Deterioration and discoloration of historical protective treatments on marble

This study integrates the complex research conducted on the sources of brown discolorations that occur on marble statues (fifteenth century) of the Church of Orsanmichele in Florence (Italy). They underwent conservative interventions in the past and the brownish discolorations on their surfaces strongly altered the clear tone of the marble. In this study, Carrara marble model specimens were treated with organic and inorganic substances (non-pasteurised milk; linseed oil; walnut oil; ammonium oxalate; microcrystalline wax; beeswax; milk + linseed oil; and milk + ammonium oxalate + linseed oil) to simulate their effects on the stone. Some of the substances were commonly used in the past (as on the Orsanmichele statues) but most of them are still used in many countries. The treated specimens were exposed to natural and artificial ageing. The main results of the research were (i) the specimens treated with linseed oil, milk + linseed oil, and milk + linseed oil + ammonium oxalate showed a severe change of colour after either artificial or natural ageing; (ii) an extensive polymerisation of the organic substances occurred; (iii) calcium oxalate and several oxidised diacylglycerols (DAGs) and triacylglycerols (TAGs) were the last chemical products of the oxidation processes induced by ageing; (iv) Maillard reaction, producing brownish coloration, likely occurred in specimens containing milk as a result of the interaction between sugars and proteins.