Here is some information on radiocarbon dating, prepared by English Heritage's radiocarbon advisor.
About 10 years ago, a new method for the radiocarbon dating of cremated bone was proposed by a research group at the Rijksuniversiteit Groningen (Lanting et al 2001). This followed some work on dating the carbonate fraction of unburnt bone from the Sahara by a group based in Lyon (Saliège et al 1998). This method works by dating the structural carbonate fraction of bone. The carbon in this fraction derives from the whole diet of the dated individual (not just the protein component that dominates the carbon in the collagen fraction), and so is much less susceptable to dietary offsets than dates on collagen. Also structural carbonate often survives in situations (such as in cremations) where collagen diagenesis makes this fraction unsuitable for radiocarbon dating.
Unfortunately, early attempts to date the carbonate fraction of unburnt bone (largely in the 1960s) gave dates that were anomalously young because the bone carbonate exchanges with humic acids in the burial environment. Consequently only in special, very dry environments, such as in the Sahara did bone carbonate give accurate radiocarbon ages. The major advance of the new method was to isolate the part of the structural carbonate in cremated bone that has had its crystalline structure altered by the cremation process in such a way as it is no longer contaminated by the burial environment (van Strydonck et al 2005). So, when dating cremated bone we are dating the time when the individual died, but the sample becomes datable because of the cremation process itself. This is why we need a 2-4g from a single piece of white, calcined bone. The crystalline structure in less calcined material has been insufficiently altered for accurate dating. Because this method was new, and was dating a sample type which had previously proved extremely problematic, it underwent extensive testing in the early 2000s in radiocarbon laboratories in many countries (eg De Mulder et al 2004; Naysmith et al 2007). It has been shown to produce accurate radiocarbon dates routinely and has now been adopted as a standard technique worldwide.
Lanting, J N, Aerts-Bijma, A T, and van der Plicht, J, 2001 Dating of cremated bones, Radiocarbon, 43, 249-54
Naysmith, P, Scott, E M, Cook, G T, Heinemeier, J, van der Plicht, J, Van Strydonck, M, Bronk Ramsey, C, Grootes, P M, and Freeman, S P H T, 2007 A cremated bone inter-comparison study, Radiocarbon, 49, 403-8
Saliège, J-F, Person, A, and Paris, F, 1998 Datation du carbonate-hydroxylapatite d'ossements Holocènes du Sahel (Mali, Mauritanie, Niger), Pré-actes du 3ème Congrès International 14C et Archéologie, Lyon 1998, 172-3
van Strydonck, M, Boudin, M, Hoefens, M, and de Mulder, G, 2005 14C-dating of cremated bones-why does it work?, Lunula, 13, 3-10
De Mulder, G, van Strydonck, M, and Boudin, M, 2004 14C-dateringen op gecremeerde menselijk botten uit de urnenvelden te Velzeke (O.-Vl.), Lunula, 12, 51-58