A method of absolute (chronometric) dating which is hoped to fill the gap between radiocarbon dates and potassium-argon dates. It is used for human and animal bone and other organic material. Specific changes in its amino acid structure (racemization or epimerization) which occur at a slow, relatively uniform rate, are measured after the organism's death. The basis for the technique is the fact that almost all amino acids change from optically active to optically passive compounds (racemize) over a period of time. Aspartic acid is the compound most often used because it has a half-life of 15,000-20,000 years and allows dates from 5,000-100,000 years to be calculated. However, racemization is very much affected by environmental factors such as temperature change. If there has been significant change in the temperature during the time in which the object is buried, the result is flawed. Other problems of contamination have occurred, so the technique is not fully established. It is fairly reliable for deep-sea sediments as the temperature is generally more stable.