14.  Linseed oil-paint and tempera-paint on masonry and plaster

Linseed oil- and tempera paints represents very old types of surface treatments of plaster and plasterwork on facades - especially used and popular in Europe in the XIX and XX Century. Many of the plastered buildings from this period is "born" with linseed oil- and tempera paint on the flat plaster layers or just the plaster decorations.
As linseed oil- and tempera paint is quite damp open materials, they are very suitable for surface treatment of masonry facades. In spite of a little shiny gloss from the beginning, the treatments soon turns totally dull and bleaches slightly, with a very nice and homogeneous appearance, which deteriorates quite beautiful.
I former times the treatment of facades with linseed oil paint consisted of both poisonous and environmentally harmful  materials such as lead white, oil of turpentine, natural resins and strong acids. Today it is possible to make a complete environmentally friendly paint type without these substances.
Linseed oil paint and tempara paint on facades last empirically 8-10 years before the surface is so deteriorated, that a new coat must be added, which is quite suitable for a facade, as the building anyway must be checked from a scaffolding in regular intervals. At this point the linseed oil- or tempera paint is adequate degraded, that a cleaning and re-treatment is rather easy and cheap.

Mixing the linseed oil- and tempera paint and colours
Linseed oil is a natural organic oil, made by pressing the seeds from flax. After the pressing the oil has to season for a half to a whole year, after which it is boiled in special boilers.
The linseed oil paint is made by grinding the boiled oil and pigments in a roller. In order to strengthen the paint layer - and make it last a longer time, it is possible to mix natural resin from pine trees or larch trees in the boiling oil - but not more than 10-15% as the paint will become too hard and tight.
Tempera paint is made by adding 50% - 75% animal glue to the linseed oil paint or the resin-reinforced oil paint - and add more pigment. 'Tempera' means "mix in right proportions" in Italian. Other used names are "composition paint", oil-glue-paint and emulsion-paint. The animal glue emulsifies the oil and makes the paint more damp open and porous. Furthermore it makes the paint perceptible cheaper. Many plaster facades, which pretend to be "oil-painted" are actually tempera painted. 
Nearly all colours and pigments are obtainable, but for facades white, light grey, light yellow, light red, rose-pink and light green are to be preferred.

Demands on the backing ground
The largest problem for linseed oil- and tempera paint on facades with plasterwork is that the lime- and cement materials are rather alkaline, which deteriorate the paint. The masonry and plaster therefore must be completely softened and neutral. That means that a period of at least one year has to pass, before the facade can be painted, for new plaster on old masonry about 1-2 months and for smaller plaster repairs about 3 weeks.

Preparing the backing ground
The plaster has to be completely dry, without any deterioration, salts, dirt or fouling. Possible loose layers of old paint has also to be thoroughly scraped of, to avoid peeling from these.
The preparation of the backing ground - that means the finished layer of plaster, is executed with a layer of tung oil (Chinese wood oil) and boiled linseed oil 1:1. 
After a days drying, the oil paint or tempera paint is applied in 2 or 3 thin coats with 2-3 days between. It is a good idea before painting the whole facade or decoration to make sample field on 1 x 1 metre on the spot to judge the colour and surface in reality.

Applying the paint
It is important that the paint is applied in absolute thin layers, wet-in-wet as the paint inevitable will wrinkle or flow in long stripes. For every new coat, 5% boiled linseed oil is added to the paint to prevent tensions and stress between the layers ("fat oil paint layers on lean oil paint layers").
The application must take place in shadow and in temperatures from + 5  C to 30  C. 
While painting whole facades from a scaffolding, the entire facade must be treated within the same day , as the different temperature and climate the next day may change the colours a little.  If necessary the day-work can cease by a frieze, string course, downpipe or pilaster. 
Furthermore the application must be carried out  wet-in-wet with vertical brush movements, across the scaffolding planks, simultaneously by two painters, so no traces of the scaffolding lines are visible in the finished work.

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