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What makes oil paint so special?
Lustrous and versatile, oil paint has appealed to artists since the middle ages. Painters from Jan Van Eyck to Gerhard Richter have been drawn to its rich sheen and vibrant colour. Many oil paint recipes used by the world’s finest paint-makers have hardly changed for hundreds of years, because there’s no need – colour simply cannot appear any richer or more beautiful.
How is Oil Paint Made?
Oil paint has been around for thousands of years; the oldest evidence of oil paint dates back to 650 AD, in the Bamiyan Valley Caves of Afganistan where the paint was used to paint Buddhist murals. The oil paint used here is thought to have been made using natural resins and gums as well as animal proteins.
It is thought that linseed oil has gained favour as the most popular drying oil since the 1400s due to the ease with which paint can be applied in layers of varying consistency; it’s versatility also makes blending and glazing easy. Linseed oil, like all drying oils, has a chemical reaction with oxygen which causes it to harden, encasing the pigment and helping to maintain colour vibrancy for hundreds of years.
Although there are still some naturally sourced pigments that have been used to make paint since prehistoric times and continue to be used today (including ochres and iron oxides), today’s oil paint manufacture uses a lot of synthetically developed pigments, such as Vermillion (made by mixing mercury and sulphur, heating it, allowing the mixture to re-condense and then removing and grinding the mercury) and Prussian Blue. It is vital that pigments used to make paint are stable: that they are lightfast and so will not fade; that they will not be broken down or damaged by solvents such as turpentine; that they have the right pH balance with the drying oil so that there is no risk of any acidity in the oil bleaching the colour; and that they are not susceptible to chemically reacting with any other pigments used in oil paint manufacture.
Each pigment has its own set of characteristics including particle size, transparency and absorbency. These characteristics determine what ratio of oil to pigment is used in the paint, and the extent to which colours are ground/milled during manufacturing. It can take up to several years for oil paint manufacturers to carry out sufficient testing and refine the process for getting the purest and most stable colours from their ingredients.
Dispersing the pigment evenly into the oil
Most oil paint manufacturers will have a machine called an ‘Automatic Muller’ – a machine that consists of two metal discs that sit on top of one another and move in a circular motion but in opposite directions, dispersing pigment evenly into a drying oil when they are placed in between the 2 discs. (NB: Artists that make their own paints at home are likely to use a non automatic glass slab and muller which requires the artist to thoroughly mix the pigment and paint on the slab in a continuous circular motion, using a lot more elbow grease!). Several samples are made, each with slightly different proportions of drying oil to pigment. These samples are then tested to find the best recipe: a small quantity is spread across a piece of card with a palette knife, while another small quantity is mixed with a little white so that the paint makers can compare the body colour of the paint as well as its tint. Once the best recipe is identified mass production commences.
Mass production of Oil Paint
The quantities of linseed oil and pigment are multiplied by the same amount to ensure that the proportions of each are right for achieving the best colour. These are then put into the mixer; a big metal drum with an automated mixing device. The purest colours are made with a single pigment, however some paints are made with 2 or more pigments. The more pigments there are in a paint the harder it is to mix it and achieve vibrant colour mixes. However paints that are made with multiple pigments are done so to meet artist demands, usually to offer popular colours that can be used straight from the tube without need for any pre mixing.
The speed and duration of the mixing time will depend on the characteristics of the pigment, such as particle size and absorbency. Once the pigment is thoroughly dispersed throughout the oil it is then transferred to the milling device. Milling involves putting the paint through 3 spinning metal cylinders which will help refine the mix – removing any ‘clumps’ of pigment particles to even out the consistency even further. Again, the speed at which the cylinders spin, the amount of pressure, and how long the paint is milled for is all part of the recipe for each specific pigment – each has a different requirement. Milling can take anything from hours to days depending on the colour.
The batch of paint is then tested before it is packaged in tubes. First of all a spread test is conducted. This involves placing a specific amount of paint between 2 pieces of glass which are then squeezed together using a brass weight. If the blob of paint does not spread far enough then it is likely that the paint will need more milling. Secondly a dispersion test is conducted: a specific amount of paint is spread across a metal gauge which measures the size of the pigment particles; if they are too large then the paint will require more milling. The final test is the drying test. Each colour has a known drying time (anything ranging from 2 days to 2 weeks), so an amount of the paint is applied to a surface and assessed regularly by a technician to make sure it is drying as it should. If the paint passes these tests it is finally poured into metal tubes which are then labelled and ready to be distributed to art shops.
