Friday, December 12, 2014

True Colors - Chemists Look at Pottery (Paper)

These guys examine the color intent of Beaker pottery in Northern Spain.

While it's not surprising that the pottery was always red and usually encrusted, it's interesting to take a fragment apart and chemically look at intended color or its post-kiln color of 4.5k years ago.

I've remarked that achieving ideal color was probably difficult for native potter houses due to European geology.  [here]  and [here] The millennia old fragments usually appear pale, pinkish, copper or blood red, as in the Mesetas, but they are always reddish.  It has only been recently that microscopic and chemical analysis has begun to reveal new details about their design.

A Pasted Bohemian Beaker replica.  "Beaker Days, 2005"


In this paper we characterise the mineralogical and elemental composition and the colour (CIELab space) of Bronze Age pottery sherds from NW Spain, using X-Ray diffraction, X-Ray fluorescence and reflectance spectroscopy, respectively. For half of the samples we also determined the content in secondary iron oxi-hydroxides (sFe, iron extracted with dithionite-citrate), using atomic absorption. The aim of the investigation was to study the relationship between the colour and the elemental and mineralogical composition, and to explore the intentionality of the resulting colour. Samples had a low luminosity and were located in the quadrant of the CIELab space ranging from red to yellow (hab: 0-90°), showing low hue variability but a wider range of variation in chromaticity. In terms of composition they showed a large mineralogical (12 different minerals were identified) and chemical (from acidic/felsic to basic-ultrabasic/mafic compositions) variation.
A principal components analysis using elemental composition and colour parameters demonstrated that luminosity (L*) depends on organic matter (OM) content and to a lesser extent on sFe content. Chromaticity (C*ab) depends on sFe content, but also on the felsic/mafic relative composition and OM content, while hue (hab) is only related to iron mineral phases. We also verified that these general trends differ to a certain extent depending on whether the pottery contains amphibole or not: the effect of sFe on L* and of OM on b* (yellowing) and C*ab was only detected for pottery sherds without amphibole, while an increase in felsic in relation to mafic minerals has a more decisive effect on the chromaticity (C*ab) of the amphibolic clays. Thus, colour seems to result from the interplay between i) the original colour of the raw material/clays, ii) compositional factors (overall composition -felsic vs mafic-, and sFe and OM content), and iii) interactions between composition and processing (sFe and firing conditions controlling yellowing). We interpret that there was an intentional selection of raw materials (felsic or mafic) and their processing (addition of iron oxides and organic matter) and a control over the firing conditions in order to give the vessels a specific colour.

The colour of ceramics from Bell Beaker contexts in NW Spain: relation to elemental composition and mineralogy.  Journal of Archaological Science.  Oscar Lantes-Suarez, Beatriz Prieto, M. Pilar Prieto-Martinez, Cruz Ferro-Vazquez, Antonio Martinez-Cortizas  (2014)  [Link]

No comments:

Post a Comment