Economic geology: Oxford Ochre.
"Oxford ochres are yellow ochres of exceptional purity, being composed almost entirely of the yellow iron oxyhydroxide mineral goethite."
WHEATLEY WINDMILL & OCHRE PITS
Dr Ruth Siddall writes... The Oxford Ochre deposits are derived from a series of quartz and iron-rich sands of Lower Cretaceous, probably Valanginian age, the Whitchurch Sand Formation. It overlies the Jurassic Purbeck, Portland and Kimmeridge Clay Formations. The Whitchurch Sands extend from Wiltshire through to Buckinghamshire and outcrop as outliers capping the hills. East and north-east of Oxford these include; Shotover Hill, Brill Hill, Long Crendon, Quainton Hill and of course at Whitchurch, near Aylesbury in Buckinghamshire. The Whitchurch Sands are variable in thickness ranging from very thin sandy seams to a maximum of approximately 20 m. The ochre deposits are clay-rich layers within the ‘iron sands’ which represent freshwater conditions, representing a river and its floodplain.
The natural historian Robert Plot wrote in 1677, “And amongst these the Ochre of Shotover, no doubt may challenge a principal place, it being accounted the best in its kind in the World, of a Yellow Colour and very Weighty, much used by Painters simply of it self, and as often mix’d with the rest of their colours. ... They dig it now at Shotover on the East side of the hill, on the right hand of the way leading from Oxford to Whately [Wheatley]... the earth is here, as at most other places, I think I may say of a bulbous nature, several folds of divers colours and consistencies, still including
one another, not unlike the several coats of a Tulip root, or onyon.”
Plott’s description of the ochres is well observed. They are concentrated in thin strata, inter-bedded with non-ochreous sandy and clay-rich layers. These were hard won as many barren layers had to be removed before an ochre seam was revealed. Goethite occurs in several bands within the Whitchurch Sands and can be remarkably pure. These layers of pure goethite were known as the stone ochres or native ochres. Second quality ochre where the goethite was mixed with
impurities was known as clay ochre and Plott records that:
“ ... because of the natural Inequality in its Goodness, they wash and steep two or three days in Water, and then beat it with Clubs on a Plank into thin broad cakes, of an equal Mixture both of good and bad: then they cut it into Squares like Tiles, and put it on Hurdles laid on Trestles to dry, which when thoroughly done ‘tis fit for the Merchant.”
Robert Plott 1677
Mining of ochre at Shotover and Wheatley continued until the early 20th Century. The Windmill at Wheatley was adapted to grind the ochres which were used as raw pigment and they were also burned to form red ochre pigments; goethite transforms to the red iron oxide hematite on heating to temperatures of 230-280°C.
Although the Whitchurch Sands also crop out on the summits of the surrounding hills, that from Shotover itself was considered the best and those from places such as Garsington, to the south east of Shotover and Witney, to the west of Oxford were considered of lower quality.
The material mined was generally known as English Ochre (the name Oxford Ochre probably being adopted in the early 19th Century). In addition to Plott’s account, various other authors refer to its use and quarrying as an artists’ pigment. The painting by William Turner (1789-1862) ‘Gravel Pit on Shotover Hill’ (Ashmolean Museum cat. WA1942.61) is also an important source of evidence. The ‘onyon’-like layers are clearly seen. Similar strata are also observed in the BGS image archives of the ochre pits at Wheatley (below).
References to English Ochre occur in a number of contemporary texts. In addition to Plotts Natural History, John Smith in 16766 remarked that English Ochre came from Shotover Hill. In the 19th Century, Salter writes that Oxford Ochre is ‘semi-opaque, of a warm yellow colour and soft argillaceous texture, absorbent of water and oil’. Other sources are quoted in Rosamund Harvey’s book on Artists’ Pigments (1982).
Secure identification and provenance of ochres in works of art is intrinsically difficult as there is no method by which the iron oxide minerals can be either visually or chemically fingerprinted. The purer the material, the harder this gets as there are no extraneous minerals which my be diagnositic of sources. All that may be done is to compare the appearrance and purity of materials used in works of art with raw and processed ochres proven to be collected in the right area. Of course, the ochres were mechanically processed which would have also improved their purity. Harley (1982) reports that there is little evidence in the British trade and custom records for the import of yellow ochres from Italy during the 16th-17th centuries, assuming that more local sources were used (this may not discount French Ochres though). This does imply that ‘local’ materials were primarily used in British painting and it is probable that the ochre was widely used. Its warm yellow colour would have made it especially suitable for the depiction of cloth of gold or jewellry on many paintings of the period, and it indeed, may have been used by William Turner in his painting of the pits on Shotover Hill.
SHOTOVER PITS BY WILLIAM TURNER of OXFORD (1789-1862)
SPECIMENS OF OXFORD OCHRES (Ruth Siddall, UCL)
BGS ARCHIVE IMAGE OF WHEATLEY OCHRE PITS)
Resources consulted by the author:
Horton, A., Stumbler, M.G., Cox, B. M. & Ambrose, K., 1995, Gelogy of the Country around Thame., Memoir of the British Geological Survey, Sheet 237 (England and Wales)., 169 pp.
Morter, A. A., 1984, Purbeck-Wealden Beds Mollusca and their relationship to ostracod biostratigraphy, stratigraphical correlation and palaeoecology in the Weald and adjacent areas., Proceedings of the Geologists’ Association., 95 (3), 217-234.
Eastaugh, N., Walsh, V., Chaplin, T., Siddall, R. (2008), Pigment Compendium: A Dictionary and Optical Microscopy of Historic Pigments. London: Butterworth-Heinemann. ISBN: 0750689803. 960pp.
Plot, R. 1677. The Natural History of Oxford-shire, being an essay towards the Natural History of England. Printed at the Theater, Oxford. 358 pp., 16 pls. A description of the outcrop occurs on p. 56.
BGS Geoscenic Image P203149; http://bit.ly/1uyPKuH
Smith, J., 1676, The Art of Painting: wherein is included the whole art of vulgar painting according to the best and most approved rules for preparing and laying on of oyl colours. London.
Salter, T. W., 1869, Fields Chromatography, or treatise on colours and pigments as used by artists., Winsor & Newton, London.
Harley, R. 1982, Artists’ Pigments c. 1600-1835., Butterworth Scientific, London.
You can read more about ochres at Ruth Siddall's blog: Orpiment