Characterisation of Czech Modern Mosaic Mortars

Abstract

The main goal of this thesis was to characterise mosaic fixing mortars of 27 mosaics from the late 19th century to the late 20th century. The mortars were examined by SEM—EDS, XRD, light and light polarising microscopy, thermal analysis coupled with EGA-MS, FTIR and gas chromatography. The mosaics studied come from different mosaic workshops and periods but all of them relate to the present-day Czech Republic. Although the analysed set is small, some trends and preferences in the use of materials can be traced in the works of different workshops. Portable sepulchral mosaics designed by Neuhauser/ Tiroler Glasmalerei workshop, the first mosaic studio operating in the Czech Lands, follow a traditional 16th century technology – bedding mortars based on lime binder + marble aggregates + high amount of linseed oil (up to almost 40%) as a plasticizer. In order to fix façade mosaics, the company applied hydraulic mortars made of lime, marble dust, random waste glass splinters and reactive ceramic aggregates (bricks, chamotte) or Portland cement based materials with carbonate aggregates. A mixture of lime and Portland cement appears to be a common binder of Josef Pfefferle´s mosaics. The works by the first Czech local mosaicist Viktor Foerster consist of sand, early Portland cement or mixtures of Portland cement and lime and crushed bricks or gypsum. Foerster´s wife Marie used blended Portland cement with blast furnace slag grains to fix the vault mosaic of the Slavín crypt. The analysed bedding mortars from mosaics produced by other early 20th century mosaic workshops also document the on-going transition from traditional calcium carbonate aggregates and cocciopesto technique to the application of sand and Portland cement based mortars. Portland cement was the main compound in the binders of the studied “socialistic” mosaics of the latter half of the 20th century. This work brought evidence of a common use of blended cements containing granulated blast furnace slag. Linseed oil turned out to be an important compound of some late 19th/ early 20th century mortars. Therefore, a methodology of linseed oil content estimation in the historic mortars was proposed based of the three methods (TG coupled with EGA-MS and TOC). The approach based on EGA-MS (identification and analysis of m/z 95 signal corresponding to “oil-specific” [C7H11]+ ion), developed on a set of model mortars and tested on authentic mosaic mortars´ samples, provided the most satisfactory results.