I had another look at the Artisan website, where the manufacturers of the water mixable oil paint also explain the proper use of their thinners and mediums.
They have been manufactured specifically for water mixable paints, so they are intended for use with that type of paint. As you mention, the fornulation of water mixable paint has been altered to eliminate turpentine and mineral spirits. And also, as you say, it is unwise to over-dilute the paint.
I don't use water to thin the water mixables, although the manufacturer says that a small amount of water can be used. even with thinner.
Here is what Artisan says: http://www.winsornewton.com/products/oil-colours/artisan-water-mixable-oil-colour/artisan-thinners-oils-mediums--varnishes/
"Although water is suitable as a diluent for the colour, its speedy
evaporation can make the colour thicken upon the palette much quicker
than conventional oil colour would when used with turpentine.
To help with this issue we have developed a Thinner to dilute Artisan
colours. This can be used instead of water or combined with it.
For fluid, transparent colour with an even sheen, dilute with Artisan
Painting Medium, Artisan Fast Drying Medium, Artisan Linseed Oil,
Artisan Safflower Oil or Artisan Stand Oil as well as water or Thinner.
Artisan can however, be thinned right down with water or Thinner for
staining a canvas in the early stages of a painting.
Important: Beware of over thinning. Water and Thinner thin the
colour by diluting the modified linseed oil. If too much is used, there
will be insufficient oil remaining to bind the pigment. The paint
surface will be susceptible to damage and appear dull and matt.
Fast Drying Medium a very popular medium as it speeds the drying of
oil colour by about 50%, allowing further layers to be applied more
quickly. The formulation has been improved and this has helped to wet
the colour better. It thins the colour and increases gloss and
transparency. When painting in layers it can substitute linseed oil and
be combined with water and/or Thinner to maintain fat over lean."
This is where I am getting my information on how to paint 'fat over lean' while using the water mixables.
Check out our blog at http://helenalanspinney.wordpress.com/
By all means, if the manufacturer makes a thinner product specifically designed to go with its line of water mixable oils, it is the ideal thinner to use. Their thinner product is formulated to allow a slower rate of evaporation than water alone.
Their statement warning about "over-thinning" resulting in "insufficient oil remaining to bind the pigment" is important to note. Whether conventional oils or water-mixable oils, the grinding of the pigment IN THE OIL is a key factor. In the case of water-mixable oils, the pigment is ground into their specially-processed linseed oil rather than straight refined linseed oil so the behavior is different (allowing it to be mixed/thinned/cleaned up with water) but the principle is the same. The dry pigment is ground in the oil, both reducing the size of the pigment particles, but mostly ensuring that every particle of pigment is completely enclosed in oil. The more mixing in of water that goes on, the more likely that this oil enclosure is disrupted and the structure of the paint layer will be affected and be detrimental to the strength of the cured paint layer that results. While made to work WITH water, the ultimate strength and adhesion of the paint layer STILL depends on the integrity of the oil-bound pigment particles. If the oil is insufficient, the strength and adhesion of the binding layer is weakened. The slight detergent effect of the water is what messes it up, and vigorous or extensive mixing exacerbates this effect.
The old rule still applies; "oil and water don't mix." While they can be manipulated and modified to work together, they still don't work in the same ways, and the differences must be respected in order to obtain the results one wants.
Wmmccoy, this is very good information to have. Although I have already borrowed Ralph Mayer's book and had read that information, it is always good to see it repeated in order to reinforce the information. The fact that the individual pigments are wrapped in the medium (oil) keeps it from cracking and peeling ( and becoming dull) if other factors are equal in using it? I am assuming that this could also hold true for pigments that are in a different medium (for watercolors, for acrylics). Perhaps that is why paintings overworked with too much water begin to look dull? Do acrylic manufacturers recommend the use of acrylic mediums as opposed to too much water? I have some watercolor mediums, but have done very little playing with them because I haven't been able to get anyone to explain why they even exist. Perhaps I should get them out and experiment. Slowly but surely the art catalogs have begun to give short explanations along with the watercolor mediums. Between that information and experimentation I should be able to get some idea of the reason for them.
All artist's mediums (here speaking of the different types of materials, not a fluid used to blend with them) incorporate some kind of "binder" that both (1) holds the pigment in a homogeneous suspension, and (2) binds it (causes it to adhere) to the support (paper, canvas, birch panel, whatever).
The artist's medium with the least amount of binder is pastel. Just enough to cause the sticks to hold together. The fixative misted over it after a pastel painting is completed, helps to bind it. Sort of a post-painting binder, externally applied, though with great care, pastel paintings CAN be framed without any fixative at all. However, since pastel is primarily a dry medium, some fixative is desirable to keep its adherence to its support.
Differently, oil paint is "bound" by the drying (curing) oil in which the pigment is ground, namely linseed, but also safflower (in commercially prepared paints). Other oils have been tried, but these are the predominant ones. A "drying oil" is any oil that cures by oxidation—that is, gradually reacting with oxygen from the atmosphere and "curing" into another form—the tough, flexible, adhesive (to the support) surface of a completely dry oil painting. The pigment particles surrounded by this drying oil are then permanently bound (or descriptively, glued) to the surface of the support, ideally, forever.
