2.5.1 On mediums.
Solvents and Mediums are used to dilute and cary color, increase gloss and transparency, reduce drying time, and avoid over-thinning. There are many mediums that the oil painter may use. I have taken my notes from the past on the mediums that I have worked with, given you a definition and my thoughts on each. I found mediums useful to create many different appearances while exploring oil painting over the years.
Poppy oil: Poppy oil is a little thicker than linseed oil, and is very pale in its color. Its better than linseed oil because it has longer non yellowing abilities, but poppy oil will crack and become brittle with age faster than linseed oil. The abstract oil painter should just stay the hell away from poppy oil. It’s delicate nature after it hardens has a tendency to crack easily and splinter into a multitude of cracks if used too heavily. If you are looking for a cracked appearance in the long run, then use poppy oil.
Rectified turpentine: Rectified turpentine, along with venetian turpentine are two of the best thinners for oil paint. Or so I was told by a traditional realist while I was taking classes in classical realism. I personally do not like it, as it is expensive and I can get the same effects with Gamblin Gamsol an odorless mineral spirit with a much lower price tag. Rectified turpentine does have an ability to pull paint out of the bristles of a brush more effectively than other thinners.
Walnut oil: Walnut oil is very thin when compared to linseed oil. And will distort the consistency of oil paint, also it is one of the weaker mediums. Think of a vanilla and chocolate swirl cake and thats what you end up with most of the time even if you are very careful. Walnut oil will slow the drying time, and because of that I feel it has no place in the use of abstract art.
Linseed oil: There are many forms to linseed oil and it is probably the most popular oil painting medium out there. It is affordable, has may uses, and is mostly consistent in its application. I like it, if i do chose to use a medium then linseed oil is the one I prefer.
Cold pressed linseed oil: It increases the transparency and gloss of paint, and slows the drying time. It is best to use it alone, or the drying time will drastically speed up.
Refined linseed oil: It is the same as cold pressed, but heat and steam are added to the process to extrude more oil. It is weaker than cold pressed. The one quality that I love about refined linseed oil is that it magically removes brushstrokes. It has the remarkable quality of reducing the marks of the tool you are using; a paint brush, palette knife or whatever you chose that day.
Sun thickened linseed oil: This is simply raw linseed oil mixed with water and then set in the sun for a year or more. It has an enamel like ability, and dries faster than the other linseed oil processes. It is best used in glazes because of its ability to resist cracking and yellowing with age. It is very expensive and unless you really need it, just go with a stand oil turpentine mix. But again as a professional, the use of sun thickened linseed oil is a part of the heavy costs of being an oil painter. It is worth the price-tag.
Stand oil: Stand oil is by far the most difficult and best medium an artist can learn to use. Treating linseed oil with heat until a slight polymerization occurs creates stand oil. It has a very thick consistency like that of honey, and is a pale or transparent amber color. Its intended use is for the artist to thin the stand oil with a solvent before he mixes it with oil paint. Stand oil is the strongest painting medium and will resist yellowing or cracking with age better than other mediums. Stand oil also when used can reduce and sometimes erase all traces that a tool was used to apply the paint. And because of that ability to rid a painting of the signs of brush stroke, it is best used in transparent glazing. It takes forever to dry, and I mean forever.
My personal favorite is a mix of three mediums. I found the use of 1/3 stand oil, 1/3linseed oil, and 1/3 damar varnish with a tiny amount (approximately 1 tenth of the total mix) of unbleached titanium oil paint, mixed evenly has the best overall effects in control of both the consistency of paint and the use of the brush. Mix this in a blender until it becomes a froth, and then cover the medium and let is stand until the air bubbles are gone. it will take about three days for the mix to settle. When this mix is added to oil paint it becomes like a creamy soup, and is always consistent. If your end result doesn't look and act like gravy then its not mixed properly.
As far as solvents go just simply use whatever you can find that works. I like gamsol but I have used many others. Whatever is on sale is what I buy. Solvents are mainly used to clean your tools with, so there is really no need to spend big money on something that you intend to pour down the drain.
I have experimented with non-painting intended solvents and oils for mediums. I have used WD40 for the effect of sharp textured painterly strokes of color with oil sticks and oil pastels. It was probably the best experiment so far simply because it didn't fall off the canvas in a year. I take that as a successful test.
I do not use mediums at the moment. I have found that if I work my oil-color on the palette I end up with a consistency that I like to work with. Working purely with oil paints has its limitations, but we can go over that later. As it goes with all for the arts, the use of mediums and solvents are a personal choice that only you can make.