Terra sigillata pottery is the most famous fine ware of the Roman period and is characterised by the red colour of the body (or paste) and slip (or gloss). The term derives from its colour, which is similar to the reddish colour of clay (terra), and the frequent use of stamps or moulds (sigilla). Its great success was due to its remarkable gloss, also called slip, which provides a bright, deep red appearance. Electron energy loss spectroscopy in a transmission electron microscope (TEM-EELS) and x-ray diffraction (XRD) techniques were used for analysing the sub-microscopic structure of the material. The paper describes the analyses made on five shard samples from two important workshops in southern Gaul. These samples appear to have the same slip structure, consisting in sub-microscopic hematite and corundum crystals in a glass matrix. The colour of hematite depends on crystal size. The largest crystals have a black colour, while the red colour occurs when the crystals decrease to submicron size. The off-white or light-yellow corundum crystals could be acting as a brightener. -- ICCROM