Oil colors are paints (coloring coating materials) that consist of drying oils as binders and pigments . Drying oils harden through oxidation and polymerisation and, together with the pigments, form the desired color layer. In European panel painting, linseed oil , walnut oil and poppy seed oil were mainly used with and without additives such as siccatives or stand oil .
As a rule, oil paints for artists are supplied as pastes in tubes . The American painter John Goffe Rand (1801–1873) received a patent for the tube on September 11, 1841 in the USA (No. 2252), and in the same year in England (No. 8863, on pewter tubes with screw-on lids and the associated filling machine) . He was annoyed about drying paints and developed a tube of lead, because until then painters in their studios only mixed the paints immediately before use. The toughness is slightly higher than that of toothpaste and can be thinned with suitable painting media . On the tubes between 20 g and 400 g, one to five stars indicate the lightfastness , and usually a square - either white, half or all black - indicates the opacity of the color. Some artists mix their own oil colors and thus have a direct influence on opacity and pastosity.
Oil paint hardens much more slowly than acrylic and gouache paints, for example . The hardening takes a few days even with thin layers and can take years to decades with thicker layers of paint. This property is an advantage when producing large-format works and the wet-on-wet technique . Using special painting media , slightly dried-on oil paint can be partially revived. The industry now offers the faster-drying alkyd oil paints that are dry to the touch after a day. The more recent developments also include water-thinnable oil paints , which avoid solvent-based thinners.
Oil paints are characterized by intense colors , some of which are outside the color space of a photographic or printing technique (e.g. cobalt blue ). High lightfastness, opacity and durability are further advantages.
Oil paints require a lot of experience from the artist. The processing is complex. A pre-planning of the work of art is necessary - oil paintings need a correspondingly long time in their completion due to the drying pauses. Oil paints cannot be mixed with water-based colorants . In most cases the painting is done on canvas or wood. Oil paint adheres well to various surfaces, but it is sensitive to mechanical deformation and peels off easily. In order to avoid the formation of cracks with thicker layers, oil paints should be applied "bold over lean". The content of pigments or other fillers in the paint should therefore decrease when further layers are applied. The painting surface should be slightly absorbent , i.e. primed with a little open pores and the subsequent layers should also be applied matt and porous so that the layers adhere to one another. Only the last layer can contain enough oil to act as a waterproof coating.
A final varnish is applied to protect the oil color layer on paintings. The most common varnishes are the soft resins dammar and mastic , which due to their yellowing process are nowadays replaced by synthetic resins.
Color pigments are added to the oil in order to obtain the desired color.
Oil paints contain hardening vegetable oils , which, depending on the oil, are also known as “ drying ” or “semi-drying” oils. The iodine number of the oil gives an indication of the oil's ability to harden through polymerization .
Frequently used oils:
- Linseed oil , yellowed, therefore more suitable for darker shades
- Poppy seed oil
- Walnut oil , not turning yellow , slowly drying
- Hemp oil
- Sunflower oil
- Safflower oil , hardly yellowing, dries slowly (the final hardness is reached after about 4 weeks)
- Soybean oil , a semi-drying oil
- castor oil
- Perilla oil
- Tung oil (Chinese wood oil)
- Oiticica oil from the nuts of a Brazilian tree, serves as a substitute for tung and linseed oil, but is significantly darker and thicker
- Tall oil is made from wood fibers
- Stand oils are particularly weatherproof and are made from linseed oil or any other oil by heating
Linseed oil, poppy seed oil and walnut oil are also referred to as fatty oils in painting .
The hardening is based on a chemical reaction of the oxygen in the air with the double bonds of the unsaturated fatty acids and a transesterification of the bonds between fatty acids and glycerol , which leads to cross-linking of the molecules; in the case of linseed oil, linoxin is produced . The oxidation produces hydroperoxides , which break down radically and react with other oleic acid molecules. So-called siccatives (e.g. cobalt naphthenate) promote radical networking. Occasionally, synthetic solvents are also added (1,4-polybutadiene oils). The solidification of the paint only takes place when a solvent is used, also partially by drying . Depending on the vegetable oil used, oil paints tend to yellow . Frequent contact with water leads to hydrolysis of the ester bonds, which reduces the durability especially of oil paints with water-based solvents and leads to a washout effect on painted surfaces. The drying time is usually longer than with conventional paints. Linseed oil paint is also used for priming of steel used, along with micaceous iron oxide as an anti-rust paint .
Oil paints with admixture of resins are known as varnish . If pigments are added in addition to resins , an oil varnish or natural resin varnish is obtained . Oil varnishes have been known in China for over 2000 years and are still used today (sometimes with synthetic binders). The proportion of solvents can be up to 60 percent, but paints emulsified with water are now also produced.
Often oil varnishes are made from linseed oil, tall oil, castor oil and perilla oil, which may have previously been boiled to form stand oils. Traditionally, natural copals , rosin or shellac were used as resins, and today synthetic resins such as alkyd and phenolic resin are also used .
The addition of resins increases the surface hardness of the coating, while the drying time, suppleness and weather resistance usually decrease.
Linseed oil paint for construction and craft
For the production of oil paints for construction and handicrafts, pigments are rubbed with so-called "boiled" linseed oil and mixed with 0.09 to a maximum of 3% dry matter. The highest quality is offered by linseed oil paints without solvents and aromatics . You do not need any further additives for processing, but can be diluted with boiled linseed oil if necessary. Since linseed oil paint is water-repellent but open to vapor diffusion, it is particularly suitable for outdoor use (half-timbered buildings, wooden facades, doors, windows, folding shutters, wooden constructions) because moisture that has penetrated the damaged areas can evaporate again. This prevents the formation of rot and mold. Aging linseed oil coatings should be treated with cold-pressed, "boiled" linseed oil every five to eight years, depending on the weather. If the paint should fade after many years and several nourishing oil coats, which is especially the case on the west and south sides of buildings, it can be painted over. The old paint does not have to be removed for this, just cleaned by brushing. In the preservation of historical monuments, the monument authorities expect the use of traditional linseed oil paint without volatile organic compounds , as pure linseed oil paint is produced according to a traditional recipe and thus falls back on authentic historical paints and linseed oil primer. In addition, solvent-free linseed oil paint does not form layers that could peel and it does not become statically charged, which means it does not attract dust.
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