The use of Chalk at Fort Burgoyne

The Explorers

Lee Evans Partnership is thrilled to have one of its buildings, The education Shelter at Samphire Hoe,  included on the Dover Arts Development  CHALKUP21 Art and Architecture Trail.

This year we are thrilled to be working with DAD at the West Wing of Fort Burgoyne in Dover.
For our building conservation of this Scheduled Ancient Monument chalk is integral to our work in conserving the structures.

Chalk is a hugely versatile material, for many of us our first thought of chalk is for art and the creation of images, or from school where it was the material used for writing on the blackboard.

Clay was, and still is, extracted from the earth to make bricks, in history the same was the case for chalk, and suitably robust strata of chalk were extracted from quarries and can be seen today in many churches and other historic buildings in Kent. The unfortunate name of this stone is ‘clunch’.

Chalks more common use, and its main use in Fort Burgoyne was for the creation of the mortar to lay the bricks to create the structures, this was a common practice, it was also used extensively for plaster. This was a practical choice based on the geology of the site, to import material would be much more expensive than to just use the resources already present on the site. Many ponds in farm yards are the result of the hole made to get the clay for the bricks used in the farm house and this use of chalk was not different from this principle.

So how could this calcium rich material be made useful? The main point to make about chalk is thats it is a lime stone, it is a material based on calcium carbonate, therefore it is capable of being transformed into lime mortar or plaster.

The process is simple, a hole is dug, the lumps of chalk are removed from the hole and placed into a make shift kiln made on site. The chalk is heated to a temperature of around 900 degrees C, this drives all the carbon and moisture from the rocks of chalk then, once cooled, this is now quicklime, a volatile material which has an exothermic reaction to moisture where huge amounts of heat are created. This is where the term of ‘hot lime’ comes from. As water is added the temperature can reach over 150 degrees C, this is called slaking, this can be either slaked to a putty, where only water is added in a container which makes what would be termed a lime putty. The other is sand slaked, where a sand bund is created, the quicklime is put in the middle, water is applied and rapidly covered in the sand. This exothermic reaction inside the sand mound drives the moisture from the sand, which has to be added later when being mixed for use. The process also causes the lime to expand up to twice its volume, creating open pours in the micro structure which allow for the movement of moisture when used in buildings.

During the Mediaeval period many of the finest buildings were built based on this tried and tested tradition of natural materials. Today mortar is termed to be ‘muck’ on site, this derogatory term is due to the lack of skill required to mix it. This was not the case in the 1860’s and throughout history, the masons who mixed the mortar were highly skilled and experienced, the skill required to mix the mortar was extremely high, and due to the nature of the mortar was often mixed weeks/moths before it was intended to be used, allowing it to mature and be at its most workable for the masons to use.

Those of you not familiar with traditional limes, here are the basics in 3 basic facts:

  1. The more pure/refined the lime stone which is burnt, and chalk falls into this category, the weaker the quicklime is created.
  2. The more contaminated (with clays etc) the stronger (hydraulic) the lime is created.
  3. The more contaminated the sands and aggregate with clays and other impurities the stronger (more hydraulic) the mortar became.

The big difference between limes and cement are that cement is fired at over 1200 degrees C and is then pulverised to create a very dense and impermeable material. A traditional mortar was little processed, it therefore worked in harmony with the materials it was used with. The number one principle of mortar is that it should be weaker than the material which it binds. Cement is stronger than almost all traditional materials, which is why the use of cement is so destructive in historic buildings.

The application of the different types of mortar are an essay in its self, however creation of the mortar at Fort Burgoyne would have been fired and then skilfully blended with sand and water, the natural contaminants of the chalk and sands would have provided the strength and hydraulic nature of the material. This mortar could then be used to bind the bricks of the fort together, making the mortar which binds the structure come from the ground upon which the structure is built.

This theory was confirmed upon the mortar analysis of the mortars in the Fort, it confirmed that the binder of the mortar was chalk, burnt on site. Therefore the repair mortar is created from chalk based lime putties blended with sands. To ensure the existing structures continue to perform and operate in the way they were intended.

Chalk! It’s such a versatile material, basic, that’s its beauty.

James Wood
Lee Evans Partnership LLP