A NEW TECHNIQUE FOR DETERMINING THE DEPTH OF PENETRATION OF CONSOLIDANTS INTO LIMESTONE USING IODINE VAPOR
RAKESH KUMAR, & WILLIAM S. GINELL
5 COMPARISON WITH OTHER METHODS
The iodine vapor technique was compared with a number of other possible methods to assess its effectiveness, applicability, and suitability for determining the penetration depth of consolidants into limestone. The same consolidants, concentrations, and stones were used for the purpose of comparison.
5.1 FLUORESCENT DYE TECHNIQUE
The fluorescent dye technique has been used as a possible staining method for the identification and localization of proteins and other binding media in conservation science (Talbott 1982; Wolbers and Landrey 1987). A high luminosity fluorescence probe has also been used to visualize restoration materials (Golikov and Zharikova 1990). However, its effectiveness has not been reported on a wide range of materials.
Of the available fluorescent dyes, an aqueous solution of Rhodamine B is commonly used and was tested for this application. Except for the epoxy-treated sample, no fluorescence was observed under ultraviolet illumination. Practically, the dye does not seem to be effective for the other consolidants. Moreover, it permanently stains the sample, therefore limiting its use for other analyses.
5.2 ELEMENTAL ANALYSIS
Elemental analysis provides information on the distribution of characteristic elements in the consolidant that are not present in the stone. Analysis of serial sections of the stone for these elements should indicate the penetration depth. In the case of epoxy or polyurethane polymers, such an element would be nitrogen. However, because of the complexity and expense of this method, it has not proven to be a practical or popular procedure for determining the migration and distribution of consolidants in stone.
It has also been suggested that electron beam microprobe analysis for the metallic elements contained in some polymerization catalysts (e.g., lead and tin) could be used to determine the location of polymers. This method was tried on a tin-catalyzed alkoxysilane consolidant in limestone but was unsuccessful owing to the low tin concentration.
5.3 SEM EXAMINATION
Freshly fractured surfaces of consolidated limestones were examined in the environmental scanning electron microscope (ESEM), and secondary electron imaging was used to locate the consolidant. Previous experience with these samples had shown that the consolidant could be morphologically distinguished from the limestone. Since the transition from the consolidant-rich to consolidant-free areas takes place over several millimeters, it proved difficult to estimate the depth of consolidant penetration exactly by examining fractured sections in the ESEM. Also, because the ESEM in its environmental mode has a lower magnification limit of 100–200x, the survey of large areas of topographically complex surfaces is difficult because the field of view is limited. Examination of polished sections might provide a method of quantifying the amount of consolidant present (using image analysis) provided sufficient contrast exists between the mounting medium and the consolidant. This technique is suitable for determining the migration and distribution of consolidants in stone but requires expensive instrumentation and a high level of skill.
5.4 CHARRING TECHNIQUE
It was thought that some organic polymers in the limestone samples would char at high temperatures, and the black charred component could be observed visually. Several limestone samples that had been treated with a variety of consolidants were charred in a nitrogen environment at 400°C. Both the treated and untreated areas became darker without any differentiation, and thus the charred organic polymer could not be visualized separately. From this experiment, it appeared that the charring method is neither suitable nor convincing for determining penetration depth of consolidants. It may be suitable for a consolidant that has a lower charring temperature and a stone that does not change color at the charring temperature. Some polymers may unzip and vaporize before charring.
5.5 ACID ETCHING
The acid etching procedure has been used to determine the resin distribution in the pores of stone (Domaslowski 1988). This method involves etching stone slices with a 5% solution of hydrochloric acid for ½, 1, or 2 hours. Calcium carbonate is decomposed in the zones where consolidant is absent.
Several consolidated stone samples were tested using this method, but it was not effective on the Indiana limestone or on the stones containing less than 3% by weight of consolidant. Both the consolidated and unconsolidated zones of Xunantunich limestone dissolved completely. This method seems to be effective only on pure limestone having consolidant concentrations greater than 5%.
5.6 WETTABILITY TEST
A simple test for determining the location of hydrophobic materials is to wet the cross section with a solution of a water-soluble dye. The test is effective only when the consolidant being tested is hydrophobic. If the treatment with a nonhydrophobic consolidant rendered the treated area nonporous, the dye solution would not penetrate but would remain on the surface of the stone, and this area would appear to be hydrophobic. In this case, the method would work.