Does this have to do with the pollen? I remember hearing or reading about how a test was done on pollen found on the Shroud. I guess it was a carbon dating. Anyway it showed the pollen was from the Middle Ages. Then a scientist discovered that there was a crust of pollution on the pollen. Once that was scraped off another carbon dating test showed the pollen was from around 100 AD and had been of plants that grew in Jerusalem at that time.
No. Shroudie is the expert on this thread, but I'll take a crack at the answer (to the extent I remember).
The individual fibers were/are coated with a shellac like substance (the details of which I do not remember). It was shown to be the residue of some biological process (the details of which I also do not remember). (Hmmm... Not so good. Now where did I put my car keys?)
The coating is clear, and thick enough to skew the carbon-14 results. If it was deposited at a linear rate from the 1st century until the date of the carbon-14 testing, it would have added enough mass to move the date from the 1st century to the middle ages. Hence my interest. (If I had known someone would ask me about it a few years later, I would have made a better effort to memorize the details!)
Not only is this substance present on the shroud, but it has been found on other ancient artifacts as well. As you probably know, there have been some highly surprising carbon-14 results on tested artifacts, and some experts have questioned the reliability of the process itself as a result. This could be part of the explanation. Simply knowing how fast carbon-14 decays may not be enough.