Arsenale in Amalfi

Medieval Mortar

Which organic additives did Medieval people use to get their mortar to bind?

Medieval mortar was traditionally made of slaked lime, sand and an additive or binder. These binders were added in order to improve the durability and hardness as well as reduce shrinkage and traction. Further, the binders helped to facilitate adhesion and increase plasticity and workability.

For some time restaurators have been interested in studying the different kinds of medieval mortars in detail, in order to be able to approach the restoration of old buildings with the original materials (see for instance about the use of ancient types of lime used in connection with the rebuilding of the medieval wall around Visby on Gotland).

The most common additives were animal glue, casein and other dairy products, beer, animal fat, linseed oil, albumen, blood and natural resins.

Recently a group of scientists have studied the mortar used in the old medieval military shipyard in Amalfi from the 9th century (but renovated in the 13th century). This particular mortar was made of lime, volcanic rocks and halite (rock salt). The last ingredient was probably caused by the nearness to the sea. Finally the scientists were able to identify a binder with a plant origin, echoing the ancient recipe of Pliny consisting of wine, hog’s lard and fig-juice.

Technology of Medieval Mortars: An Investigation into the Use of Organic Additives
By M. P. Rampazzi, C.  Colombini, C. Conti, C. Corti, A. Lluveras-Tenorio, A. Sansonetti and M. Zanaboni
Article first published online: 20 JAN 2015
DOI: 10.1111/arcm.12155

ABSTRACT:

This work proposes a multi-analytical approach to determine the additives in historical mortars, the use of which is widely described in bibliographical sources, but has rarely been reported in the literature. A protocol to thoroughly analyse mortars was created (optical microscopy, X-ray diffraction, infrared spectroscopy, thermal analyses and gas chromatography – mass spectrometry). These techniques, which had already been carried out on samples from various sites from the Roman to the modern era, determined that additives had only been used in the mortars from the internal masonry at our sampling site: the medieval military shipyard of Amalfi (Italy). The investigations yielded information on the production technology, and FT–IR and GC–MS revealed a saccharide material-based additive in the mortars, of plant origin. The FT–IR spectra suggested the presence of a natural gum, which has been used since ancient times to strengthen the cohesion properties of mortars and their resistance to tensile stress.

ABOUT THE AUTHORS:

M. P. Rampazzi,Laboratorio di Analisi dei Materiali Antichi, Università IUAV di Venezia, Venezia, Italy
C.  Colombini, Dipartimento di Scienze della Terra, della Vita e dell’Ambiente, Università degli Studi di Urbino ‘Carlo Bo’, Urbino, Italy
C. Conti, Dipartimento di Scienze Psicologiche, Umanistiche e della Terra, Università ‘G. d’Annunzio’ 66100 Chieti Pescara, Italy