A Novel Approach to Carbonate Stone Conservation: Induced Calcium Oxalate Formation Through the Application of Ammonium N-Ethyloxamate (AmEtOxam) on White Carrara Marble

TL;DR

A new method for preserving white Carrara marble uses AmEtOxam to form a protective calcium oxalate layer, improving strength and reducing damage.

Contribution

AmEtOxam is introduced as a novel, highly soluble precursor for calcium oxalate formation on carbonate stone.

Findings

Spraying AmEtOxam improves mechanical strength and reduces water absorption with minimal color change.

Brushing increases surface hardness with low aesthetic impact.

Immersion effectively reduces porosity and increases surface tension.

Abstract

Ammonium N-ethyloxamate (AmEtOxam) was synthesized, fully characterized by microanalytical and spectroscopic means, and assayed as a precursor of calcium oxalate, acting as a protecting agent for white Carrara marble. The monohydrate form of AmEtOxam shows a water solubility of 1.5 mol·L−1 (~23% w/w), significantly higher than that of common calcium oxalate precursors (CaOx), such as ammonium oxalate (0.4 mol·L−1, ~5% w/w). While AmEtOxam is stable in water solution and in the solid state in its monohydrate form, during the application on carbonate stone it undergoes a complete hydrolysis resulting in the formation of a uniform weddellite layer (CaC2O4·2H2O) on carbonate stone surfaces. Application of 5% w/w aqueous solutions by spraying, brushing, and immersion resulted in different effects. Spraying yielded the most balanced performance, improving mechanical strength, reducing water absorption, recovering superficial tension, and limiting chromatic alteration. Brushing achieved significant gain in surface hardness with minimal esthetic impact. Immersion most effectively reduced porosity and increased surface tension. These results, coupled with the negligible chromatic changes induced in all cases, make AmEtOxam a promising candidate for the conservation of stone cultural heritage.

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