Antique masonry mortars, plasters and wall paintings

Historical mortars, which are among the most durable materials of our cultural heritage, provide a great amount of information about the current state of technological development of a civilization. It is therefore not surprising that the characterization of historical mortars, plasters and stuccoes has been the focus of interdisciplinary research for more than 200 years. Lime has been used as a binder for mortars for several millennia and is one of the oldest man-made materials, alongside ceramics.

Despite the apparent simplicity of the production engineering, which includes burning the raw rock materials (calcining), adding water (slaking), mixing as well as processing the mortar, these tasks allow a wide range of possibilities that lead to very different results. These include, for example, the choice of different raw materials, the many variants of lime slaking, but also the addition of reactive minerals, which can lead to a fundamentally different hardening process. Accordingly, even modern lime standards, which largely refer to industrial products, differentiate between different types of lime: those that are air-dried or those that set by their interaction with water, known as hydraulic or pozzolanic lime. In historical lime techniques, the production process varies to an even greater extent, so that in such mortar samples the detection of special characteristics caused by varying production techniques is of particular interest. Thus a wide number of analytical tools are employed in order to discover the historical methods employed to produce a mortar, how the mortar has changed over time, and, at the same time, offer insight into the connections between physical-mechanical and chemical-mineralogical properties of the mortar.

Mortars are systems of several components – a binder and variety of aggregates combined with water. Microscopic and micro-analytical analysis methods are particularly suitable for the investigation of such systems, because it is possible to observe and analyze the components without separating them and thus also to study the characteristics of their microstructural relationships to each other. Early on, we recognized the potential of microscopy when investigating historical mortars and have been applying it at a high level for many decades. Always on the lookout for fingerprints that can provide us with information on the raw materials and their mixing, firing and extinguishing conditions, we have recently expanded our range of spatially resolving imaging methods to include microscopic FTIR analyses.

We have primarily dealt with two groups of mortar, based on their historical use. The first of these are ancient and historical masonry mortars with varying hydraulic properties, which they owe either to differing raw material characteristics or to pozzolanic additives. Here we mainly focus on respective sources of hydraulicity in order to understand the full spectrum of old lime techniques and, at the same time, to contribute to basic research on the reaction mechanisms that created these materials. Another point of interest is to find out how the materials change over time in different environments.

The second group of mortars we are dealing with are wall plasters from different eras, which are most commonly used as foundation for mural paintings. These usually non-hydraulic sequences of plaster layers are examined and documented in their initial form from the layer of paint to the layers attaching the plaster to the underlying masonry (arrico layers). They provide insights into time- and site-specific craft and art techniques and are particularly suitable for comparing different practices of plaster preparation and application as well as the execution of high-quality wall paintings from different excavations of a cultural epoch. In this sense, we have been able to study Roman mural paintings from Ephesus and Ostia with provincial Roman comparative examples from Central Europe and document the respective deviations from Vitruvian techniques. We frequently use false-color processing on stacked sectional photomicrographs, which enables us to quantify mortar parameters layer by layer.

Selected Project History:

  • A scientific investigation of antique wall-building materials from Terrace House 2 in Ephesos. - in cooperation with the Austrian Archaeological Institute (F. Krinzinger, S. Ladstätter, N. Zimmermann) and the University of Leoben (W. Prochaska, E.-M. Maurer), 2002-04 (Dipl.-arbeit E.-M. Maurer of the same title.)
  • Scientific investigations of plaster and wall painting fragments from Carnuntum (in cooperation with T. Bayerova and C. Gurtner, on behalf of the Carnuntum Archaeological Park). 2003-04
  • An Examination of the Hellenistic backfilling mortar on the mausoleum of Belevi. - in cooperation with the Austrian Archaeological Institute and the Institute for Cultural History of Antiquity of the Austrian Academy of Sciences (P. Ruggendorfer, R. Heinz) and K. Bayer, 2004-05
  • Petrographic analyses of selected Roman wall plasters from Ostia (in cooperation with S. Falzone, N. Zimmermann), 2004-09
  • Technical material investigations on the wall and ceiling decoration of a Roman villa in Saalfelden/Wiesersberg (in cooperation with B. Tober), 2006-07
  • Scientific characterization of masonry mortars and opus caementitium from the area of the ancient theatre of Ephesus (in cooperation with T. Köberle and A. Baragona, commissioned by G. Styhler-Aydin), 2011-13
  • Analysis of mortar and plaster samples from of "The 'Case a Giardino' in Ostia - Archaeological Context and Virtual Archaeology of a Large Roman Housing Complex, a FWF-Project (Project Management: P.Ruggendorfer, Austrian Academy of Sciences), starting in 2019

Relevant Dissertations:

Anthony Baragona: Pozzolan-Lime Mortar from Antiquity to Present: the
Utility of Experimental Archaeology and Image Based Analysis for Understanding Historical Mortar

ao.Univ.-Prof. Dr. phil. Johannes Weber
Univ.-Prof. Dr. Norman Weiss (Columbia University, N.Y.)

Hazal Özlem Ersan Eruş: Mortar Technology in the Byzantine and Ottoman
Construction of the City Wall of Istanbul

ao. Univ.-Prof. Dr. phil. Johannes Weber