Yorkshire Philosophical Society

Event Information

‘Pavement, Pots and Pigments: Applied Geology in Cultural Heritage’

Dr Ruth Siddall, UCL

Abstract

Applied geology is a poorly recognised discipline in the UK. However, a great variety of geologically derived materials have and are being used in art, architecture and functional design. This talk will present the author’s experience of applying geological techniques and experiences to the analysis of archaeological and art historical cultural materials.

Lecture to be held in the Tempest Anderson Lecture Theatre, Yorkshire Museum,
YO1 7DR at 7pm

Member’s report

Geology is usually thought of as the study of rocks in the landscape, but these rocks have provided the building stones and minerals used throughout history. Dr Siddall began her career cataloguing a collection of rocks in Athens, and was inspired to work across a number of different disciplines to research our cultural history. In the lecture she used three case studies to illustrate how geological techniques can be used to examine materials found in a variety of settings, providing valuable information for archaeologists and art historians.

Case Study I, The Cosmati pavement   This was topical, as the pavement was on prominent display at the recent coronation of King Charles III. This is an intricate mosaic floor commissioned by Henry III and probably constructed in 1268-69. The elaborate design consists of different shaped pieces of coloured stone, arranged in geometrical patterns, rather like a patchwork quilt, within a framework of Purbeck Stone. The coloured stones include green volcanic rock from Sparta, purple porphyry from Egypt, and yellow Roman marble. The stones are not closely fitted and each is surrounded by mortar, which forms part of the design. Over the centuries, a number of repairs have been made, using different mortars, which are distinctive, and show a range of colours. The original mortar is referred to as Primary, while that used in the 17th century repair as Secondary, and mortar from the late 18th and early 19th Centuries as Tertiary. Additionally, further repairs were done in 1868 by Sir George Gilbert-Scott who attempted to replicate the appearance of the original mortar by mixing special additives in contemporary Portland cement.

Case Study 2, Pots from Kythera   Kythera is a Greek island in the Mediterranean. Much very early pottery has been found there, including fragments of Pithoi, which are typical storage jars, and tripod cooking pots. Similar pottery has been found distributed around other sites in this area of the Mediterranean. Microscopic examination reveals that they contain red mica. Geological studies show that Kythera was an important production site exporting to other areas.

Case Study 3, Painting Roman Britain   Colourful wall paintings were very popular with Romans throughout their Empire, including in Britain. The coloured components of paint are pigments, and are obtained from many sources, which can be traced using scientific techniques. Mineral pigments, especially earth pigments such as ochres, were often the easiest to obtain. The mineral used must retain its colour when ground, and should not be soluble in water. Alternatively natural dyes can be used for colour. The majority of these are processed from plant sources. Direct dyes give good colour when used alone, but many dyes require the use of a mordant.  Mordants combine with the dye to give an insoluble organic precipitate, called a ‘lake’ pigment. The Romans used a wide variety of pigments, both mineral and organic.

Wall-painting fragments found at the Romano-British Villa discovered at Sudbrook, Lincolnshire, in 2008, have been studied. A variety of pigments, including Egyptian Blue, hematite and cinnabar have been identified. In Scotland, two rare Mithraic altars discovered in the Lewisvale Park in Musselburgh in 2010 were found to have traces of pigment, including Bone White, red ochre and the organic red pigment Madder.

Margaret Leonard