2013-07

2013-07

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Marble is used in many of our most precious monuments and sculptures, but it deteriorates when exposed to water, owing to its relatively high solubility. Consequently, researchers have sought methods to enhance the durability of marble through the use of environmentally friendly chemical treatments. A particularly promising route, introduced by our research group, is to expose marble to a solution of an ammonium phosphate salt, which results in the formation of an invisibly thin coating of hydroxyapatite (HAP), which is the very durable mineral in tooth and bone. In the present study, we explored several aspects of the application of this chemistry. The phosphate salt chosen was diammonium hydrogen phosphate, which produces coatings about ten microns thick in one day. The solution is nontoxic and reasonably inexpensive, and the rate of reaction is satisfactory for field application by brushing or poulticing. By adding small concentrations of a calcium salt to the solution, the rate of formation is enhanced, and dissolution of the marble substrate is suppressed. Unfortunately, the acid resistance conferred by the film is far less than expected, because the film remains porous. Future work will attempt to eliminate the porosity by using organic templates to control the growth process.

The performance of the phosphate treatment as a consolidant for marble and limestone has proved to be exceptionally good. After being damaged by thermal treatment, the dynamic modulus of marble and limestone is returned to its original value by a single treatment with the phosphate consolidant. Direct comparison shows that it is superior to commercial silicate consolidants. Moreover, the HAP treatment does not substantially reduce the moisture transport through the stone, and does not confer hydrophobicity. Therefore, we expect this procedure to become widely used for treatment of weathered carbonate stones.

For quartzitic stones, silicate consolidants are superior, but they render the stone hydrophobic, which prevents moisture transport and hinders additional treatments, such as application of mortars or renders. We discovered that the hydrophobicity of the consolidated stone could be strongly reduced by a simple rinsing with an aqueous solution containing only about 16% ethanol. This allows the stone to be made hydrophilic within a week or two of consolidation, without excessive release of VOCs. We expect this procedure to be of great interest to conservators.

This research was made possible through MT-2210-12-NC-08 from the National Center for Preservation Technology and Training (NCPTT).

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