Although charring doesn’t completely destroy cereal grains and other plant parts, it does cause some changes. Lots of experimental work has shown that heating modern cereal grains to no more than 230˚C for more than 8 hours produces grains that look like their archaeological counterparts. Any hotter than this and they start to “popcorn”, bursting out of their epidermis and distorting their shape so that archaeobotanists can no longer identify them.
Knowing this means that we can investigate the effect of charring on modern grains at 230˚C to get a real handle on any changes that the charring might be making to the isotope values. In my previous work, we found that heating causes a loss of gases, containing both carbon and nitrogen. The loss of carbon and nitrogen means that there is a potential for a change in the isotope values of the charred grains. Remember that the lighter isotope moves faster and will therefore be more likely to leave the grains.
This is exactly what Fraser et al. (2013) found: charred grains had a heavier nitrogen isotope value than uncharred grains. The carbon isotope value stayed the same, suggesting that carbon from different molecules in the grains was lost – some with a heavier isotope value and others with a lighter. Having established that there is a change in the nitrogen isotope value with charring, we need to test this for all relevant crop species, so that we have an exact offset that can be subtracted from the archaeological grain values to give us their uncharred isotope values.