title: Analysis of ochres from Clearwell Caves: the role of particle size in determining colour.
abstract: Three ochre samples (A (orange-red in colour), B (red) and C (purple)) from Clearwell Caves, (Gloucestershire, UK) have been examined using an integrated analytical methodology based on the techniques of IR and diffuse reflectance UV-visible-NIR spectroscopy, X-ray diffraction, elemental analysis by ICP-AES and particle size analysis. It is shown that the chromophore in each case is haematite. The differences in colour may be accounted for by (i) different mineralogical and chemical composition in the case of the orange ochre, where higher levels of dolomite and copper are seen and (ii) an unusual particle size distribution in the case of the purple ochre. When the purple ochre was ground to give the same particle size distribution as the red ochre then the colours of the two samples became indistinguishable. An analysis has now been completed of a range of ochre samples with colours from yellow to purple from the important site of Clearwell Caves.
authors.affiliation: School of Chemistry, The University of Reading, Whiteknights, Reading RG6 6AD, UK.
authors.forename: Lisa-Jane R
authors.initials: LJ
authors.lastname: Marshall
authors.forename: Joanne R
authors.initials: JR
authors.lastname: Williams
authors.forename: Matthew J
authors.initials: MJ
authors.lastname: Almond
authors.forename: Samantha D M
authors.initials: SD
authors.lastname: Atkinson
authors.forename: Samantha R
authors.initials: SR
authors.lastname: Cook
authors.forename: Wendy
authors.initials: W
authors.lastname: Matthews
authors.forename: Joanne L
authors.initials: JL
authors.lastname: Mortimore
mesh_terms: D002119:Calcium Carbonate / Q000737:chemistry; D003116:Color; D004058:Diffusion; D005290:Ferric Compounds / Q000737:chemistry; D007477:Ions / Q000737:chemistry; D008274:Magnesium / Q000737:chemistry; D008670:Metals / Q000737:chemistry; D010316:Particle Size; D010860:Pigments, Biological / Q000737:chemistry*; D013057:Spectrum Analysis; D006113:United Kingdom; D014961:X-Ray Diffraction
chemical_list: D005290:Ferric Compounds; D007477:Ions; D008670:Metals; D010860:Pigments, Biological; C000499:ferric oxide; D002119:Calcium Carbonate; D008274:Magnesium; C028042:calcium magnesium carbonate