13.  Lime-wash on masonry and plaster

Properties and experiences of lime wash
The properties of lime-wash, compared to plastic paint, silicate paint etc. is very often discussed. The 3 most important technical properties of lime-wash, which make this product the most suitable surface-treatment on masonry are:
1 We have very long experiences (over 1000 years) with lime-wash, protecting the masonry in a technical perfect way, allowing the construction to "breathe" and to get rid of moisture. 
2 The lime-wash is a very weak building-material, and therefore more suited for weak and deteriorated materials on old buildings, than many, too strong modern materials.
3 It is very easy and simple (and cheap) to maintain lime-wash. Contrary to modern surface- treatments the lime-wash do not need expensive removal of all the previous layers, before/when maintaining/adding new coatings.
Many people prefer the lime-washed surfaces for aesthetic reasons, because of the very beautiful and unsurpassed shine and texture of the surface.
There are also 3 limitations and disadvantages of the lime-wash: The very rigid demands to weather, temperature and moisture-conditions during the application, the strict demands to experienced craftsmanship and the demands to a perfect, un-defect, dry and "cement-less" ground. All 3 disadvantages can complicate the optimal use of lime-wash.
The lime-wash is partly washed down during the years, which has the effect, that the rain-washed surfaces are kept quite clean from dirt and so on. This gives the old lime-washed surfaces another characteristic look.
The experiences of correctly made lime-wash are a maintenance- or re-treatment-time of 5-8 years in many cases 8-10 years. 

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 different. The lime putty can also be mixed with water 1:5 to a thin surface treatment: Lime-wash. 

Lime colours
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 become coloured lime-wash, or lime-colours.
The hardening of the lime, both in lime-wash and in lime mortar, occurs always 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.

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 underneath). This is called lime-water-glazing, as the colour will be slightly transparent.

Water
The water, used for lime-wash, shall be plain water from the tap.

Pigments for coloured lime-wash/ lime-colours
Pigments for coloured lime-wash, also called lime-colours, shall be lime-resistant, that 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.

Mixing the lime wash
The lime-putty, used for white-wash or lime-colours has to be wet-slaked in the pit for at least 2 years, in order to be fine-grained enough for the purpose.

White-wash
The white-wash should be produced at least one day before use. You mix 1 part wet-slaked (at least for 2 years) with 5-6 parts of water. The mixture should equal to a concentration of 15%. The mixture is stirred well, for example with a machine-wiper. Before use, you should wipe the lime-wash careful again, and you also wipe the liquid in the meantime, during the use.
This liquid may seem very thin, with no colouring or whitening effect, but it will turn white as soon as it carbonates within a few hours.

Lime-colours
If the lime-wash has to be coloured, the normal un-pigmented, natural coloured whitewash is added lime-resistant pigments in the ratio of maximum 7% pigment-pasta (volume) to the finished lime-colour. 
Before mixing in the whitewash (lime-putty and water 1:5), the pigments have to soak in water or lime-water as a very thick and well-mixed pigment-pasta, for 24 hours.

Storing
The white or coloured lime-wash should always be stored in an non-freezing environment and also with an air-tight cover. If the lime-wash gets frost, the liquid will be grainy and must not be used. If a thin membrane of hardened lime covers the stored lime-wash liquid because of the influence by the air, it has to be removed before use.

Demands for weather and temperature
Lime-wash must under ideal circumstances take place during a relative humidity in the air at 65-75 %. The temperature should be minimum +5 Celsius and maximum +15-18 Celsius. Lime-wash must never take place in direct sunshine or on masonry, warmed up by the sun for long time. The best result is achieved by lime-washing in the early morning and/or under a constantly overclouded sky. Another possibility is to make a totally covered scaffolding, possibly with some vessels  with evaporating water or with some thick, wet jute sacks at the ground.
In Denmark, the traditional lime-wash-periods goes from March to mid May and again from September to mid November. Never lime-wash in the summer, unless you uses a totally covered scaffolding with respectively evaporating water-vessels, or heating in the winter.

Demands to the ground
The ground, on which you can lime-wash with an optimal result, has to be:
1 Masonry, at least 3 weeks old, and made of yellow or red bricks with a not too hard or smooth surface. The masonry must not have been cleaned/washed with acids.
2 Plasters and renders, at least 3 weeks old, and made of air-lime-mortar or hydraulic mortar, without any addition of cement-powder. The plaster may be coloured in the same colour as the lime-wash (see Technical Instruction 7.2.3: Plaster on Masonry).
3 Limestone, marble, lime-sandstone etc. with slightly rough/rugged surfaces.

With a less optimal result, you can lime-wash on:
4 Earth-constructions - with a suitable wet prepared surface, before lime-washing.
5 50 years old cement-plaster with a very porous surface -  well-wetted for the purpose.
6 Un-planed, sawn wood, carefully wetted beforehand.
7 Sandstone or granite, with rough/rugged surface - also well-wetted.

