Among the different compositional groups of Roman glasses, one group of unknown origin is characterized by low levels of aluminium, magnesium, iron and phosphorus as well as by the use of a decolorizer : antimony (compositional group 4). This compositional group was first identified in Great Britain. The results of our analyses undertaken on the cargo of a shipwreck and on a lot of samples coming from numerous Mediterranean sites but also from two secondary workshops in Gaul reveal/show the same composition or variants. The latter may be due to multiple origins, which still has to be proved. But they would rather be due to natural variations within the sands of a same area or to pollutions or mixtures caused by the remeltings in the secondary workshops. This compositional group, named group 4, is important mainly in the 2nd century AD and in the beginning of the 3rd century AD. Its existence means that others production centres of perfectly colourless glass seriously competed with the production centres of the syro-palestinian coast, which up to this period had almost the monopoly of glass trade towards Occident.

With the development of micro-LIBS imaging, the ever-increasing size of data sets (sometimes >1 million spectra) makes the processing of spectral data difficult and time consuming. Advanced statistical methods have...

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