Intro: I was talking with an artist this last week. I noticed her paintings looked pretty wet, so I asked her if she used any medium, or if she had varnished the piece. She said she couldn't use medium because she reacts to turpentine (aka mineral spirits, well kind of) so she cant use any while she is painting in oil. I asked her if she had tried painting with Acrylic, or water based oils (both of which I can't stand, but I don't react to turps). She said she doesn't like them. I asked her what she used to thin her paint and she responded "I use baby oil" I was taken back so I asked her where she learned to use mineral oil, she had heard it from another artist. So I am posting some information on oil and its painting properties, what should be used and what should be negated.
Oil Paints: Without going to deep in the subject and if you would like to learn more please read from Max Doerner's "The Materials of the Artist" pg. 96-142 he goes into great detail about Oil Mediums. Oil is a binding medium we place in pigment. If too much oil is mixed with the pigment it will create wrinkles in the paint. If too little oil is in the pigment then it is hard to paint with (even though some artists like Lucian Freud places his paint on a paper towel to dry it, then paints with it, or at least that is what someone told me). Some Oil's don't dry at all, some dry quickly, some yellow, some go black. Oil's dry simply by evaporation much like water except at a much slower rate. If you want your oil paint to dry faster place a fan by it, warm it up, and stick it in the sun (as a side note I would never stick my painting in the sun, this is how paint companies find out how permanent their paint is, the sun will yellow your painting quickly so unless that is what you are going for I wouldn't do it). If you want it to dry slower then place it in a room that is dark, cool, and has no air flow.
Here is the breakdown.
Cold Pressed Linseed Oil: This is the most common vehicle for oil painting pigment. It doesn't yellow that much when painted in thin layers, but when painted in thick layers like with a palette knife it does show signs of yellowing over time.
Linseed Oil can also be in a Sun-Thickened consistency, it is Linseed oil dried in the sun until is has the consistency of honey. It dries more quickly than standard linseed oil and gives it more body. Sun-Thickened oil is better than boiled oils and remains very elastic and rarely cracks.
Stand Oil (boiled oil with carbonic acid) is also usually made from linseed oil. It dries more slowly than raw linseed because they have absorbed no oxygen and it weatherproofs a painting when added to the paint.
Walnut Oil: A little more fluid than linseed and does not yellow as much, so it is better to use with light pigments, it usually dries a little slower than linseed, the downside is that it divides more easily from the pigment compared to linseed oil, so you get the watery look. Robert Doak, Blue Ridge Oil Paint and some other paint makers mix walnut oil with linseed to get the best of both worlds.
Poppy Oil: Dries slower than walnut oil and doesn't yellow as much as linseed. It gives a nice buttery consistency so it is good to use with Alla Prima painting. It is not recommended to be used in layers because it is known to crack. It takes too long to dry for a great under painting. It has also been known to turn soft after it is dry and go dark in value, and even remain sticky. So only use it with Alla Prima Painting.
ALL OTHER OILS SHOULD NOT BE USED! if you don't believe me read the book and do some experiments. Never use mineral oil with your paints, it never really dries and linseed does dry so you will get cracks and a dust magnet to the never drying oil (have fun cleaning it too). Castor Oil is the same, never use it as a substitute for painting or even cleaning your brushes. Use Turps to clean your brushes when painting, then dry off the brush using a paper towel so you don't thin out your paints too much.
Rule of oil painting: Paint lean to fat. Now what does that mean. You use mineral spirits to thin your paint (less oil, more evaporation) when starting a painting. The more paint you add to the wet surface the more oil (or medium) you can add to the paint. eg:
step 1: Oil and Turps (thin or fast drying paint)
step 2: Oil (straight oil and pigment or a little bit of dryer)
step 3: Oil and medium (slower drying medium)
step 4: medium and Oil (aka glazing, but this should be done when painting is dry)
If you are working multiple layers wait until the paint is dry, not just to the touch, but dry all the way through. A simple way to test is lightly press your fingernail in the thickest area of the painting if it makes a dent it is still wet.
Another way to look at lean to fat is fast to slow drying. If you use a color and mix a fast drying medium then your paint will dry faster and can be placed in the lower layers so long as you place a slower drying medium in the layers on top, like stand oil. If you use fast drying on top of slow drying then you will crack your painting, so paint thin to thick, fast to slow, or lean to fat.
Summary: So now that you understand why certain oils are used and others are not we can get to the topic at hand. What can you use as a substitute for turpentine to quickly clean your brushes between strokes. You can use linseed oil, walnut oil, or Poppy Oil (I personally would not use it). I would not use anything else (Castor oil, mineral oil (or baby oil), turenoid natural, or any other substitute), if you do you will be placing your painting at risk to crack or never cure. Just make sure if you are using linseed oil or walnut oil that you dry your brush before dipping it in your paint. Cleaning your brushes at the end of the day will be another topic of discussion. Please comment if you have found any thing different from what I have written by reading and personal experimentation.