Usually the painter doesn’t use the colour as it is when it has just came from the tube. The colour has to be mixed with an appropriate medium. A medium is an additive the purpose of which is to change the feature of the colour, e.g. It edits the consistency or the time of drying. Usually a medium is composed by turpentine, a vegetable oil or the combination of the two.
Solvents are used in order to dilute the paint and to wash the brushes from it. The most used solvent is turpentine but there is a multitude of alternative solvents which are suitable for oil painting, e.g. odorless solvents.
Turpentine is toxic and very volatile so it has to be handled with precautions. Never use turpentine to wash your hands because it can cause irritation and it is dangerous for health. If you fear of coming in contact with turpentine use gloves while you paint. I use disposable latex gloves.
Don’t leave the brushes immersed in turpentine for too much time because it dissolves the glues and spoils the bristles. Moreover, it’s better to purchase turpentine in an art store because the one purchased by a hardware store may be unsuitable for oil colours.
The purpose of oils is to be adhesive, that is to stick the pigments on the surface. When the oil dries it creates a film in which the pigments are suspended in order to preserve them. Moreover, the light can pass through the various films and create sophisticated effects which improve the quality of the painting.
There is a lot of different oils that can be used to paint. These oils can be used alone to dilute the paint or in combination with turpentine. The most used oil are linseed oil, poppy oil and walnut oil.
This vegetable oil is the most used one in oil painting because of it stability over time, the high quality, and the price. Indeed, Linseed oil dries fast and doesn’t make the colour opaque. However, it tends to make the colour yellowish over time, and when mixed with some pigments it dries too fast and causes cracks. Its smell may result unpleasant.
Linseed oils is extracted form line seeds using various techniques. The cold pressed oil is the one of the higher quality because it contains few impurities. There are many other kinds of linseed oil:
- stand linseed oil: It’s produced by polymerizing the linseed oil. It has an elevated viscosity. Compared with the cold pressed one It dries more slowly, turns the colour less yellow over time and gives a brilliant finishing.
- Boiled linseed oil: It is produced by heating the linseed oil. It darkens the colours over time. It may contain additives that can soil the pigments.
- Sun-bleached linseed oil: more viscous and clearer than the cold pressed oil. It dries very quickly.
- Refined linseed oil: It dries slowly and turns the colours yellow over time. Usually it is produced using low quality oil.
These types of oil have different features when compared with the cold pressed linseed oil, so some painters mix them.
If you don’t want to use linseed oil there are a lot of alternates, the most famous are:
- poppy oil: Compared to linseed oil it turns the colour less yellow over time, it is more brilliant and it dries more slowly. However, it forms a weaker film and over time may create cracks.
- Walnut oil: this oil is of a lower quality if compared to poppy and linseed oil. It must be used fresh because over time it turns rancid.
Every medium has its own features. The main types of medium are:
- Standard: It is composed by linseed oil, turpentine and a resin. It is very recommended to the beginners.
- Glazing: This medium is used to create very transparent (glaze) or translucent (washes) films of paint which allows the light to penetrate to the underlying layers. It dries very fast.
- Fast drying medium: It reduces the drying time. Usually by using this medium the painting dries overnight. However, if the colours dries too quickly they may form cracks.
- Impasto: Whereas the other medium make the colour more liquid this medium turns it more thick. It used to create full-bodied layers of paint and it is employed it the namesake technique.
- Alkyds: This category of medium is composed only by synthetic substances.
Varnishes and resins
Varnishes and resins are classified depending on their solvent (alcohol, water, essentials oils, etc..). Varnishes are used as component of the medium, as “final varnish” or to adjust the painting.
- The retouching varnish is used to adjust areas of the painting which have already dried.
- Final varnishes have to be apply after six months (the painting has to be completely dry). They improve the painting brightness and make the surface uniform. Moreover, they protect the painting from factors that could ruin it.
How to use oils and solvents
In order to paint tow jars are needed. The first one is used to wash the brushes and contains only turpentine, whereas the second one contains the medium. Before washing the brushes with the turpentine they have to be polished with a slag in order to leave a consistent part of paint.
As said before the brushes must’t be immersed in the turpentine for a too long period.
The turpentine can be recycled: it is enough to allow the impurities to sediment and then transfer the turpentine which will appear clear.
Vegetable oils are used not only to create the medium but also to preserve brushes. Over time brushes tends to become “chewy”. This can be avoided just by putting a small amount of oil on the bristles after having washed them. Some painters recommend to use olive oil when the brushes aren’t expected to be handled for a long period.
- Wet-on-wet (“alla prima”): With this technique the painting gets performed in only one passage so it is more quickly than the “fat” over “lean” one. As the noun explain the paint is spread over wet layers of paint. Usually the painter uses only linseed oil as a medium, but some painters prefers a mix 50/50 of linseed oil and turpentine.
- Fat over lean: With this technique new layers of paint are placed over layers which have already dried. Turpentine is defined as “lean” whereas oil is “fat” so as the number of layers grows the medium has to become more “fat” (it has to contain a bigger amount of oil). The number of layers depend on the subjects and on the tastes of the painter. Never put “lean” layers over “fat” layers the dries more quickly and creates cracks. This is an example of the realization of a painting following the fat over lean technique in five steps:
- The first step consists in the drawing. It is recommended to use the charcoal or directly a thin paint ( the pencil can be difficult to cover with the paint so it could still be visible even if the painting has been completed)
- The second step is the “underpainting”: A layer of a very thin paint is put on the canvas. The medium is usually composed only by turpentine because it dries very quickly. Normally a single colour is used to do the underpainting, but it depends on the subject. This layer of paint allow the painter to have “general idea” of how the painting will look like when it will be finished and it is easy to edit.
- In the third step the painter creates the “chiaroscuro” by using a darker or brighter shade of the underpainting colour.
- In the fourth step the artist paints the subjects using a thin paint so that the “chiaroscuro” is still visible.
- The last step consists in painting the subjects using a thick layer of colour.
It is possible to accomplish only the drawing, the underpainting and the last step if one isn’t interested in following all the steps. As said before the part of the medium composted by linseed oil has to increase as the number of layers grows.