Ammonium oxalate treatment for the conservation of weathered globigerina limestone

Abstract

The aim of this research was to investigate the efficacy of ammonium oxalate treatment on Globigerina Limestone. The study was structured into three phases, laboratory treated samples in Phase 1, identical samples exposed to natural site conditions for one year in Phase 2 and in situ wall samples in Phase 3. This study therefore progressed from a laboratory based, controlled environment, to uncontrolled site conditions. Laboratory samples included quarry and artificially weathered franka and sol/ samples - two types of local stone. These included salt free samples and samples artificially contaminated with sodium chloride, sodium sulphate and sodium nitrate respectively. Phase 2 samples included quarry and artificially weathered franka with the same salts as Phase 1. Phase 3 wall samples consisted of naturally weathered franka. Treatment was applied using a 5% ammonium oxalate monohydrate poultice. Testing was carried out throughout the three phases, and this was targeted at evaluating aesthetic changes, depths of calcium oxalate formation, properties of newly formed calcium oxalate and porosity/water transport properties of untreated and treated stone. To this end, the testing programme included visual observations, colorimetric analyses, adhesion (tape) tests, Drilling Resistance Measurement System (DRMS), X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM & EDS), acid resistance tests, salt crystallisation tests, salt mobility testing, open porosity tests and pore size distribution tests and water absorption tests by capillarity. Results showed that whewellite was formed in all treated samples, even in the presence of the soluble salts, and that no deleterious by-products were detected. Treatment resulted in aesthetic improvement and surface consolidation while still retaining adequate Wf..ller lrdnsport properties. The type and rlPpthr, of whrw~llite formed, which overall ranged from 0.7mm to l.6mm, were found to be related to the soluble salts present during treatment. In general, it was concluded that as part of the overall conservation process, ammonium oxalate treatment to weathered Globigerina Limestone containing soluble salts is very valid, although further in situ investigations are needed.