A mortar maker’s guide to evolving mortar specifications in 18th and 19th C France and England and their implications today

In France and England, the late 18th and early 19th centuries saw the publications of essential treaties on construction and building materials, such as “Cours d’Architecture” by Jacques-François Blondel (1777), “Traité sur l’Art de Bâtir” by JeanBaptiste Rondelet (1803), George R. Burnell’s “Rudimentary Treatise on Limes, Cements, Mortars, etc.” (1850) and Valentin Biston’s “Manuel théorique et pratique du chaffournier” published (1836). In all of them, the way to burn lime is detailed along with methods to mix lime with sand, and recipes of a range of mortars are compared and given. It was a time of reflections and research on what constitutes a “good mortar” and how to improve it. Conservation practice is currently undergoing similar considerations and discussions. This work aims to explore the reflections and recommendations exposed in these treatises and guidance and how they could inform the choice of materials and the methods of mixing for the design of the conservation mortars used today, such as how to mix sand with fat lime and whether and when to add quicklime. By combining a detailed literature review of these treaties with knowledge of current mortars in conservation this work links history of construction and conservation science to help inform conservation decisions. These treatises reflect the evolution of thoughts and considerations on the composition of a good lime mortar at that time and show that many aspects, such as the quality and type of the burnt limestones, the sand, and especially how the mortar is mixed, are most important to obtain a good mortar. Conservation practice today could reflect those evolutions by adapting to the period of construction of the fabric being repaired and by being authentic about the methods and the overall type of mortar that was used at the time of the buildin