The Lime Cycle:
The image below shows what is known as the Lime Cycle. Much like the circle of life (see ‘Lion king’) but with less death and animals (fossils excluded), the lime cycle is the circular chemical process that naturally occurring limestone goes through to become a usable lime putty before returning to the same chemical composition as it was to begin with.
- The quarried limestone(Calcium Carbonate)is burned in a kiln. -Lime kilns are iconic on historical landscapes around the country and were once the backbone of regional communities for the local and commercial provision of lime for various uses, principally building and for farming as a fertiliser.The lime is burnt at temperatures of around 900C which drives off the carbon content of the stone.
This process yields what is known as quicklimeor Calcium Oxide.
- The Quicklime is a strong alkaline and is then either laid on the ground to neutralise acidic soils, or added to water to go through a process known as Slaking or Slacking. Slaking is an exothermic reaction which can reach temperatures well in excess of 100C,and can be quite violent causing the water to begin boiling almost instantly. This reaction transforms the Quicklime into lime putty or wet lime, or fat lime (Calcium Hydroxide).
- This lime putty can then be diluted with water to make lime wash or added to (or slaked with) sand to produce a mortar or plaster. This process can produce anything from fine plasters for polishing to course renders for external protection. Local sands would have been used historically and the variable chemical composition of these would have cause the resulting mortars to have quite different characteristics.
- The lime plaster or lime wash is then left to dry. Once it has dried, it can begin curing. It gets its strength by curing using the carbon from the air around us. This is a slow process which cannot not be rushed. As the lime takes carbon from the air it transforms back into Calcium carbonate which completes the circle.
Comparison over modern limes
Qualities vs cement
What is Cob?
Cob is an earth-based building material used extensively across the south west of England, although not exclusively. It can be found in many corners of the country, especially where a ready supply of stone may have been lacking historically or where labour was readily available.
Cob, as we call it down ‘yer is essentially just a clay rich sub soil mixed with straw. The clay content hardens to provide a dense lump and the straw provides tensile stability to the mass. There are many forms of earth-based building material in the world and they all have slightly different characteristics with regards to their composition, manufacture technique or application methods. Rammed earth and Adobe to name two.
Traditionally, the earth used to build these walls was dugout locally which leads to a diverse range of ingredients, such as sand, stone, clay, silt etc, being present in old constructions in varying quantities. Subsequently, there is broad range of characteristics which lead to variations in the ability for Cob and Earth mortars to handle moisture and damp ingress and structural weaknesses such cracks and subsidence.
The generally low skill of the traditional builders combined with an apparent lack of a need for accuracy during the building process of cob walls, often leads to inconsistencies in the coherence of the materials within a historic earth structure.
Cob houses today have often evolved from lower grade buildings which may not have originally been living accommodation. Many houses today have incorporated structure which were animal sheds which may not have had windows or a basic workers house which may not have had a first-floor level and the standards to which these alterations were carried out are often dubious at best.
Click Here to see how moisture is managed naturally in a traditional cob house.“