Charakterizace malt Juditina mostu

Zprávy památkové péče 2013, 73(6):548-554

Characterizing the mortar in Judith Bridge

Petr Kuneš

Archeological research on the relics of the Judith Bridge, carried out in 2010-2013 on the Malá Strana and the Old Town banks, provided access to, among others, the authentic mortar of the oldest stone bridge in our country. The research presented made it possible to examine the mortar from the filled walls of the pillar and the core of the bridge, the divided segments of the bridge deck, and the tiled surface of the bridge roadway. The mortar from the core of the Charles Bridge, taken as part of research on the condition of the mortar wall filling in 2007, was studied as a reference. The research primarily focused on documenting, characterizing, and mutually comparing individual samples. The samples were visually examined and macroscopically documented, and a binocular microscope was used to establish the portion of carbonate and to carry out a sieve analysis of the aggregate. The mechanical characteristics of the mortar were evaluated empirically, and pressure and bending strength was measured. The bulk density was determined, as was the overall porosity of the mortar, and in selected cases the pore system of the malt was characterized using mercury porosimetry. The polished stone cuts were studied using a polarizing and electron microscope, the mortar matrix was separated, and discovered unbaked or overbaked limestone particles were instrumentally analyzed. The mortars of the filled walls of both Judith and Charles Bridge are represented by a single type. In terms of aggregate composition, matrix structure, and the physical-mechanical characteristics of the mortar, there are no fundamental differences visible, and similar procedures were evidently used to prepare them from similar materials. The mortars of the roadway and deck of Judith Bridge, in contrast, had a richer proportion of crushed slate, a larger proportion of hydraulic components, and generally a higher proportion of carbonate. The particles of "opuka" (sandy marlstone) found in these mortars suggest an interaction with the lime binder. These differences likely indicate the intention to increase the resilience of the mortars used on the most highly stressed parts of the bridge deck. The findings also show that during the time when Judith Bridge was under construction, i. e. the middle 12th century, hydraulic lime was produced in Prague by firing local argillaceous limestone.