Patination of Cultural Flints

Abstract

All flints containing unstable impurities are susceptible to patination. The rate of patination varies with many factors: (i) the texture and microstructure of the flint; (ii) its permeability; (iii) the kind, proportion and distribution of impurities; and (iv) environmental factors, such as temperature and soil chemistry. The thickness of the patina varies also with time. Two contrasting types of patina can develop: a chalky white patina and a ferruginous brown patina. Both types are observable primarily as a color change, and study of these types is facilitated by a clear understanding of the causes of color in flint. The color of most flints is the result of repeated refraction and reflection of light at numerous intergranular surfaces, whereby part of the light is internally absorbed and part is reflected back to the observer. The ratio of reflected to absorbed light governs the lightness of the color, or its value. The preferential absorption of certain wavelengths by natural pigments (such as iron oxide and hydrous iron oxide) disseminated through the flint determine the hue of the color. The color changes produced during patination relate to changes in texture and impurity content occasioned by the attack of weathering agents on impurities in the flint. The creation of voids by the dissolution and leaching of carbonates, the loosening of quartz crystallites, and the dispersal of clays all modify the reflectivity of the flint. Chemical changes involving the epigments, their dispersal along intergranular surfaces, or removal by leachig modify both reflectivity and capacity to preferentially absorb. Attempts to correlate patina thickness with age, and thus to use flint patinae chronometrically, have proven unsatisfactory because other factors, whose importance in some cases exceeds that of age, have not been taken into account. The texture and microstructure of flint, its permeability, and the kind, proportion, and distribution of impurities can be evaluated by regular petrographic techniques. Environmental factors can be assumed constant for artifacts from the same types of soil in a given climatic region. Only after allowances have been made for these additional variables does the age-dependence of flint patination become clear.