Medieval Mortar Recipe

(A 600-year-old still-standing section of a castle wall, built using the method described below)

This Medieval Mortar Recipe has been made the same way for over 1,500 years, with evidence of it first appearing in Britain as early as the 4th century. If you’ve ever been to a medieval castle then this is the type of mortar that was used to fix the stones in place, it was also used on the later roundhouses that people lived in up until the late medieval period.

From the middle of the 1800s onwards, cement took over as the primary material to use when it came to building with brick or stone, but everything before this would have been built using the method described below, and evidence of mortar works can be seen in churches, walls, and old stone bridges all around the country.

But what exactly is Mortar and how is it made?

Mortar is made by mixing sand, water, and a type of binder, which in this case was Slaked lime. All along the coastline of Britain, you can find many of the kilns used to create slaked lime from its raw material, which was limestone. The firmness of the mortar comes from using a calcium-based mineral which causes the mixture to set into something close to stone, and the only calcium-based mineral available in huge amounts was limestone rock.

There is a common belief that sea shells were used as the calcium base for mortar, and even though this would be possible it was never the case. Gathering billions of shells for something as big as a castle and being able to grind them into a powder simply wouldn’t be possible, especially with medieval levels of technology. The shell fragments often seen in the mortar of very old buildings would have come from gathering sand from the beach to use within the mix.

(All castles in the UK that were built during the medieval period were built using limestone-based mortar)

Limestone rock is the only thing that can be mined in quantities big enough to produce enough mortar powder for a church or castle and is also one of the softer rocks which makes it much easier to break off in chunks. The rock would be taken to a kiln and placed in alternating layers of rock and coal until it was full up.

The heat from the burning coal causes all of the carbon dioxide to be burned off and converts the limestone into calcium oxide, also known as quicklime. This is a very dangerous substance and the smoke from burning limestone can blind people, so the poor local village boy working on a kiln would have to risk losing his eyesight every day in exchange for pitiful wages.

Quicklime is a strong alkaline and works in the same way as a weak acid, but when water is added to it, the quicklime (calcium oxide) turns into Slaked lime (calcium hydroxide) where it loses its burning properties but retains a very high calcium content. The slaked lime rocks are then smashed in a powder and mixed with sand and water to create mortar, a very early form of what most people call cement.

The carbon dioxide doesn’t burn out of the rock until the temperature reaches above 900°C which is too hot for any open fire that doesn’t use coal and can’t be done in any large quantity unless contained within a kiln. Here’s a quick step-by-step process of how you’d create slaked lime when working on a kiln.

(The remains of an old limestone kiln)

 A simple Medieval Mortar Recipe

Step one – The delivery

Kilns are far more common along the coast than inland as the limestone was normally brought in from far away, with the only practical method being by ship. When the delivery was unloaded and brought to the kiln, the next stage would be to smash it into small enough chunks to heat.

Step two – Layering the kiln

The bottom of a kiln will slope inwards so there’s only a small space at the bottom for pieces to drop out of. The bottom layer would be coal and would be up to 12 inches thick, the kiln would then be filled up with alternating layers of coal and stone and ending with a coal layer on top.

Step three – Burning process

After the kiln was full up a fire would be started at the bottom and the burning process would last for up to two days depending on the size of the kiln. As the coal reduced in size more would be added from the top and possibly more limestone since it was more practical to just keep a lit kiln going instead of doing individual burns.

Step four – Raking and slaking

The coal would eventually turn into ash and start to fall out the bottom, along with pieces of limestone. Some poor locals who couldn’t find a safer job would have to stand at the entrance with a long rake-type tool and pull out all the white-hot quicklime chunks as they fell out the bottom.

If you let the limestone cool naturally then you have quicklime, but it doesn’t matter when the water is added in order to turn it into Slaked lime and so this is done at the site of the kiln while the rock is still white hot. Cooling it when it’s hot also helps to make it more brittle and easier to smash into a powder. If you ever see a kiln near the coast keep an eye out for a water source other than the sea, they were always built next to freshwater streams or rivers.

Step five – Powdering

There would normally be an area where the raker could just pull out a piece of rock and shove it into an area where water would flow through, letting the stream or river do all the cooling work. The next job is for someone to remove the cooled pieces of stone from the water and put them into a grinding area, which is usually nothing more than a clear piece of ground.

After heating and cooling the limestone rock becomes very brittle and is easy to smash into a powder, even with wooden mallets. One of the kiln workers would spend all day fetching pieces of rock out of a stream before smashing it with a mallet, only to shovel it up and bag it before doing it over and over again.

The powdered slaked lime would then be delivered to the site of construction where an equal amount of sand would be added and enough water to give it the right consistency. This method was used to build castles and homes for hundreds of years and since many of those castles are still standing it shows just how effective this early building technology was.

Apart from being used to make mortar, limestone had two other main uses. The first was as quicklime which was used to sprinkle on dead bodies to help them decompose faster and stop the spread of disease, and the second use was in the form of slaked lime on farm fields.

If you ever see soil that has a thin layer of slimy green moss on it then that happens when the acid level in the soil is too high, something that can prevent most crops from growing. After the last crop harvest, slaked lime would be sprinkled onto the soil and plowed in, and since slaked lime is a strong alkaline this lowers the pH level of the soil back to a more neutral level that crops can be grown in.