Archaeology Research

The origin of Melian alum

A non-technical summary prepared for the NERC
(NERC award NER/B/S/2000/00301)

Aluminium sulphate (alum) and sulphur mineralisation of archaeological significance, Melos, Greece: a stable isotope study

A.J Hall and A.E. Fallick

There are many minerals which play a 'discrete' yet important everyday role in modern life and this was undoubtedly true in Antiquity. Minerals can have industrial uses, such as quartz sand for glass manufacture or domestic uses, such as absorbant clay for cat litter. Many are produced on a major scale for worldwide marketing and most have diverse uses.

We know from Pliny and other classical authors that two thousand years ago alumen was such an industrial mineral being used as a chemical throughout the Roman world to fix bright dyes and as a medicine, with "thirty-eight remedies" related to its astringency. Alumen (now known as alum) is said to have been at the basis of the modern chemical industry because of its economic importance in dyeing. The small island of Melos in the southern Aegean produced particularly good alumen in Roman times. Melos is particulary rich in industrial minerals related to volcanic processes and has a long history, nine thousand years, of mineral production. There were indications of possible mineral processing sites but no conclusive evidence for Roman workings. Our research set out to understand the nature and origin of Melian 'alum' as a basis for future archaeological surveys for evidence of Roman mineral production on Melos.

We suspected that alumen was an aluminium sulphate mineral like modern alum and we knew from our earlier work that this occurred in SE Melos. There are many very similar sulphate minerals impossible to identify in the field, so we sampled deposits of white minerals, often efflorescences and some related minerals and rocks.

We analysed our samples to obtain their mineralogical and chemical compositions. Analyses of sulphur and oxygen isotopes helped us to understand the origin of the minerals. Both sulphur and 'alum' form at sulphurous fumaroles which are common in the geothermally active post-volcanic landscape of Melos. The process is rather like enhanced weathering because of the sulphuric acid formed in the hot ground. 

Pliny said that pomegranate juice could be used to test for contaminated alumen. We have demonstrated that pomegranate juice is a very sensitive test for iron impurity in alumen. Iron would darken dyes fixed using impure alumen. We used computerised theoretical chemical methods to demonstrate how iron and other impurities could be removed from alumen 'harvested' from around fumaroles.

Our research has given us a greater understanding of the nature of Melian 'alum', how its formation requires an unusual environmental setting, and how it may have been worked in Roman times. This geological/mineralogical research will guide future archaeological surveys for Roman mineral workings, and potentially enhance the archaeological record of early mining on Melos where a Museum of Mining History has recently been opened. Our geoarchaeological research is also contributing to the history of the chemical industry as well as the interpretation of classical texts.

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