 was bought bleached by an unknown process, and 6 is a part of 4 bleached by the authors using a natron recipe. Natron appears to have had no effect.

8 and 9 are German, the lighter one being part of the other, bleached by the authors using hydrogen peroxide.

All the samples were subjected to FTIR ATR analysis, and apart from the 16th century cloth, this is a generalised graph of the results:

When sodium hypochlorite or natron (mostly sodium carbonate) was used to bleach the sample, the bump at A shot up to a sharp spike, which led the authors to speculate that the bump was associated with natron bleaching, but in fact all the unbleached samples except the American ones also showed a similar bump. In general, bleaching reduced the intensity of the curve under C, and increased the intensity of the curve above A. What this means is anyone’s guess; suffice it to say that all these samples were modern.

The only old textile was the 16th century one, and its spectra were compared with data from the Shroud, as measured by the Gilberts, producing quite a close match.

The “natron peak” is missing, and there is a prominent bulge between 1550 and 1600 which is not present in the other samples. The authors intepret this as due to the absence of any bleaching process. “The band 1550 and 1600 cm⁻¹ in the spectrum of the 16th century sample must correspond to the lignin content, consistent with the absence of any bleaching process.” On the other hand, the same bulge does not appear in any of the other unbleached samples. The authors try to claim that a similar bulge also appears in Ancient Egyptian linen fabrics, but the paper they reference actually suggests that Ancient Egyptian fabrics did not have a similar spectrum. The paper was ‘First Evidence of Fractionation on Stable Organic Carbon Isotopes in Ancient Egyptian Linen Textile Fibers during Degradation,’ by Antonino Biagio Carbonaro, and contains a number of FTIR transmittance spectra like this (coloured lines mine):

a) This spectrum has a range of 500-4000 cm^-1, while the spectra above range from 1500-1800 cm^-1, so only the area within the green lines above is comparable.
b) This spectrum above is more or less the inverse of the spectra above, so must be inverted before a comparison can be made. Having done that, the spectra can be superimposed.

The Ancient Egyptian spectrum is in blue. The only major difference between it and the 16th century spectrum is in the slope of the graph between 1650 and 1800 cm^-1, and the Shroud corresponds much better to the 16th century than to Ancient Egypt. Interestingly, both the historic textiles show a decrease in intensity between 1500 and 1590 cm^-1, while both the Shroud spectra show an increase.

Actually I think this comparison is meaningless, but if an inference had be drawn from it, it must be that this evidence supports a medieval provenance much better than a first century one.

All the results associated with natron were disappointing. The authors tried to bleach their raw American cloth by following the very complicated process described by Robert Forbes in ‘Studies in Ancient Technology.’ It produced a huge spike in the FTIR spectrum at 1740 cm^-1 without bleaching the fabric at all, and the 16th century cloth (and the Shroud), which might have been expected to have been bleached with natron and to produce a pronounced 1740 cm^-1 peak, in fact produced nothing.

[I might digress slightly here and mention that nowhere in Forbes’ monumental work (9 volumes) is there any mention of ‘hank-bleaching,’ but several of the laying of the cloth in the fields to bleach in the sun, or steeped in vats of various chemicals.]

Sadly, this paper contributes nothing to the scholarship of the Shroud, but it could possibly lead to something if further studies based on it were undertaken.

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‘Bleaching of Historical Old Linen Fabrics: the Case of the Shroud of Turin,’ César Barta & Elisa María Ruiz-Navas, Heritage Science, 2026

‘Ultraviolet-Visible Reflectance and Fluorescence Spectra of the Shroud of Turin,’ Roger & Marion Gilbert, Applied Optics, 1980

A Chemist’s Perspective on the Shroud of Turin, Raymond Rogers, 2011

Naturalis Historia (Liber XIX), Pliny the Elder, c. 50AD

‘L’Industrie Textile dans l’Egypte Romaine, Ewa Wipszycka, 1965

Studies in Ancient Technology (Vol 4), Robert Forbes, 1964

‘First Evidence of Fractionation on Stable Organic Carbon Isotopes in Ancient Egyptian Linen Textile Fibers during Degradation,’ Antonino Biagio Carbonaro et al., Journal of Cultural Heritage, 2023