In the past, many organic additives – especially proteinaceous and polysaccharide compounds – were added to mortars to improve their performances (Sickels-Taves and Allsopp, 2005). Oxblood was one of the most common protein materials added. Apart from improving mortar properties, its use in architecture could also be considered as a means of slaughterhouse waste disposal to avoid environmental contamination, especially in big cities. The aims of this research are to approach the subject of blood addition in mortars through a historic perspective and, more specifically, to understand historic knowledge of mortars with blood addition through preparation of model samples based on historic recipes, to verify, for example, credibility as well as feasibility of the chosen recipe, and to have a preliminary understanding of possible modifications in mortar microstructure, which could be induced by the blood addition.

The use of animal blood in mortars was mentioned in many ancient texts from both Europe and Asia, e.g. in Palladius (4th–5th century AD) and Plat and Boate (1653). With the emergence and development of modern chemistry in the 18th and 19th century, historic accounts documented recipes with more precise compositions and more specific applications of such mortars. According to these recipes, they could be used as mastics for ceramic, vitreous and metal items (Croker et al., 1765; Tilloch, 1803), but moreover blood addition was also used in mortar mixes to produce plaster. For instance, a series of lime–pozzolan mortars added with oxblood gave a waterproofing quality, which was recorded in French and Italian treatises (Encyclopédie méthodique, 1791; Pegoretti, 1843; Brey, 1844; Corso di costruzioni civili e militari di Alberto Gabba, 1870). According to some English sources, oxblood beaten together with lime produces great flooring and plastering material (Aikin and Aikin, 1807; Breymann et al., 1885). Similarly, oxblood and clay tempered together could make a strong binding material for floors and walls (Plat and Boate, 1653; Neve, 1726; Chambers, 1728). In China, pig blood together with tung oil added to lime and brick dust mixtures has been used for applying plaster on wooden board substrate since the later part of Qing dynasty (1644–1912). When blood is added to mortars, the mortar is often described as quick-setting (Brey, 1844; Purgotti, 1857), water-repellent (Good, 1813; Ascolese, 1832), adhesive (Plat and Boate, 1653; Jones, 1827), fireproof (Turriano, 16th century; Brannt and Wahl, 1887), hard (Tilloch, 1803), and frost-resistant (Polini, 1850), etc.

To choose the recipe to be further studied experimentally, some requirements needed to be fulfilled: the mortar should be lime-based; blood should be the only organic additive in the mortar; the mortar should be used for construction purposes, with specific mortar function given; the ingredient amounts in the recipe should be given as precisely as possible. The final choice of the historic recipe used for samples preparation is quoted from a 19th-century Italian manual (Pegoretti, 1843), which is reported as a “common mastic” for masonry plaster and stucco, or for pavement connections. It is described that the materials needed to compose 1 kg of such mastic are the following: “0.12 kg of quicklime in powder; 0.25 kg of oxblood, in which the lime is slaked; 0.70 kg or 0.00054 m3 of pozzolana; 0.0275 kg of iron filings”. This recipe not only accounts for the precise amount of each ingredient but also recalls other similar historic recipes from 18th- to 20th-century literature. To be specific, the recipe that we have chosen, along with a series of recipes with similar compositions and similar mortar functions, could have all been derived from a single recipe in Encyclopédie méthodique (1791). This presumably French-origin mix was cited in many later publications (e.g. Pegoretti, 1843; Brey, 1844; Corso di costruzioni civili e militari di Alberto Gabba, 1870). In addition, the credibility and practicality of the chosen recipe can be established by the literature source – Manuale pratico per l'estimazione dei lavori architettonici, stradali, idraulici e di fortificazione – which is a work that bears a lot of diligence from the Italian engineer Giovanni Pegoretti, and it is written with information as detailed as possible to guide builders and engineers.

