Erosion on the surface of stone works creating a very granulated or “sugary” surface appearance, which can be indicative of a certain brittleness or tendency of the stone to break up or dissolve. Most commonly noted with limestone or marble, as the material breaks up into small crystals that look and feel like coarse sugar. Stones generally have to be replaced when sugaring progresses into crumbling occurs.
Synonyms in English
|Spanish||sacarificación del mármol|
|German||Verzuckerung von Marmor|
Causes of sugaring include:
- An inherent weakness in the stone
- Gradual breakdown of the binder from acidic rain, freezing and thawing, etc.
- External factors affecting the strength and durability of the stone such as de-icing salts (called salt fretting), or any other source of salt migration (like rising damp).
- Acids produced by accumulated biological growth such as moss or lichen may result in localized sugaring of limestone surfaces. The mechanical action of the microscopic rootlets of these lower plants also contributes to sugaring as they penetrate the stone surface. (Trinkley 2013)
No completely satisfactory consolidant for sugaring marble currently exists, according to Sassoni, et.al., however, the use of ammonium phosphate to form hydroxyapatite (HAP) inside marble micro-cracks has recently been proposed by this team. Their study aimed at: (i) investigating different treatment formulations and characterizing the new Ca–P phases; (ii) systematically testing mechanical effectiveness and compatibility with the substrate of the most promising formulations, in comparison with ethyl silicate and ammonium oxalate; (iii) performing a pilot application of the most promising formulation to a real marble artwork affected by sugaring. The results of the study indicate that application of a 3 M DAP solution ( 3 M aqueous solution of diammonium hydrogen phosphate (DAP), followed by limewater poultice application, produces remarkable consolidation of weathered marble, with only slight alterations in pore size distribution and color change, thus providing much better results than both ethyl silicate and ammonium oxalate. Notably, while the above-described treatment conditions caused formation of tricalcium phosphate and octacalcium phosphate in artificially weathered samples, the same conditions caused formation of HAP in the historic artwork, presumably because some gypsum residues were present and calcite grains had high surface roughness. (Sassoni et.al 2014), (Sassoni et.al 2013)
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Enrico Sassoni, Gabriela Graziani, Elisa Franzoni, "Repair of sugaring marble by ammonium phosphate: Comparison with ethyl silicate and ammonium oxalate and pilot application to historic artifact," Materials & Design, Volume 88, 25 December 2014, Pages 1145-1157, ISSN 0264-1275, http://dx.doi.org/10.1016/j.matdes.2015.09.101. or http://www.sciencedirect.com/science/article/pii/S0264127515305116 (accessed 15 October, 2015).
Enrico Sassoni, Elisa Franzoni, 1287 Built Heritage 2013 Monitoring Conservation Management, Evaluation of hydroxyapatite effects in marble consolidation and behaviour towards thermal weathering, http://www.bh2013.polimi.it/papers/_bh2013_paper_24.pdf (accessed 15 October, 2015).
Michael Trinkley, "CONSERVATION TALK: Marble and Its Deterioration". ACS QUARTERLY / VOL. 37, NO.2 / SUMMER 2013 http://www.chicora.org/pdfs/AGS%20Conservation%20Talk%20-%20Marble%20Problems.pdf (accessed 15 October, 2015).