2.    Materials

Lime
Lime is made by the burning of natural limestone (calcium carbonate CaCO3) in a kiln at 900-1000 Celsius. After the burning (calcining), the so called "quicklime" (CaO) is slaked with water to produce lime putty (Calcium hydroxide Ca(OH)2). The putty is then sieved to remove any unslaked lumps or extraneous materials and Running into a pit to mature.
This wet slaked lime putty can be mixed with sand of different roughness/size of corns to produce lime mortar or -plaster, which actually is the same, but used differently. The lime putty can also be mixed with water 1:5 to a thin surface treatment: Lime-wash. 
Normal lime-wash will always be white as the lime and the putty, and is therefore also called whitewash. If certain colours (pigments soaked in water) are added to this, you get coloured lime-wash, or lime-colours.
The hardening of the lime, both in lime-wash and in lime mortar, always is obtained through a carbonation, a chemical process involving the atmospheric carbon dioxide, but also with water as an important solvent. The process forms calcium carbonate  - the original limestone, firmly integrated in the building construction.

Air-lime mortar
The name air-lime, is due to the hardening process by the carbon dioxide from the air.
To make mortar or plaster, which actually is the same, but used differently, the above mentioned lime putty may be mixed with sand of different roughness/size of corns to produce coarse mortar or fine mortar.
The traditional mixing of lime mortar is lime putty and sand  1 : 3 (volumes).The mixing must be done by a machine for 20-30 minutes, while adding a suitable volume of water.
The air-lime plaster has the best water-draining properties, if the stratified layers of coarse mortar and the fine mortar are added correctly. Compared with the Portland Cement (OPC), the air-lime mortar has a higher plasticity and workability. The released hydration heat is lower and develops more slowly, giving origin to less water evaporation and retraction. After been stiffened, the air-lime mortar keeps an auto-regeneration capacity for filling cracks since its carbonation by the air occurs slowly along the time.
The air-lime plaster may be used as a sacrificed layer to remove/draw out salts from the masonry. Therefore the air-lime mortar/plaster must be preferred on most repairs on old buildings - also because its relatively weak material properties.

Hydraulic lime
Not all limestones are "clean" calcium carbonate. Certain limestones contain "impurities", mostly clay and silica. When this "impure" limestone are burned, the clay decomposes at between 400 - 600 Celcius and combines at 950 - 1250 Celcius (the top of the burning temperatures for hydraulic lime) with some of the lime, forming silicats and aluminates, especially tricalcium silicate and dicalcium aluminate.
When the burned lime is slaked, and water is added,  it immediately, and very quickly, depending of the strongness of the hydraulic effect, carbonates to a mixture of calcium carbonate and very hard cementitious materials and some clay. 
This hard lime, already known and used by the Romans, and therefore later called Roman Cement, was also called water lime, according to its ability to harden only by water, and even under water. Today this lime is known as Hydraulic lime.

Hydraulic lime mortar
The hydraulic lime hardens faster and the plaster becomes often harder and stronger than the air-lime-plaster. But the water and moisture from rain or snow will remain for longer time in the masonry and plaster, which may be fatal. On certain extreme places, chimneys, plinths,  windy west-gables etc., it may be appropriate to use hydraulic lime mortar/plaster.
The "classic" hydraulic lime mortar for exterior plaster is made of wet-slaked lime, hydraulic lime and gravel/sand in proportion 1:1:6 (volume measures). The hydraulic additive can be natural hydraulic lime, volcanic ash (pozzolans, trass) or finely powderet brick/roof tile dust. For more exposed areas or purposes, for instance plinths, ashlar surfaces and friezes, or repair works on 100 years old Portland Cement plaster or plaster details, a slightly harder and more resistant 1:2:9 hydraulic lime mortar is recommended. 

Portland cement (OPC)
The "impurities", which makes the hydraulic lime carbonate very quickly can also be added in the form of clay to the calk or limestone before or under the burning. This was invented in 1811 in England and, to distinguish this product from the often reddish Roman Cement, it was named ìPortland Cementî, from its supposed appearance and similarity to the white limestone of that name. 
First in the late 1850s the process for the industrial production of Portland Cement was ready. The ground limestone and the clay are mixed into a fine slurry with water. The mixture is fired in temperatures of 1300 - 1500 C, which is a lot higher than air lime and hydraulic lime, and the burnt lime (quicklime) and the clay unites chemically to form a clincer. After regrinding and firing, the white hot clincker powder is allowed to cool and a small amount of gypsum is added to lengthen the setting time of the Portland Cement. 
From the 1860s the Portland Cement turns to be the dominant material for plaster and plaster decorations on facades. In 1891 a Danish firm, F.L.Smidth, bought the British patent and managed to spread Portland Cement plants all over the world. 

