The influence of contaminating carbonate, not due to the normal carbonation process, on the age of the mortar is discussed. In most cases the activity of the sample and the isotope fractionation approach the real values at lower yield of the preparation reaction of the counting gas. The real activity and the age can then be calculated by extrapolation to the theoretical zero-point of the reaction. See also AATA 8-448, 11-12, and 21-131.