Both lime and clay are breathable renders so allow the passage of moisture vapour and air thought them. This means they can be rendered on a material that you do not wish to rot and will protect it from rotting if it has not be sealed with paint. The main use of this is in straw bales where you need the bales to breath to stop moisture build up which would make the bales not.
One of the other main advantages of lime and clay is that they are flexable, which means they are able to move slightly to accomodate the swelling and shrinkages of summer and winter.
Lime plaster has one other main advantage that makes it immensily useful for warmer climates is that it alternatively repells insect and bugs protecting your home from small creatures.
Lime has been the binding material (mortar) for a long time in England and Ireland , long before cement came along.
Lime renders have been know to last for hundreds of years but however it is a very delicate process with lots of factors such as drying time, variables in sand that can seriously effect the durability of the plaster.
The raw material for all lime mortars is alternative orccuring limestone, shells or coral which is calcium carbonate (CaCO3)
Calcium Carbonate needs to be turned into lime putty, this happens in a special kiln as it needs to reach between 900 – 1200 degree centerigate for 12 hours to drive off all the CO2
heat + CaCO3 = CaO + CO2
heat + Calcuim carbonate = calcium oxide and carbon dioxide
What you have left from this reaction is a white calcium oxide (often called quick lime) and black and burnt pieces which you need to remove. This white quick lime is very reactive and can be dangerous and needs to be stored safely.
Always wear protective clothing, goggles and a mask when working with quicklime as can be dangerous.
To make lime putty from quick lime you need to add the quick lime back to two parts of water. Be very careful as this reaction can produce a lot of heat and either needs to be done in a metal bath or in a pit.
Always add quicklime to water and never water to quicklime as it can explode.
This mixture needs to be sived before use. Alternatively lime putty can be purchased from suppliers in most countries.
Lime putty render mix can be made by mixing 1 part lime putty with 3 parts sands. The sand must be well graded and best bought off a supplier as this is going to be your final coat. The only difference between render and plaster is the finest of the sand so to make a plaster mix you just need to get finer sand. The longer the lime putty has matured the stronger it is. It needs to be left for 3 months either as lime putty or as made up render. If the mixture is too tough it can be beaten which losens it up back to workable state.
When the lime render has been used it is important to understand what is going on with the lime after it has been rendered and exposed to air as it continues to carbonite for a while. It is not uncommon for renders that have been set for hundreds of years to not be fully carbonated yet.
Ca (OH)2 +CO2 = CaCO3 + H2O
Calcuim hyrdoxide + carbon dioxide = calcuim carbonate + water
This means for this reaction to happen the carbon dioxide must be disolved in water. The idea conditions for lime and high humidity and good ventilation.
In the days after applying the render it is important to protect the render from direct sunlight, driving rain, forceful wind and frost. Often this can be done by placing bags on scalfolding infront of the wall to protect it from the elements.
Lime renders needs to be limewashed every two to three years at first but this reduces as time goes on as the render becomes stronger .
Lime wash is made from lime putty and is diluted with water. The limewash seals up the cracks in the render. Lime wash should be applied in thin watery coats, leaving it to dry overnight between coats. Pigmants can be added to the lime to colour the building to your choice.
Active silicate paints can be used as long as they don not have any sealant chemicals in them. These provide much better weather protection that lime washing and do not need to be reapplied for about 15 years.