Application of analytical chemistry in the early stages of an archaeological excavations

Analytical chemistry is the branch of chemistry that deals with the qualitative and quantitative identification of specific components called "analytes". It can be useful, in geoarcheology, to investigate the composition of the geological matrix of a given sample.

This science makes use of recognition techniques based on the reactivity and analyzes the tendency of a particular chemical species to react in the presence of specific reagents; in other words, under a practical aspect, it is possible to induce a reaction and to exploit its sensitive aspects to identify the presence or the absence of the analyte we are looking for.

During an archaeological excavation, it can be useful to know if, in the sediment we are removing, are present some components which can be considered as markers of specific past human activities. This kind of archaeological markers are organic substance, carbonates and phosphates and they may indicate the presence of paleosurfaces, middens, agricultural activities (like fertilization) or craftsmanship sites.

The organic substance is composed by chains of carbon, with oxidation state lower than +4, and hydrogen, that bind a number of heteroatoms (mainly oxygen, nitrogen, sulfur and phosphorus); the diagnosis involves the use of hydrogen peroxide [H₂O₂], as a liquid solution, which is poured directly on the soil sample; if some organic substances are present, they generate bubbles with effervescence; the more elevated is the organic concentration, the greater is the effervescence and vice versa.

The carbonates are chemical compounds which contain molecules made of a carbon and three oxygens [CO₃^2-] that bind other elements; the diagnosis involves the use of hydrochloric acid [HCl] in a liquid solution, poured directly on the sample. Also in this case, if the test is positive, it generates effervescence with bubbles (with a direct proportion with the carbonates presence).

This process is illustrated in the following video (H₂O₂ and HCl test from an old Arc-Team's excavation).

The phosphates are chemical compounds which contain molecules made ​​of a phosphorus and four oxygens [PO₄^3-] that bind other elements; the diagnosis involves the use of a reagent, in form of powder, consisting of ammonium molybdate [(NH₄)₆Mo₇O₂₄.4H₂O], antimony potassium tartrate [C₈H₄K₂O₁₂Sb₂.3H₂O] and ascorbic acid [C₆H₈O₆]; such a reagent is mixed with the solution of distilled water in which has been previously dissolved the soil sample: if some phosphates are present, the aqueous solution turns from azure to blue, depending on the concentration of this chemical element.
The process is illustrated in the following videos (phosphates test from an old Arc-Team's excavation):

1. Preparation of the soil sample

 2. Preparation of the analysis

 3. Reading the results

PS
For Italian readers, sorry for the jargon in the videos... everything is recorded from reality