State of Research
The International Mortar Dating project, initiated in 1994 with the churches of the Åland Islands, has gathered a large databank with samples from different parts of Europe.
At the present we have performed more than 530 analyses of 465 individual mortar samples all over Europe. Those marked in orange on the map are sites with lime mortar, those in green mark pozzolana mortars. The majority of samples come from the Åland Islands, but other parts are steadily growing.
Mortar dating statistics
Over the years different types of mortar have been tested. The diagram shows the proportions of different types of mortar, the type of hydrolosis used, and the proportions between samples from Classical and Medieval Archaeology.
In the Åland churches it was possible to validate the method comparing the results with dates known from other methods and other materials. With age control available the Åland samples turned out very successful, 96% coincided with the known age with the first CO2 fractions. The chronology of the medieval stone churches in the Åland Islands was established through mortar dating.
Analyzing pozzolana mortars from Rome, however, proved more problematic. The age profiles were more difficult to interpret, some of the age profiles pinpointed the known age with the first CO2 fractions, others with a horizontal plateau in the middle of the profile. Different variations of the age profiles were present within the identical building constructions. Feasible results were yielded by 50 % of the samples. The remaining 50% presented unsuccessful results, but for reasons well known. Samples from Pompeii and Herculaneum continuously went wrong.
In the preparatory procedure mainly phosphorous acid has been used within the project. In addition to that, for experimental reasons some 60 samples were separated with hydrochlorid acid, both with pozzolana and with lime mortars. The result of the experiment was that phosphorous acid remains the acid used by the project.
In 1994 the first lime lump was analyzed, from Hammarland church in Åland. The result was convincing, showing the same age for the two fractions analyzed. In this case the rate of success of the Åland mortars had an effect of postponing the development of the method. It is only recently that the research has focused on dating lime lumps. With the 28 samples so far analyzed, from all parts of Europe, from different types of mortar and pozzolana, only two went wrong, those from Pompeii. Lime lumps from fire-damaged mortars were also successful. We therefore think that dating lime lumps is the way to proceed, especially with Roman pozzolana, where the first CO2 fractions so far hit the known age.
What we learned from analyzing lime-lumps:
- Differently from earlier presumptions – Analyzing lime lumps embedded in the mortar is NOT necessarily the most reliable method.
- Apparently differences between different types of mortar. Lime lumps taken from sheltered places in Medieval Scandinavian structures (Åland and Sweden) generally yield reliable results, whereas lime lumps from Classical or Medieval structures in the Mediterranean area are less suited for dating.
- In Classical archaeology (The Colosseum, Torre de Palma) the results of lime lump analysis is less convincing, regardless of sampling situation. Regardless of whether unsheltered ruins open to erosion, or if samples have been safely covered.
- Lime lumps also need to be analyzed in successive fractions to avoid contamination of insufficient burning.
- The importance of comparative age control from different independent dating methods.
Dating of bulk mortars continues to be the most important and reliable method IF:
Lime mortars are analyzed in complete age profiles, and the results meet with the different demands of Criteria I and II.
- H3PO4 hydrolysis is used in the preparatory process, and the sample is analyzed in successive fractions.
- Here the first CO2 fraction tends to provide the most plausible result.
- The most reliable result: if the result of bulk mortar analysis can be confirmed by analysis of lime lumps within the same mortar sample, and the two agree.
Therefore the present state of research is focused a combination of lime lumps in two CO2 fractions, and bulk mortars, both lime mortars and in hydraulic pozzolanas. Chemical separation with hydrochlorid acid is clearly suited to some mortars, whereas it consistently yields wrong result in medieval Scandinavian mortars. The reason for this can hopefully be solved with the inter-comparison of different preparation patterns. The development of the method continues in close collaboration with the international and interdisciplinary network, in joint publications and joint presentations at scientific conferences.