Tuscan black glosses: A mineralogical characterization by high resolution techniques

Tuscan black glosses on ancient pottery have been characterized using high resolution techniques. Ancient (4(th)-1(st) century B.C.) pottery from southern and northern Tuscany is coated with black glosses which show different visual appearance. Misfired red-black, matt, shining, metallic, and bluish black glosses have been studied with X-ray diffraction, scanning and transmission electron microscopes to establish their mineralogical, textural and chemical characteristics. The glosses are chemically homogeneous and they are all comprised of a Si-Al rich, amorphous matrix where nanometer-sized crystals of different nature are embedded. Crystals occur both as euhedral grains and as grains with a high aspect ratio. The misfired gloss contains hematite as the only mineral phase, both in the red and black portion of the coating. The matt black gloss contains mainly homogeneous hercynite crystals. In a small portion of the gloss, few magnetite crystals have been detected. However, no gradual variations either along or perpendicular to the gloss layer have been observed. Magnetite is the only mineral phase occurring in the shining metallic black gloss. The bluish-black gloss shows spinel grains with variable characteristics. Most grains are comprised of homogeneous hercynite with magnetite in solid solution. Other crystals are zoned: the core has hercynitic composition with relatively high content in magnetite, ulvospinel, and spinel s.s. in solid solution; Al and Mg increases towards the rim. These results clearly indicate that microanalytical techniques provide accurate mineralogical and microtextural characterizations at the nanometer scale. It has been shown that visually different glosses have different mineralogy which are mainly caused by different oxygen fugacities (f(O2)) during firing.