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Building a Castle: Mortar

You need mortar makers as well as well as quarrymen and stonecutters

Once you have the stone to build your castle, you need mortar to fix it together. As well as the quarrymen and stonecutters for the stone, you need mortar makers to prepare the mortar and mason layers to put the walls together using trowel, plumb line and mason’s level.

Mortar was made from lime, sand and water, using different proportions depending on the quality of the materials. They kept the recipes secret and passed them down from father to son. Just as they used three different types of stone (see last week), they used three types of mortar: a flexible type for arches and vaults, a fine type for facing walls, and a coarse type for the rubble core of the walls. This last type can take hundreds of years to set, to allow for the stones to settle over time. Archaeologists have found mortar in the centre of some castle walls that is still not set, centuries after it was mixed.

At Guédelon they originally used sand from the River Loire and industrially produced hydraulic lime, which was not very authentic and looked wrong. They called in an expert who analysed local medieval mortar and they learned to make mortar using local materials, including making lime from scratch.

How to make lime

The people at Guédelon built a kiln against a bank, and built a dome on top from limestone offcuts. The dome is covered with an insulating layer of straw and clay and a fire is lit in the kiln. For the first few hours the heat drives the moisture out of the stone, then the temperature is raised to 900-1000°C (1650-1800°F) and maintained for 3 days. I should imagine they had to work shifts through the night. When the clay is removed the limestone has turned to quicklime, and is plunged into water to finish the process.

How to make mortar

Basketloads of sand from the quarry at Guédelon are laid out on boards. The lime is thinned with water to a smooth, creamy consistency. The lime is poured over the sand and they are mixed together using long-handled hoes.

The mortar for the rubble cores of the walls is made from one part gravelly earth, one part sand and one part lime. The mortar for the facing walls is two parts sand to one part lime.

[adapted from The Medieval Castle Haynes Manual by Charles Phillips]

Ann Marie Thomas is the author of four medieval history books, a surprisingly cheerful poetry collection about her 2010 stroke, and the science fiction series Flight of the Kestrel. Book one, Intruders, and book two Alien Secrets, are out now. Follow her at