The use of mortars and plasters has been widespread in many cultures for thousands of years and these materials are found in the vast majority of built cultural heritage. They play a crucial role in the preservation of immovable heritage and must be taken into great consideration when conserving historical buildings. Plasters and mortars have been extensively studied from a chemical and mineralogical point of view. The main causes and mechanisms of damage and deterioration are understood and various methods and materials have been developed for their preservation. Treatment development, testing and evaluation in the laboratory, under controlled conditions, has produced a high level of knowledge. However, there is a divergence between the results of academic-scientific studies and practice in the field. Laboratory studies typically tackle only a few variables at the time and the results cannot always be directly applied to address the complex problems that restorers face in their daily work practice. In addition, in situ conservation-restoration is extremely challenging because many of large number of variables involved cannot be controlled or modified. Following a description of the ideal “value-based” conservation process, this compilation work discusses aspects related to understanding deterioration mechanisms and planning of preventive and remedial interventions in a way that may be useful to update conservators-restorers on the status of scientific research in this field, and to guide conservation-scientists to identify unresolved issues, which require future research efforts.

The monastery of St. John in Müstair, a UNESCO world heritage site, preserves archaeological remains and stone structures dated from the 8th century to the present. It has been extensively studied archaeologically so that numerous samples of historical materials, including mortar, are available for study. In addition to that, some of the structures have been precisely dated with dendrochronology. The monastery is located in a region characterized by dolomite rocks and the mortars are therefore of dolomitic nature, being perfectly suited to test the possibility of being dated with 14C. Furthermore, the presence of embedded carbon fragments has provided additional independent data to support or deny the results of mortar dating. A comparison of the results obtained from radiocarbon (14C) dating of bulk mortars, sieved fractions enriched in binder, lime lumps and carbon fragments, for two samples is presented, in relation to the petrographic characterization and the mineralogical phase content. This preliminary study shows that the dating of 14C can potentially be applied to the mortar of Müstair, as results in accordance with the established chronologies have been obtained for one sample. However, if the dolomitic sand contamination is very high, further studies are needed to develop a specific sample preparation technique.

From the 16th to the 18th century, many artists and craftspeople coming from villages in the Italian-speaking part of Switzerland, well known for its stucco tradition, worked actively in Central Europe. Baldassarre Fontana (Chiasso 1661–1733) was one of them; after a period in Rome, he was active in Moravia and Poland. This contribution presents an analytical insight into stucco works in the Ticino canton and Czech lands in order to provide an understanding of how artistic and technical skills were transferred according to local traditions, materials, or other requirements found abroad. By comparing two works that Fontana realized in his homeland and two works that he realized in today’s Czech Republic, the historic techniques used to make stucco are characterized, with an emphasis on technological aspects and materials. The detailed analyses provide information on materials and their production and processing methods in relation to the specific sites. They also reveal certain specific technological approaches which were common in all four cases that were studied. The most significant was the layered system of high-relief pieces, where the ground layer was made of lime–gypsum mortar and the finishing layer was pure air lime mortar. The construction of the finishing layer differed between the Czech and Swiss sites, suggesting some other influential factors.

This work investigated the techniques used for creating a three-dimensional stucco decoration. During the multidisciplinary project "The art and industry of Ticino plasterers from the sixteenth to the seventeenth century", some stucco works mainly situated between the south of the Ticino region and the province of Como were studied. These artists traditionally known as "Masters of the Lakes", were invited across Europe to decorate the most important architectures at the time. The diffusion of this art and in particular of these workers was probably due to their unique ability to master the complex art of stucco. In particular, the scientific research was addressed at understanding the structure, composition and original “recipes” of the high relief stucco and statues, by using different complementary analytical techniques: the internal structures were analyzed with the radiographic technique, the mortars were characterized by polarized light microscopy, SEM–EDS and FT-IR spectroscopy. The identification of organic additives was possible thanks to a pre-treatment sample with the reactive sulfur tetrafluoride gas (SF₄). Our results indicated that the stucco works of the considered authors were made using almost all recurring components, although differences emerged in the assembly of the internal structure, in the presence of gypsum in the ground mortar layers and in the use of the aggregate in the finishing.

After the last damaging earthquake in 2012, an anti-seismic reinforcement project of the cathedral of Modena was designed giving us the opportunity to investigate and date the building materials. Radiocarbon (¹⁴C), optically stimulated luminescence (OSL), and thermoluminescence (TL) dating techniques were performed on the vaults with the aim to (1) clarify the construction timing, (2) define the history of the restorations, and (3) explore the possible correlation of the main restoration works to the earthquake chronology deduced from the historic catalog. Preliminary results show that medieval older bricks were reused for most of the original construction. Only lime and non-gypsum mortar was used for the original construction in the 15th century and for later repair of damage caused by earthquakes in the 16th and 17th centuries. Gypsum mortar was used for later repair in the 18th century. The results show much stronger damage due to earthquakes than previously thought.

Archaeological and historical research combined with material science help to understand the development of building material and construction technology of the Middle Ages. The natural sciences, especially mortar characterisation and scientific dating have allowed new insights into mechanical mortar mixing which has been introduced and used mainly in the early medieval period between AD 500 and 1000. This paper combines the results of the pan-European archaeological research project RESTOMO with that of the interdisciplinary SNF-project “Mortar technology and construction history” at the UNESCO-world heritage site of Müstair monastery. Müstair so far is the only site with six mortar mixers from two periods. The comparison of the mortar samples from the mixers with samples from the building remains has proven to be challenging and partly contradicts the previous correlation of mortar mixers with archaeologically established building phases. GRAPHICAL ABSTRACT