[lepton]

Radiocarbon dating is not interested in the total amount of carbon, but rather the relative quantities of carbon isotopes. If the shroud was burnt, and its carbon impregnated into other portions of the shroud, that is not likely to make a significant difference in the test results...however if other cloth or wood was burnt with it, or the water was fetid or otherwise contaminated, that introduction of "new" carbon would affect the tests.

[aruanan]

There was a lot of microbial plaque on the cloth from over the centuries. This would tend to skew the C14 date toward the present.