On occasion there are other ingredients in your tube of oil paint.
Beeswax is added to help the pigment stick to the drying oil and also help prevent the pigment from sinking to the bottom of the tube.
Driers are added to help speed the curing process (although most professional ranges avoid driers as they can impair the strength of the paint film)
Alkyd resin is sometimes added to add lustre and speed drying of the paint.
A drying oil is a natural oil that oxidises when exposed to air, causing it to contract and harden into a solid layer. Linseed oil is the most common drying oil used in oil paint manufacture; however Poppy, Walnut and Safflower are all used as well. Each oil brings its own characteristics to the personality of the paint.
Linseed Oil is essentially the same as the flaxseed oil you might come across in your local supermarket, but it is not edible! The extraction methods used for artist’s linseed oil use petroleum which makes it only really suited for art purposes. Conversely flaxseed oil will have impurities in it that will cause a greater degree of yellowing over time. Linseed oil dries very thoroughly and so creates very stable paint. Both Refined Linseed Oil and Cold-pressed Linseed Oil are known to be used in oil paint manufacture. Which is superior is a case of personal preference; while cold pressed linseed oil has not undergone any chemical treatment and so is therefore often considered the more stable of the two varieties of oil, refined linseed oil is likely to have less impurities and so will have less tendency to yellow over time. Ultimately you can get varying qualities of both, but what paint manufacturers (and possibly you!) will look for is an oil with the least impurities; these oils will look clearest in the bottle.
Poppy Oil is a very pale, more transparent drying oil that is less likely to yellow than linseed oil. It is much slower drying than linseed oil – on average 5-7 days – which makes it ideal to use when working wet-into-wet. At one time Poppy oil was used in the manufacture of some whites but more recently the use of less expensive Safflower oil has become more common.
Walnut Oil has been used to make oil paint for longer than linseed oil and again is used to make whites on occasion, as it yellows less than linseed oil. As a result paler and cooler colours suffer less change as they dry. The film of walnut oil when dry is stronger than Poppy oil (though still not as strong as linseed) which makes it a better oil to use in the initial layers of paint. It is a great oil to use when painting detail and it has a similar drying time to linseed oil. Walnut oil paints have a rich, silken texture with refined handling which is thought to have helped baroque painters achieve wonderful effects when painting the ruffs of collars and other detail in opulent clothing.
Safflower Oil is also used to make whites in some brands because it is bright and clean with less tendency to yellow than linseed oil. It takes 2-3 days longer to dry than linseed so is recommended only for use in the final layers of a painting.
How oil paints dry and the fat over lean rule
Oils can be extended with drying oils and thinned with solvent. The drying process is different to other paints; acrylics and watercolours dry as their water content evaporates into the atmosphere, while the oil in oil paint reacts with air causing it to solidify – this process is called oxidisation. In the meantime the solvent in the paint evaporates. While the evaporation of the solvent doesn’t take very long at all, the oxidisation process is very slow and never really stops. As oil paint oxidises it also contracts. If anything that dries faster is put on top of not-sufficiently-dry oil paint it is likely to crack. This explains why oils can only be painted over acrylics but never the other way around, and why oils should always be painted fat over lean: the more oil (or fat) in the paint the longer it will take to dry. Always put paint with more oil over the top of paint that has less oil in it (more diluted with solvent).
What is the difference between Traditional oils, Water-mixable oils, Alkyd oils and Oil Sticks?
- Traditional oil paints are made using finely ground pigment particles suspended in drying oils, usually linseed but sometimes pale colours are mixed with poppy, safflower or walnut. The buttery consistency can be made more fluid by whipping the paint up with a palette knife once squeezed from the tube. Transparency, sheen, drying times and thickness can all be altered by adding mediums, available pre-mixed or made using drying oils and solvents.
