Surface Analysis Of Architectural Terracotta: Including New And Soiled Examples, And Pieces Treated With A Hydrofluoric Acid-Based Cleaning Solution

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Authors: Moynehan, C.R., Allen, G.C., Brown, I.T., Church, S.R., Beavis, J. and Ashurst, J.

Journal: Journal of Architectural Conservation

Volume: 1

Issue: 1

Pages: 56-69

eISSN: 2326-6384

ISSN: 1355-6207

DOI: 10.1080/13556207.1995.10785126

The great quantity of buildings faced with architectural terracotta and faience, principally constructed between I860 and 1930, now present a major problem in terms of repair, maintenance and conservation. As part of a study into the nature of soiling on terracotta, and the effects of different cleaning methods, examples have been studied using a variety of surface analytical and conventional chemical techniaues. These include scanning electron microscopy with energy-dispersive X-ray analysis (SEM-EDX), Secondary ion mass spectrometry (SIMS), X-ray diffraction (XRD), atomic absorption (AAS) and X-ray photoelectron spectroscopy (XPS). Aluminium and silicon have been identified as the major elemental components of terracotta, with iron and calcium present in smaller amounts, and trace levels of potassium, magnesium and titanium. More iron is present in red terracotta than the buff variety; the reverse is true for titanium. The major crystalline species are cristobalite and muUite. Soiling may be attributed to the elements calcium, sulphur and chlorine, although pulverised fuel ash (PFA) and iron-rich particles have also been found incorporated into the surface layer. The latter are possibly generated by blast furnace activity. The treatment of terracotta with a proprietary hydrofluoric acid cleaning solution has been shown to enlarge the surface pores, increasing the potential for damage due to water penetration. Large amounts of fluorine remain on the surface after this treatment, even following prolonged rinsing, penetrating to a considerable depth. Silicon and potassium appear to have been most affected by the treatment; calcium, iron and titanium less so. © 1995, Taylor and Francis Group. All rights reserved.

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