ASSESSMENT OF THE PRODUCTION OF ANTIQUITY PIGMENTS THROUGH EXPERIMENTAL TREATMENT OF OCHRES AND OTHER IRON BASED PRECURSORS

Abstract

In this work we explore the effects of various grinding and thermal‐oxidative treatments applied to natural and artificial iron‐based materials available (or related to those available) during GraecoRoman antiquity. The raw materials examined are: (a) commercial natural iron pigments (ochres, natrojarosite, caput mortum), (b) artificial melanterite (FeSO4. 7H2O), (c) mineral pyrite (FeS2) and mineral metallic hematite. Additionally explored are: (a) the non‐attested in surviving sources, yet highly probable during antiquity, route of pigment preparation from iron (or steel) plates exposed to vinegar vapors, (b) a Vitruvius recipe for purplish pigment via vinegar quenching of hot ochre.   We obtain oxide pigments with colors ranging from yellowish and red to brownish and purplish. The puzzling variation of colors obtained by subjecting iron‐oxide con‐ taining materials to identical oxidative heat treatments is found to be explainable on the basis of starting grain size and possible size modifications. We also show, by using highly purity starting materials, that purplish colors obtainable in certain cases by heat treatment do not necessitate, as often claimed, the presence of impurities such as manganese etc. A framework of antiquity color possibilities for iron‐oxide based pigments obtainable under the conditions explored is included. All samples prepared are exam‐ ined via scanning electron microscopy for micromorphology coupled with EDAX for composition, and X‐Rays Diffraction for mineralogy.