- Water-mixable oil paints offer the charm of traditional oils with the added advantage of not requiring solvents for either thinning or the clean up operation. Water-mixable oils can be a little more stringy or fluid than regular oils, but for many the advantages vastly outweigh the disadvantages – no need for the heady fumes of solvents, and a much safer and easier clean-up operation with soap and water. Special water-mixable oil painting mediums are also available. It’s possible to mix water-mixable oils with regular oils but in doing so they lose their water-solubility.
- Alkyd oil paints contain the same alkyd resin used in painting mediums to speed drying; a godsend for painters who have little time to spare.
- Oil Bars are also known as pigment sticks and are a mixture of pigment, oil and wax. Use them to draw straight onto canvas – a direct, tactile and expressive way of working. Marks can be thinned or made more fluid with solvent and oils, just like regular oils.
All brands of oil colour are intermixable. Traditional oil paints are classified as being either ‘Professional’, ‘Artist’ or ‘Student’ quality, with a couple of exceptions that straddle these classifications. Price and the name of the range usually indicate the paint’s quality.
- Professional paints pack in as much pigment into the oil as possible. Pigments are ground to a precise particle size that optimises the visual qualities of the colour, and then milled carefully into the drying oil binder. Occasionally the pigment to oil ratio is so great that the weight of the pigment sinks to the bottom of the tube, causing the binder and pigment to separate (this can be rectified by stirring the contents of the tube with a straightened paper clip). This allows the unique characteristics of the pigments to have a greater influence on the behaviour of the paint – properties such as sheen, transparency, tinting and staining capacity. These vary from colour to colour and contribute to the dynamism that can be achieved on your canvas. Professional paints comprise a greater number of single pigment colours; colours look purer, more luminous and are easier to create vibrant mixes with. On the website paints that are classified as ‘Exceptional’ at jacksonsart.com are generally considered Professional quality.
- Artist quality paints have a little less pigment in the mix but are usually made using very similar processes as professional paint. The characteristics of each pigment are maintained but with less intensity than the professional paints. Paints that are classified as ‘excellent’, at jacksonsart.com are considered Artist quality.
- Student quality paints have less pigment in them in order to keep production costs down, and may have added fillers to make the paints more uniform in tinting strength, viscosity and covering power. Driers are often added to slow drying pigments so that all colours dry at the same rate – the differing drying times of colours in professional and artist ranges can be a surprise for painters that have upgraded from student paints. Combinations of less expensive pigments are sometimes blended to replace the most expensive single pigment colours (these have the word ‘hue’ in their colour name). Colours may appear more chalky than Artist and Professional colours, and the choice is sometimes more limited and conventional. However, the affordability of student grade paints make them a popular choice for painters who are new to oils as well as those who need to stick to a budget. Paints that are classified as ‘good’ at jacksonsart.com are considered Student quality.
Same Colour Name, Different colour…
It’s worth noting that paints with the same colour name but made by different manufacturers may look different. This can be down to different pigments being used or differences in production methods – one brand may use different binders or fillers to another brand, and they may mill the paint differently too. Most oil paints (and certainly professional and artist grade brands) will list the pigment codes on their label, but again, if there are multiple pigments in the paint there is no guarantee that the quantities of each pigment will be the same in the different tubes. Ensuring the pigment codes match may not guarantee that the paint will look the same as it is squeezed out, but at least how the paints mix with other colours will be similar. Finally, colours may even have subtle differences within the same brand. Colours are made in batches and although manufacturers will try to match batches with what’s been made before as much as possible, if the pigment itself is slightly different (maybe as a result of being sourced from another location) then this will result in subtle differences in the batches of paint produced. If colour matching is of absolutely vital importance (as it would be if you were a picture restorer or if you were painting a highly detailed work) then we would always suggest you test the appearance of the paint on a separate panel prior to using it on your work in progress.
What mediums do I need?
