Tacit Knowledge and Classical Technique in Seventeenth-Century France: Hydraulic Cement as a Living Practice among Masons and Military Engineers

Tacit Knowledge and Classical Techniquein Seventeenth-Century France

Hydraulic Cement as a Living Practice among Masons and Military Engineers

Roman technical practices that went out of common use in northern Europe during the Middle Ages were long thought to be forgotten after the fall of Rome. But in limited areas some of these techniques remained part of living traditions. One of them is the subject of this paper: the formula for hydraulic mortar. Its "rediscovery" in the eighteenth century was assumed to be a recovery of a lost art, but was really part of a longer process of reproduction and formalization. The "secret" of hydraulic mortar was apparently maintained throughout the Middle Ages as tacit knowledge, and techniques for making it were already starting to be formalized in the seventeenth century.

Hydraulic cement had been used extensively by the Romans. It was employed, for example, to build a new seaport for the city of Rome at Ostia. Masons added a volcanic ash called pozzolan to the conventional mixture of lime and sand to create cement that would harden under water. This material was durable as well as versatile, so the Romans also used it for construction on land, and even added pozzolan to their plaster. ¹

After the fall of Rome, masons in most parts of northern Europe stopped using this durable cement for construction. This led to the assumption that the secrets of Roman cement had been lost. There seemed few ways to explain how the technique could have survived the Middle Ages. Based on this supposition, eighteenth-century experimentalists "rediscovered" hydraulic mortar. But the canonical story of technical amnesia has been challenged by new evidence of continuity from classical times ² and the growing literature on medieval memory techniques. ³ Roman engineering did not disappear simply because Rome fell. Some techniques entered oral culture, moving outside the textual realm of classicists, ⁴ and were stripped of their provenance. In this paper, I will examine hydraulic cement as a case in point, drawing on data about the use of this mortar for building the Canal du Midi.

Remembering Hydraulic Cement

In 1670, the entrepreneur for the Canal du Midi, Pierre-Paul Riquet, imported pozzolan from Italy to make hydraulic mortar. He wanted it to erect a strong seawall for the terminus of the canal—the port at Sète (then Cette)—as the military engineer and chevalier Louis-Nicolas de Clerville had recommended. ⁵ The entrepreneur wrote to Jean-Baptiste Colbert, his patron and minister of the treasury, to say that a ship was on its way to Italy to acquire the (volcanic) sand. ⁶

Riquet's letter raises the fundamental questions for this paper: How was hydraulic mortar remembered and used before its "rediscovery"? What did Riquet, Colbert, and the Chevalier de Clerville know about pozzolan? Did literate Frenchmen understand this technique as Roman? How did Riquet know where to acquire this ingredient? What did French masons know about the material? And how did they mix their cement? ⁷ The limited evidence of pozzolan use on the Canal du Midi cannot provide clear answers to all these questions, but it can address them at least obliquely. It sheds light on the power of vernacular or tacit knowledge as a carrier of classical culture, and even suggests how the technique for making hydraulic cement began to be formalized in seventeenth-century France.

There were a half-dozen instances by which pozzolan appeared (or seemed to appear) to be cited in canal documents: 1) in the "Bail et Adjudication des Ouvrages à Faire pour La Continuation du Canal et du Port de Cette, 20 Août 1668," where "terres porcelaines" were specified for the mortar in the seawall; ⁸ 2) in a letter from Riquet to Colbert in 1670 saying that he had sent a ship to Italy to obtain pozzolan; ⁹ 3) in the devis or contract...