Bowing of marble slabs: can the phenomenon be arrested and prevented by inorganic treatments?

Abstract

Bowing of thin marble slabs is a phenomenon affecting both historic monuments and modern buildings. In spite of the ubiquity and destructiveness of this phenomenon, no fully satisfactory treatment is currently available to arrest and/or prevent bowing. In this study, a treatment based on formation of hydroxyapatite (HAP) was investigated as a possible route to arrest and possibly prevent bowing of Carrara marble slabs. Four different formulations of the HAP-treatment were tested and compared to ammonium oxalate and ethyl silicate (widely used in the practice of marble conservation). The treatments were applied onto pre-weathered and unweathered specimens to investigate their ability to arrest and prevent bowing, respectively. Marble behavior was studied in terms of residual strain and bowing after thermal cycles up to 90°C in dry and wet conditions. Marble cohesion was assessed before and after the thermal cycles by ultrasound. The HAP-treatments exhibited promising results, as the residual strain and the bowing after the cycles were always lower or equal to the untreated references, while marble cohesion was always higher. Surprisingly, ammonium oxalate caused marked worsening of marble thermal behavior. In the case of ethyl silicate, most of the initial benefit after consolidation was lost after the thermal cycles. In general, the results of the study point out the importance of evaluating marble thermal behavior to assess the suitability of any conservation treatment and suggest that treatments able to strengthen marble without causing excessive pore occlusion and stiffening are preferable to enhance durability to thermal cycles.

Comparative Study Between Ammonium Phosphate and Ethyl Silicate Towards Conservation of Prehistoric Paintings in the Magura Cave (Bulgaria)

“…After treatment with "DAP", small new bands appeared at 602 and 561 cm −1 , which indicated the formation of new calcium phosphates [61]. The fact that these new bands were very small was due to the low concentration of DAP used for the treatment (0.1 M), which was consistent with a previous study where FT-IR bands of new calcium phosphates were found to be more pronounced for more concentrated DAP solutions [60]. To distinguish between formation of hydroxyapatite [HAP, Ca 10 (PO 4 ) 6 (OH) 2 ] and octacalcium phosphate [OCP, Ca 8 H 2 (PO 4 )6·5H 2 O], which both could be formed as the reaction product between calcareous stones and DAP solutions [51] and both have FT-IR bands at 560-563 and 600-602 cm −1 [61], the position of the other strong bands (at 1031 cm −1 for HAP and 1023 cm −1 for OCP [61]) should be considered.…”

“…After treatment with "DAP", small clusters of the new calcium phosphates were visible on the stone surface, which, also in this case, was not continuously coated with the new phases. Generally, new calcium phosphates formed in carbonate stones from the reaction with ammonium phosphate solutions show a flower-like morphology [25], but, in the case of dilute solutions (such as that adopted in the present case), the formation of small clusters has been reported before [59,60].…”

Hydroxyapatite and Other Calcium Phosphates for the Conservation of Cultural Heritage: A Review

“…Coatings formed at even higher DAP concentration (3 M DAP, followed by limewater application [ 42 ]) exhibited diffused cracking [ 43 ] but still a remarkable consolidating effectiveness. Because the consolidating action derives from the ability of the new CaP phases to seal the tips of cracks among grains (even though a discontinuous coating is formed), a steady increase in mechanical consolidation (assessed by ultrasound) was found for increasing DAP concentration, both in porous limestone [ 1 ] and in marble [ 44 ].…”

“…Application of a limewater poultice after treatment with a 3 M DAP solution has been investigated as well [ 42 , 43 , 44 , 45 , 49 , 50 , 51 ]. In this case, the limewater poultice, applied after treatment with the DAP solution and drying, has the twofold purpose of (i) providing additional calcium ions for the reaction with unreacted DAP and (ii) removing all the unreacted DAP during drying, as the poultice is left to dry in contact with the stone [ 42 ].…”

Editorial to the topical collection in Environmental Earth Sciences “Stone in the architectural heritage: from quarry to monuments—environment, exploitation, properties and durability”

“…The use of consolidants improves the drainage of the water during the drying process and increases the durability of the materials (Molina et al 2018). The use of a treatment based on the formation of hydroxyapatite do not cause excessive pore occlusion and stiffening, and therefore enhances marble durability to thermal cycles and bowing (Sassoni et al 2018). Weathering crusts can have significant differences in hydric and thermal expansion in relation to the stone and the conservation treatment should promote the reduction of this heterogeneity (Wedekind et al 2018).…”