Mortars are heterogeneous building materials whose raw materials, manufacturing processes and application conditions have evolved continuously throughout history. The fact that apparently small changes in the components or dosages of mortars can affect their overall performance in the masonry makes the study of historic mortars a complex task that needs to be tackled via a multidisciplinary approach, with the support of complementary analytical techniques from the field of chemistry, mineralogy, physics and engineering, among others. This review is intended to be a useful tool for researchers working in the field of archaeology and/or cultural heritage conservation, as it offers a complete overview of the most widely accepted analytical techniques and physical-mechanical tests used in the characterisation of historic mortars and plasters. Although the methods described here are common to both air-hardening and hydraulic mortars, we focus above all on the latter, paying special attention to aspects relating to the chemical, mineralogical and petrographic investigation of the calcium silicate and aluminate hydrated phases that may indicate the use of one or other hydraulic binder in historic mortars, all this taking into account and discussing the practical aspects, drawbacks and limitations of each technique. European standards for the study of mortars are also addressed in this paper.

Mortars and plasters—how to characterise hydraulic mortars

This paper offers a broad and critical overview of current discussions on the potential uses and the characterization of pigments in prehistory, with a special focus on prehistoric rock art. Today, analytical approaches to pigments and paints allow us to go beyond the identification of the elemental and molecular composition of these archaeological remains, to explore also raw material procurement, transformation and use strategies of interest to investigate the technological and socio-cultural practices of prehistoric artists and their change over space and time. The paper also summarizes the palette of prehistoric artists, as well as the techniques and analytical strategies used to date to characterize prehistoric pigments and paints (colours, raw materials, binders and recipes) used in prehistoric rock art.

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Characterizing the pigments and paints of prehistoric artists

This article summarises the history of cinnabar, from its first uses in burials to modern oils on canvas. After a brief introduction on mercury and contamination issues, the article gets to the heart of the topic. First, mercury-based minerals significant for studying pigments, i.e. cinnabar, metacinnabar, hypercinnabar and calomel, are presented. Structural information and properties precede an overview of the geographic distribution of cinnabar deposits. The following section addresses the multiple uses of cinnabar, divided into funerary use, decorative use, lustre and Chinese lacquer production. The use of cinnabar for writing (ink), medicine and cosmetics is briefly described, and a shortlist of uncommon finds is further provided. The following section approaches inherent but less known topics such as cinnabar procurement, trade, production technology, application and alteration. An entire section is dedicated to calomel before concluding with an overview of the analytical methods for the characterisation and provenance investigation of cinnabar.

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Pigments — Mercury-based red (cinnabar-vermilion) and white (calomel) and their degradation products

Since antiquity, various copper-containing substances have been used as green and blue pigments. Their exceptional diversity, reflecting their various chemical and phase composition, chemical stability as well as their origin, makes their correct identification challenging. The review focuses on copper-based pigments used in ancient and mediaeval works of art, especially in wall paintings and/or related polychromed decorations or statues—siliceous copper pigments (Egyptian blue and green, Han blue and purple, chrysocolla), copper carbonates (azurite, malachite, blue and green verditers), copper chlorides (atacamite-group, cumengeite, calumetite), copper sulphates (posnjakite, brochantite) and—to a lesser extent—copper acetates (verdigris) and other organometallics. Particular attention is given to the necessity of the detailed study of accompanying phases which can serve as useful indicators of natural and/or artificial origin of copper pigments. Factors affecting the stability of copper pigments in wall paintings—salt attack, oxalic acid, alkalinity and heat—are overviewed. A suitable analytical approach based on complemental combination of in situ and laboratory analyses for proper identification and differentiation of copper pigments is proposed.

Pigments—copper-based greens and blues

The present contribution deals with the use of organic colourants in wall paintings, polychrome pottery and painted stone artworks, i.e. painted artworks on inorganic supports. The term organic colourants is referred to the chemical nature of these compounds but not to the way of application; therefore, organic colourants can be dyes, lakes or pigments. The use of organic colourants in wall paintings has been given little attention in the past, perhaps on the assumption that they were rarely used by ancient artists. Recent diagnostic studies, however, brought evidence that their use was not fragmentary; on the contrary, there seems to be continuity in the centuries, at least with regard to the most widely used such as madder, Tyrian purple and indigo. Sources, alteration phenomena, identification methods and analytical evidence is given for the main organic colourants with concern to red, yellow, green, purple and blue hues. Drawbacks and issues are discussed with concern to the identification techniques.

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Pigments—the palette of organic colourants in wall paintings

Mortar is of fundamental importance for the building technology. It is used to bind together masonry units and ease the building process. Several building techniques evolved to take advantage of the capacity of mortars to improve cohesiveness and form sound structures. In this paper, I discuss how lime and gypsum mortars were employed from the Antiquity to the Middle Ages. Gypsum mortars offered several advantages due to their adhesive properties and quick setting. Their use as a structural binding agent developed in regions rich in gypsum, and was particularly significant in vault construction. Lime mortars offered different advantages over gypsum ones, particularly in terms of mechanical resistance and resilience to humid conditions. The massive use of lime mortars started with the ancient Romans and continued throughout the centuries as the foremost binding material, until the introduction of Portland cement mortars in the nineteenth century.

Paolo Vitti

Papers

Mortars and masonry—structural lime and gypsum mortars in Antiquity and Middle Ages