Other potential contaminants include paper, cardboard, cotton wool, string and cigarette ash.
Researchers had previously thought that many ideas spread by diffusion through the continent, or by invasions of peoples bringing new cultural ideas with them. In addition, a sample with a standard activity is measured, to provide a baseline for comparison. The quantity of material needed for testing depends on the sample type and the technology being used. The older a sample is, the less 14 C there is to be detected, and because the half-life of 14 C the period of time after which half of a given sample will have decayed is about 5, years, the oldest dates that can be reliably measured by this process date to around 50, years ago, although special preparation methods occasionally permit accurate analysis of older samples. Contamination with modern carbon causes a sample to appear to be younger than it really is: The ratio of 14 C to 12 C is approximately 1.