SEM/EDS was performed using a Field Emission Gun Tescan Mira 3XMU-series FE-SEM equipped with an EDAX microanalytical system (accelerating voltage 20 kV, beam current 40 mA, working distance 15.8 mm). Attenuated total reflectance (ATR) spectra were recorded by Thermo Scientific Nicolet iS10 instrument equipped with a diamond crystal (wavenumber 4000–600 cm-1, resolution 4 cm-1, 32 scans). X-ray diffraction (XRD) was conducted using a Rigaku Miniflex 300 instrument (30 kV, 10 mA, Cu–Kα radiation – λ=1.5418 Å), 5–85∘ Theta/2-Theta, step scan 0.02∘, scan speed 3∘ min-1). Thermogravimetry and differential scanning calorimetry (TG/DSC) was carried out with a NETZSCH STA 409 PC instrument (room temperature – 1000 ∘C, pure N2 atmosphere, heating rate 10 ∘C min-1).

As mentioned in Sect. 3.2, there is a colour difference between the top surface and the bulk of the specimens with blood addition (A, C, D). SEM images on the cross section of these specimens show THE presence of “film” covering their top surface (Fig. 2). This film seems to be thicker (∼70 µm) in specimen A and appears to have gradually merged into the bulk; in specimen C and D the film is thinner, gathering on the very top of the specimens, with a thickness of around 10–20 µm.

According to Fig. 3, in specimen B, rhombohedral calcite particles can be clearly distinguished (B1); in specimen A, C and D, the CaCO3 particles appear to be in less defined or A rounded shape (A1, C1, D1). Additionally, in specimens A, C and D – taking specimen C as an example – the CaCO3 particles appear to be connected with bridge-like structures (C2, C3), which could be due to the oxblood addition to them. Figure 3 shows some more differences between the hydraulic phases in specimens A and D and specimens B and C. In specimen A and D (A2, D2), no obvious signs of hydraulic phases can be observed; instead, the CaCO3 particles and pozzolana are covered and connected by a network of membranes (A3, D3). EDS spectra of these membranes (A4, D4) show higher contents of carbon, indicating their organic origin (i.e. the oxblood). Moreover, considering the compositions of the specimens (Table 2), the organic membranes could be specifically resulted from the quicklime and the oxblood, an issue which needs further study. In specimen B and C, however, hydraulic phases in cubic shape were observed in both specimens (B2, C4), which might be attributed to hydrogarnet (C3AH6) (Madej et al., 2013).

In order to further investigate the nature of the CaCO3 formed in the specimens, analyses such as ATR, XRD and TG/DSC were performed. For specimens A, C and D, which contain oxblood, their ATR spectra show low intensity of the calcite ν4 band at ∼713 cm-1; XRD patterns show low intensity of the main calcite peak at 2θ≈29.5∘; TG and DSC curves of specimens A, C and D indicate relatively lower decomposition temperatures of CaCO3 than those of specimen B, which is without blood. All these results seem to support the fact that amorphous calcium carbonate could be formed in the specimens with oxblood addition, which is also suggested by the less-defined shape of CaCO3 particles in the SEM images (Fig. 3 – A1, C1, C2, C3, D1). The details of the analysis results will be elaborated on in future publications.

In this study, model samples prepared according to a relatively precise historic recipe were investigated. During sample preparation, the feasibility of the ancient recipe was discussed, and a better understanding of historic knowledge on lime mortars with blood addition was achieved. Some characteristics of mortars with blood addition were noted, such as the distinctive dark red colour on the top surface, the presence of “film” covering the top surface, the presence of organic membranes, as well as the less-defined shape of the calcium carbonate, and they could serve as some first indications for the characterization of historic mortar samples with organic – or even blood – additives.

This study intends to emphasize the importance of understanding properties and characteristics of lab-prepared model samples taking into account the historical framework as a crucial point; the model samples were in fact set up on the basis of a careful survey into relevant ancient sources and literature. Such a framework is of great significance, since when trying to understand a traditional material (such as the “common mastic” presented in this study), historic investigations were often partly neglected. Even when fully completed, their significance was not framed in a scientific experimental project. A great deal of scientific data have been collected from a series of analyses (e.g. ATR, XRD, TG/DSC) performed on these model samples, and the results, which are organized under various topics, will be published in several future publications, referring to the historic framework setup in the present study. The challenge of the present work relies on a full multi-discipline approach to bridge the history of architecture and building materials to conservation science.