The following is from pages 15 and 16 of Scientific Method Applied to the Shroud of Turin: A Review by Raymond N. Rogers and Anna Arnoldi published at shroud.com.
(D) Lignin amounts vary among Shroud locations. X-ray-transmission[4,22], contrast-enhanced, ultraviolet, and transmitted-light photographs of the Shroud all show specific, discrete bands of yarn with different x-ray densities and corresponding color densities (figure 3). Both warp and weft yarns show this property. Some areas show darker warp yarns and some show darker weft yarns. In some places bands of darker color cross. In other places bands of lighter color cross. The effect is somewhat like a plaid. Many photographs of the Shroud can be viewed on the Shroud web site: http://www.shroud.com.
Linen is bleached to remove the lignin in an attempt to render it pure white. The more quantitative the bleaching process the whiter the product. The bands of different color on the Shroud are the result of different amounts of lignin left from the bleaching process. The tape samples reflect this variation as observed differences among quantitative measurements of lignin on the fibers.16
A conservator at Turin’s Museum of Egyptology, Anna Maria Donadoni, pointed out locations where batches of yarn ended in the weave and new yarn had been inserted in order to continue weaving. The yarn ends were laid side by side, and the weave was compressed with the comb. The ends are often visible, and the overlaps appear to correspond to zones of different color in the weave.
I believe that the observations of bands of different colors agree with Pliny the Elder’s description of ancient linen-production technology. 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; indeed, different parts of the same hank show slightly different colors, a little like variegated yarn. The warp yarn was protected with starch during the weaving process, making the cloth stiff. The final cloth was washed with "struthium," Saponaria officinalis, to make it more supple.
Medieval linen was bleached as the whole cloth. Most commercial bleaching took place in "bleach fields" in the Low Countries, the genesis of the name "Holland cloth" for the Medieval backing on the Shroud. Considerable material was lost during the bleaching process, and the newer linens are less dense than ancient linens, as can be seen by comparing the Holland cloth and patches with the main part of the Shroud. The newer linens are also homogeneous. They do not show bands of different-colored yarn in the weave.
A phloroglucinol-hydrochloric-acid reagent detects vanillin (4-hydroxy-2-methoxybenzaldehyde) with great sensitivity. Fresh lignin evolves vanillin in the reagent. You can often smell the vanillin that is evolved from the lignin of warm pine-tree bark. The lignin loses vanillin with time and temperature. The lignin on older samples of linen gives progressively weaker tests for vanillin as age increases. The lignin on Shroud samples does not give the test. That fact could indicate either significant age for the Shroud or accelerated aging of the lignin as a result of heating during the fire of AD 1532. Differences between amounts of lignin on linen fibers in the Raes samples and on Shroud fibers are significant. There is probably a similar difference between the radiocarbon samples and the main part of the Shroud.
A little over a year ago a reader of this blog wrote to me saying:
I hate to inform you but Ray Rogers was wrong. Pliny the Elder never said that Saponaria officinalis was used in the bleaching of linen. What he said is, “There is another kind of wild poppy (a spurge rather, Euphorbia esula of Linnaeus), known as "heraclion" by some persons, and as "aphron" by others. The leaves of it, when seen from a distance, have all the appearance of sparrows; the root lies on the surface of the ground, and the seed has exactly the colour of foam. This plant is used for the purpose of bleaching linen cloths in summer.” — Pliny the Elder, The Natural Histories, Book 20, Chap.79
The heraclion plant is not Struthium (soapwort, Saponaria officinalis).
I wrote back at the time:
It was Theophrastus who said that struthium was used for bleaching linen. Pliny, according to some sources, misunderstood Theophrastus to mean heraclion.
Was Saponaria officinalis used for bleaching linen 2000 years ago? Yes. Do we need to correct about a million web pages at this point? Probably.
It is a bit curious where Rogers got his information.
Now as for the truth about banding. My guess is that Rogers is right; Medieval linen, particularly in Europe, was field bleached because the technology and the linen production industry had advanced by that time. How do we prove it?