Lapillus, Pumice and Lime. The Neapolitan 'lastrico' Technique in French Architectural Literature (18th-19th Century)

This study investigates the transnational reception of the 'lastrico' technique – a Neapolitan method for waterproofing vaults and terraces using lime, lapillus, and pumice – within French architectural literature between the eighteenth and nineteenth centuries. Originally developed in Southern Italy and rooted in Roman construction traditions, 'lastrico' (or 'battuto di lapillo') is distinguished by its use of volcanic aggregates and the process of ‘battitura’, which imparts high impermeability and mechanical resistance. While the technique remained in continuous use in Naples and surrounding regions well into the twentieth century, its dissemination beyond local contexts was primarily facilitated through foreign observation and scholarly reinterpretation. The paper draws on a broad range of primary sources, including travel writings, geological surveys, scientific memoirs, technical dictionaries, and architectural treatises. Among the earliest and most influential observers were Nicolas Desmarest, Auguste-Denis Fougeroux de Bondaroy, and Pierre-Jean Grosley, who encountered the technique during travels to Southern Italy in the 1760s and 1770s. Their accounts, which highlighted the hydraulic properties of volcanic mortars, emphasized the potential of French volcanic materials as local alternatives. These observations laid the foundations for later theoretical and experimental investigations. A pivotal moment came with Jean-Baptiste Rondelet’s 'Traité théorique et pratique de l’art de bâtir', which marked the first systematic codification of the 'lastrico' within the French architectural canon. Based on direct observation during his 1783-84 study tour of Italy and supplemented by laboratory trials in Paris, Rondelet reframed the technique as a scientifically validated and theoretically transferable model. His formulation became the principal reference for nineteenth-century French and Italian technical literature. Through a comparative analysis of French and Italian sources, the study situates 'lastrico' within broader Enlightenment-era efforts to recover and adapt ancient building practices. It examines how the technique was redefined to align with emerging theories on hydraulic binders and cement, yet ultimately found limited application in practice. Despite recurring proposals to adapt it using local resources, lastrico remained largely confined to descriptive and speculative discourse in France. The paper also explores the phenomenon of “returning knowledge”, whereby nineteenth-century Italian authors reengaged with the technique through French codifications rather than direct local transmission. In doing so, the study reveals how a regionally embedded construction method, shaped by geological specificity and artisanal expertise, was abstracted into a conceptual model within transnational architectural discourse – highlighting the complex processes through which technical knowledge is appropriated, reframed, and disseminated.