Since the end of 2010 our laboratory has a Fourier-transform infrared spectrometer (FTIR spectrometer) for the analysis of chemical compounds. Our instrument is a Bruker ALPHA-P with diamond ATR.
FTIR spectrometry is based on the absorption of infrared radiation by the analyzed material. It allows the analysis of chemical compounds present in the material via the detection of the characteristic vibrations of their chemical bonds. To each material with its specific chemical composition and structure belong a series of absorption bands at characteristic wavelengths that show themselves as more or less large peaks in the measuring curves (infrared spectra). The specific composition of the examined material can be derived by comparing the spectra with reference spectra of known compounds.
Application example:
In the following figure, the infrared spectra of two unknown materials taken from a Roman ruin are shown.
FTIR-Spectra of two materials sampled on a Roman ruin.
The rapid comparison of the spectra shows that the materials of the two samples have to a large extent the same chemical composition.
After the customer having indicated that the ruin in question had been covered with bird nests in the past, we compared the spectra of the samples with the spectrum of hydroxyapatite a calcium phosphate with the chemical formula Ca5(PO4)3(OH) and the similarity between the three spectra could immediately be seen (see figure below). This analysis is also in accordance with the optical and chemical properties of the material.
Comparison of the sample spectra with the spectrum of hydroxyapatite from the website http://rruff.info/hydroxylapatit/display=default/R050512
Therefore we could inform our customer that the Roman ruin was covered, not with polychromies, but with hydroxyapatite, which was most likely formed by the reaction between bird droppings – rich in phosphates – and the lime present in the materials of ruin.
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Bird droppings can form coatings on buildings, which can look very similar to the paint remnants.