They contain carbon in which the radioactive decay process has started far earlier and where the content of 14C is a practically non-existent, and affect the radiocarbon dating indicating a higher age than the correct one.
Different carbonates will have different rates of dissolution, so that CO2 from the most soluble carbonates will be isolated in the first CO2 fractions. We have modeled the effects of different kinds of contaminants under different circumstances (i.e.
fire damaged mortar) and created models for the interpretation of age profiles and identification of the CO2 fraction(s) relevant for the dating of the sample.
The age BP (Before Present= AD 1950) of the relevant CO2 fraction(s) can then be converted into calendar years using a standard calibration curve for 14C dating.
Mortar is made by burning crushed limestone (calcium carbonate, Ca CO3) in a high temperature until it forms unslaked lime (calcium oxide, Ca O), whereby carbon dioxide (CO2) is emitted.
The process of making mortar Mortar is an inorganic material, but the principle behind 14C-dating of mortar is the same as in 14C-dating of organic materials.
The calcium oxide is slaked with water and converted into calcium hydroxide (slaked lime, Ca(OH)2).
It is then mixed with water and an aggregate (usually sand, gravel or volcanic pyroclastic materials) to form mortar.
When the mortar hardens it absorbs carbon dioxide from the atmosphere and forms calcium carbonate.
In principle, the hardening process corresponds to the death of the living organism; the point in time when no new 14C is formed and when the radiometric clock starts to tick.