There are more oil paint mediums on the market today than ever, giving you the opportunity to work with paint that has the exact qualities that you’re after. Here’s what you can do to modify your paint with oil mediums:
- Increase transparency
All mediums extend colour and therefore increase transparency. If you’re looking to keep the drying times nice and slow to allow for blending, add a little drying oil such as linseed, stand, poppy, walnut or safflower oil. The more you add and the thicker you apply it the longer the drying time, which can be anything from overnight to months. If it’s too thick there’s a risk of the top layer wrinkling, caused by the paint on the surface contracting at a faster rate than what’s underneath. Linseed oil is amber coloured and pourable like double cream, Stand oil is linseed oil that has been thickened through a special heating process, and is particularly treacle-like in colour and consistency. It’s very useful in glaze mediums. All varieties of linseed oil will give a yellow tint to pale mixes. Poppy and Safflower oils are both much paler than linseed oils and are often added to pale colours as they hardly tint colour at all, but they do tend to take even longer to dry. Walnut is also used with paler colours, and is faster drying than poppy or safflower. It dries with a more flexible paint film which means it’s great to use if you are layering colours. Alkyd resin mediums, such as Liquin or Jackson’s Fast Drying Oil Painting Medium will speed drying. Glaze mediums are specially formulated to increase transparency and gloss for glazes that layer beautifully. Solvents such as turpentine or Zest-It dilute rather than extend paint, so add a little of either to your medium to stop your paint becoming too gloopy. Household white spirit is too abrasive, can kill the vibrancy of paint and is not recommended for oil paint mediums. Remember too that some colours are naturally more transparent than others and that nearly all colour charts will specify the transparency of each colour in their range.
- Alter sheen
When used neat, oils usually dry with a subtle velvet sheen. Adding one of the drying oils or an alkyd medium such as Jackson’s Fast Drying Oil Painting Medium will increase gloss. For high gloss glazes try adding a small quantity of Dammar or retouching varnish to your medium, or try our pre-mixed Jackson’s Glaze Medium. Beeswax mediums will reduce or keep gloss at the original level of the paint. Adding a little more artist’s solvent to mediums will also help to bring down the shininess!
- Alter drying time
Drying oils (linseed, stand, walnut, poppy, safflower) will slow drying times, while alkyd mediums (Liquin, Galkyd mediums and any fast drying oil painting mediums), artist’s solvent (turpentine, Zest-it, Shellsol, Gamsol, Sansodor, oil of spike lavender) and driers such as cobalt (found in siccative) will speed drying. Many oil painting mediums available are made of a mixture of solvent, oil and resin and are designed to subtly increase gloss, give a workable consistency and dry faster than neat oil paint. It’s possible to alter pre-mixed mediums – reduce sheen and speed drying times by adding a few more drops of solvent, or increase gloss by adding a bit more oil. You can also make your own mediums by combining drying oils with solvents. A couple of drops of siccative, dammar or retouching varnish all speed drying and increase gloss when added to painting mediums. Siccative should only be added in very small quantities (10% of the paint mixture) to ensure that the paint doesn’t dry so fast that it cracks, and it’s best to avoid mixing so much solvent into your paint that it forms the majority of the overall recipe – this is because it breaks the paint down and very dilute layers of oil paint are much more fragile as there is not enough oil to harden to a firm layer.
- Thicken or thin the paint
Make your oils pourable by adding linseed oil and solvent in near equal measure (a touch more oil than solvent). If you want your oil paint to have a treacle texture and gloss try adding stand oil. Liquin Impasto or beeswax based mediums will thicken the paint even further without layers wrinkling or taking a lifetime to dry. To learn more about how to thicken or thin your paint it pays to try a little bit of this and a little bit of that – add a little solvent, a little oil, try more or less paint in the mix… so long as you stick to the ‘fat over lean’ rule (always apply mixtures with more oil in them on top) you can’t really go wrong.
Solvent based varnishes will alter sheen and protect oil paintings. Varnish is made of natural or synthetic resin crystals dissolved in solvent. There are 2 main types of varnish.
- Retouching varnish – for touch dry paintings. It is more dilute and forms a thinner layer than picture varnish. It is fast drying and will unify the sheen of your picture and give the surface some protection if you need to show it before it is fully dry (complete drying of the paint usually takes many months). Spray retouching varnish dries more reliably. Later, after the painting is fully dry, simply clean the surface of dust and apply picture varnish on top of the retouching varnish.
- Picture Varnish – for fully dry paintings (depending on the thickness of the paint, the support the paint’s applied to and the atmosphere, it can take anything from a week to a number of years for a painting to be sufficiently dry for varnishing). Contains more resin and when dry forms a much less porous and inflexible layer. Picture varnish is naturally gloss – matting agents or beeswax are added to make matte picture varnish. Spray varnishes are available for thin coats, otherwise varnish is best applied with a brush.