Carbon dates on ancient mortar

Phillip Bigelow (
Wed, 08 Jan 1997 21:39:30 -0800

fmurray@pobox, frank murray wrote:
> it leads to the next it known
> where the materials for the mortar were obtained??...and were they
> obtained from a site or sites where stratification of the organic
> materials by age might be expected??..if so, might not the carbon
> dating be taken more as an indication of the level from which the
> materials were obtained, rather than of the date at which the mortar
> was laid??...

I have no knowledge of the mortar used in north Africa at that time,
but the following may be useful:
Most low-tech mortars that are used by peoples of the world today
contain a substantial amount of organic carbon. This carbon,
depending on manufacturing techniques and purpose, can be
accidentally-introduced or purposefully-introduced.
In lime-based mortars, most of the organic carbon is accidentally
introduced, and is mainly contamination from the soil.
This contamination is usually in the form of decomposed pieces
of grass, fungi, roots, and animal remains (such as feces from
burrowing organisms and worms) and lastly from microbial organisms
themselves. In short, "soil contamination" introduces recycled
organic carbon, which gets it's C13/C14 ratio PARTLY reset.
Grass particles, on the other hand, are always going to be
contemporaneous with the age of construction of the mortar,
because grass remains cannot survive for more than a few years
in soil.
On the other hand, if large pieces of logs are incorporated
into the mortar (highly unlikely!), then there is some room to
suggest that the logs could be much older than the construction.
With mortar, this issue isn't a problem. Large "suspect" pieces
of wood are not dated (or should not be dated!).

In mud/grass construction (not a "real" mortar),
the organic material is purposefully-incorporated into the mix.
The grass is mostly of a monospecies, and
is usually dried before incorporation into the mix. The grass
is usually collected in the late summer of the year (it is tallest
at that time).
In this case, radiocarbon dating would probably give close-
correlations between the contamination of the mud matrix and
the grass reinforcement. Both ages would be closely contemporaneous
with the date of the construction.