3.    General principles for the crafts techniques

Demands on weather and temperature
Lime mortar-repairs must under ideal circumstances take place during a relative humidity in the air at 75-95 %. The temperature should be minimum +5 Celsius and maximum +18-20 Celsius. A total absence of sun and wind is also recommendable. These circumstances, moisture, low temperature and no sun, promote a slow-acting carbonating and hardening of the mortar, which is necessary for an optimal binding of the mortar to the ground and prevention of small (crocodile-skin) shrink cracks of the surface. 

Watering the ground
Before adding new plaster, the masonry has to be watered carefully with plain tap-water. The carbonating and hardening of the mortar needs, besides CO2 from the air, also water. If the ground is too dry, the binding of the mortar to the ground will take place too fast, which will cause a poor fixing of the mortar and often larger shrink cracks of the surface.

Thin layers on hardened ground
Layers of mortar should always be applied in a fairly thin layer - and always on a backing layer which has had time to carbonate. By this procedure, the inevitable shrinking cracks in all lime- and cement mortars, will be covered by a new, thin layer of mortar, with less shrinkage. If the layers are too thick or the ground is not fully carbonated, before adding new layers, this on the contrary will cause large shrinking cracks in the surface.
That is why the running moulds in the drawings and descriptions in this material are prescribed made in two sizes - a smaller mould size for the backing coats, to allow this layers to harden, including shrinkage, and a larger and true size mould, for the last coat. 

Fine pores on larger pores
Layers of mortar shall always be made in three coatings or layers: 
  1 A thin, rough backing coat of hydraulic lime-mortar with quite coarse sand, thrown on the masonry, keeping a fairly rough surface. This layer should harden 2-3 days.
  2 A 2-3 cm thick second coat of coarse mortar, drawn to an even layer and after that allowed to harden for one week. 
  3 A 1 cm thick finishing layer with fine grained sand, thrown on and scoured to an even coating. 
As water inevitable moves from larger pores to smaller, a construction with fine pores on larger pores, this construction ensures, that intruding water will move out again relatively fast. 
The thickness of the rough, first layer must not be more than 2 cm, as thicker layers will cause internal pressure between the interior areas and the area more exposed to the weather conditions, where the water evaporation and the quick cooling of the mortar occurs. This effect favours the retraction effect which often causes cracks in the surface.

Traditional materials - without modern additives
It is important to use the traditional materials as original and pure as possible. Remember, that the now deteriorated old lime materials, often has lasted for 200-250 years. No modern additives, such as acrylates, casein, resins has yet proved to last more than 25 years. 

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