Portland Cement mortar
The Portland cement mortar is even stronger and harder than the hydraulic mortar - in most cases too strong and moisture absorbing for the old weakened and vulnerable masonry materials on old buildings. Even if the plaster originally is made of Portland cement, it is recommended to use air lime mortar or hydraulic lime mortar to repair, because also the cement materials has also been weakened during the years.

Gypsum
Once mined from large gypsum quarries near Paris (thus the name plaster of Paris-), gypsum in its natural form is calcium sulfate. When calcined (or heated), one-and-a-half water molecules are driven off, leaving a hemi-hydrate of calcium sulfate. When mixed with water, it becomes calcium sulfate again. Gypsum begins to cure as soon as it is mixed with water. It sets in minutes and completely dries in two to three weeks. 
Gypsum was primary used as interior plaster, as finish coat and decorative plaster. For finish coats, gauging plaster was added to lime putty; it causes the lime to harden. Historically, gypsum made a more rigid plaster and did not require a fibrous binder.
Gypsum can also be used for cast decorations on exterior facades. For instance for bas-relief friezes, for "teeth" at dentils cornices or for other decorations. To this purpose it is necessary to paint the gypsum with linseedoil paint, as the gypsum decorations on facades needs protection from water.

Sand
The sand used in lime mortar must be totally clean from organic materials, humus, clay and salts. The two types of sand are "hill-sand" and "sea-sand" - both carefully washed, dried and sieved. The grains in the sand have furthermore to be sharp and of various size - from maximum 8-10 mm to very fine sand-dust. 
In order to minimise the retraction effect of new mortar, the sand should have a continious granulometric curve, with both thin and thick grains. As mentioned above, it is also very important that the different layers/coatings of the plaster has different grain-sizes, which are decisive for the size of the pores in the mortar. The pores have to be finer and finer from the ground and out, which will lead the water in the plaster out. If the opposite is the case, the water will soak straight into the masonry.

Lime-water
If the well-mixed lime-putty and water (1:5) is stored for 24 hours, the undissolved lime is sinking to the ground as a white sediment with some clear water above. This clear water is water saturated with lime (pure Calcium hydroxide Ca(OH)2), visible by a light shine of blue. This so called lime-water can be tapped and used for preparation of the ground before lime-washing or to a finishing and fixing coat on the lime-wash. 
Lime-water can be coloured with lime-resistant pigments, in a ratio of 7 % vol. pigment-pasta (se beneath). This is called lime-water-glazing, as the colour will be slightly transparent.

Pigments for coloured lime-wash/ lime-colours
Pigments for coloured lime-wash, also called lime-colours, shall be lime-resistant, which means resistant to basic/alcaline exposition. The "classic" lime-colours are therefore oxide-red, yellow-ochre, raw or burnt Terra di Siena, lampblack, raw or burnt umbra, earth-green or azurit-blue, Egyptian-blue etc. - always beautifully "whitened" by the white colour of the white-wash.

Laths
Laths provided a means of holding the plaster in place if the back ground is very deteriorated, brittle or weak. Wooden lath is nailed at right angles directly to the structural members of the buildings (the joists and studs), or it is fastened to nonstructural spaced strips known as furring strips. Three types of lath can be found on histo-ric buildings.

Wooden Laths is usually made up of narrow, strips of split willow twigs or branches with spaces in between. The plasterer applies a slight pressure to push the wet plaster through the spaces. The plaster slumps down on the inside of the wall, forming plaster "keys." These keys hold the plaster in place. 
Metal laths, in form of a steel net (galvanized chicken wire), needs less space than wooden laths, and is often better able to hold plaster.

Brick laths, steel net with lumps of burnt clay.

The laths will often have to be combined with additional key-arrangements into the wall, small iron nails, carved holes in the backing. Furthermore the backing coat mortar can be reinforced and the adhesion improved with fibrous materials such as cow hair, sisal fibres etc. 

General use of the materials on plasterworks on facades
The materials used for the plaster decorations are, as mentioned above lime-materials: Air lime mortar and hydraulic lime mortar (Roman cement). Further gypsum and lime-gypsum mortar. From about 1850, when the Portland cement was industrially produced, the "cement-mortar" becomes the dominant material.
The appearance of the Portland cement enabled among other things the introduction of the small or larger cast balcony at the facade, as a very popular element. As balcony balustrade, wrought or cast iron was very often used. The general rule is that:

• Decorated flat plaster is made of air lime mortar or natural hydraulic lime mortar. 
• Sunken batten decorations are made of natural hydraulic lime mortar or Portland cement mortar. Present repairs must use hydraulic lime mortar. 
• Running mould decorations are mainly made of air lime mortar or hydraulic lime mortar, more seldom Portland cement mortar. 
• In-situ modelled decorations are always made of air lime mortar or hydraulic lime mortar. 
• Cast decorations on facades are either made of gypsum or Portland cement mortar. 

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