You are not able to lime-wash on:
8 New cement-mortar/plaster/render, concrete, eternite etc.
9 Bricks,  tiles, terra cotta with very smooth surfaces, as well as slates, 
10 Masonry saturated with or with remains of raising damp, hygroscopic salts, rust, crust, tar, dirt etc.

Tools for lime wash
The right tools for lime-washing of big surfaces is a big broad brush with stiff animal hair from cows or pigs. For smaller details applies minor brushes, also with animal hair.

Preparing the ground
Before the lime-washing, the ground has to be prepared with a coat of lime-water, but before adding lime-water, it is very important that the ground is well wetted with ordinary water. This can be done with a big broad brush or from a water hose with a effective scatter. The ground should be so wet, that ìshinyî water stands for some seconds on the surface, while adding new water with a big brush.
Immediately after the water-wetting, the ground is treated with a good layer of lime-water, applied with a brush and massaged well in all corners and hollows of the ground.

First coat
The first lime-wash coating is made 24 hours after the watering and the lime-water-treatment, because the lime-water has to carbonate. Also the lime-wash, coloured or uncoloured has as already mentioned to be mixed and prepared the day before the lime-washing.
If the ground has dried out in the meantime, which often happens, it has to be watered again with plain water, to obtain the moisture-condition, described under "preparing the ground".
The lime-wash is applied with a lime-brush in a layer/coating as thin as possible. The lime-wash is well massaged in all corners and hollows of the ground, criss-crossing  with the brush. After a suitable area of lime-washing, the surface is drawn up with parallel, horizontal light touches with the brush.
It is important to stir in the lime-washing-liquid for every brush-coat. The brush is splashed lightly in the bucket before coating, to prevent long "ropes" of lime-wash from the brush, on the wall, when applying. 
It is also important to apply the lime-wash in one process, wet-in-wet, without any stops. Natural stops can be made at house-corners, plinths, cornices or other natural borders on the facade. Furthermore, the gang-bridges on the scaffolding must not be the slightest visible in the lime-wash-surface.
After the first coating the lime-wash has to carbonate in 12-24 hours before the further treatment.

Second and third coat
If the ground has dried out between the first and second coatings, it is watered again with plain water. In order not to hurt or tear the newly made lime-wash, the water this time is applied by splashing the water carefully from the brush or watering from a water hose with a very soft scatter.
After this, the second layer of lime-wash is applied in the same way as the first coating, that means a very thin layer, but this time with a "lighter hand" without massaging and criss-crossing with the brush. Also this layer has to rest for 12-24 hours, before the third coating.
When making a lime-colour on masonry or plaster, it is usual to make the first coting white (whitewash), before 2-3 coloured coatings, as the white coloured ground will "shine" a little through the coloured layers.
The third coating, and if necessary a forth and fifth coatings, if the lime-wash is not forming completely satisfactory covering layer, is applied exactly the same way as the second coating. 
This judgement is made after drying of the 3. layer/coating.

Finishing treatment
After additional 12-24 hours, the lime-wash is fixed with a thin coating of lime-water. 
This can possibly be carefully sprayed on, with a special spraying-tool, not to disturb the carefully made lime-washed surface.

Other things
Horizontal plinths, decorations, wooden framework structure etc., which are not going to be lime-washed in the same colour or to be speckled with lime-wash, can be covered with a thin brush-coating of a specially made clay-gruel of red-clay. After finishing the lime-wash, the clay-gruel can be brushed and flushed off.
Normally, lime-wash should not be mixed with any additives such as casein, buttermilk, glue etc. (Max. 10 vol. %) in order to "improve" the properties and binding of the lime-wash. But as these materials changes the important binding of the lime-wash from an advanced chemical binding to a pure glue-binding, this can not be recommended on masonry. It can be appropri-ate in some special cases, for instance on wooden framework constructions, but not in all.

Maintaining lime wash
When the lime-washed surface looks worn, and the masonry or plaster ground is still intact and in good shape, the lime-wash-coating can be maintained more or less the same way as making a new lime-wash.
It is important first to remove all possible loose layers, parts or bits of the old coatings. This is most effectively done by hand with a metal-scraper (with rounded edges). Besides this all dirt, dust, growth of algae, etc. is carefully removed from the surface, for instance with a metal scraper, a steel brush or a hard broom.
If the lime-washed surface consists of more than 25 layers, or is more than 1_-2 mm. thick, it is recommended to remove all the old lime-wash for technical reasons, by using a very careful low-pressure wet-sandblasting - leaving a representative _ squaremeter untouched, for instance 10 cm just below the cornice, for historic and documentation reasons.

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