Anything that compromises this process is detrimental to the integrity of the final result. Bad technique or misuse of materials can cause the paint layer to release from the support, bubble up, crack, bloom, or even mold, among other problems.
The dullness you refer to can be caused by many things, but in OIL painting, it has more to do with the relative amount of oil that a pigment can absorb; in other words, the characteristics of the dry pigment determines how much oil is required to completely enclose all the pigment particles. Some of that has to do with how absorptive the actual pigment itself is. The more oil a pigment holds, the glossier or wetter it will appear. Since colors don't cure at the same rate, parts of a painting may look dull while other parts appear wet, and if touched, the dull parts will by dryer than the glossier parts.
In a previous post, I mentioned that burnt umber oxidizes quickly, while ivory black oxidizes slowly. Burnt umber will not absorb much oil, therefore, it tends to cure, or oxidize, more rapidly, and will look dull in a day or so, unless extra oil is mixed with it (I'm not advocating that, mind you, just as an observation), whereas ivory black holds a lot of oil, and takes a long time to cure completely, particularly if used in a significantly thick layer. Every pigment has its own characteristic of absorptiveness. Also, the source of the pigment has a bearing on how absorpitve it might be. Pigment sources can be mineral (the umbers, siennas, ochres, cadmiums, true ultramarine blue, chromiums, leads, zincs, cobalts, etc), organic (any of the colors known as "lakes", indian yellow, ivory black, etc.) or synthetic (created in a lab or as a by-product of another process; anilines are the major ones, phthalos). Most of the mineral-sourced pigments dry more rapidly than those of organic or synthetic sources, because they will hold less oil (less absorptive).
Read the extensive section in Mayer's book about pigments. He lists all the sources and their characteristics, and where they rate on the Munsell color scale, and how much oil they require. It will begin to make sense to you why they don't all work just alike, even though they are oil paints, and they came in the same set from one manufacturer.
Acrylic paints dry by evaporation rather than curing. The acrylic medium itself is water-soluble, which is why water is the primary "thinner" but once the water is evaporated from it, acrylic forms a very tough and impervious layer, holding it's suspended pigments permanently bound to the support. The various acrylic "mediums" impart other qualities, most notably, the degree of "thickness of texture". Mediums can be gloss or matte, and range in the "thickness" quality from "fluid" to "heavy molding or modeling paste". Because it dries by evaporation, acrylic paint tends to shrink as it dries, which some painters (transferring techniques from oil painting) find annoying because acrylics won't hold the effect of brushwork to the same level. Hence, mixing into the colors some of the modeling paste lends the possibilty of impasto brushwork. Acrylic also has the ability to be made to look like traditional water color when it's thinned out to a truly aqueous consistency, so it is a very versatile medium.
As to the watercolor mediums, I have never used any, but I suspect they affect the evaporation rate of the water, allowing more fluid washes without creating lines where the edges of wet areas dry. A close reading of the labels should clarify their use; or go to the manufacturer's web site and look up or request technical specifications for the product. There's also probably a phone number or interactive link for your technical questions to be answered.
I just discovered your conversations about cracking of oil paint (thanks also to Margo5 and watermixableguy) and very much appreciate your concise explanation of causes of cracking of oil paint. You're obviously experienced with the chemistry behind the behavior of the paint and I wondered if you might have an opinion about a painting I did in the mid 80s of two men. I was using traditional oils at the time and worked on it for many days. Towards the end of that time frame (not months later) I decided to add "heft" to one of the bodies. That area has now cracked badly - fat over lean failure on my part. I used Permalba white w/yellow ochre and probably some burnt sienna under a ultra blue/burnt umber/burnt sienna mix. I did not as a rule use any medium when I painted, just a bit of turps in burnt sienna to do a quick gesture for layout purposes, but I seem to remember adding medium to the dark mixture to fatten it up - wrong move on my part as it was either not enough or wrong type obviously.
The cracking has become more noticeable in the last few years. The cracking is not through the ground (traditional rabbit skin glue and gesso on heavy-weight cotton duck) just in the dark surface showing the white layer beneath. The painting has never been varnished and is in good condition otherwise. So my questions are: Could/should I fill the cracks with traditional oil and a super fine brush to darken them to make them less noticeable or should I just leave it the way it is now? (As this is the only painting I've done over the years where this has happened, I'm not really sure what to do about it.) If I do darken the cracks, do you think I am opening myself up to additional problems in the future?
The painting is on my web site www.janetkondziela.com in the Portraits Gallery under the "Works" tab and entitled "Two Friends". The cracking is very apparenty under the arm of the man on the left although it goes along the upper arm too.
Thanks for any opinion/advice you might have. You can also email me at firstname.lastname@example.org if you like, since this is such a specific question and might not be of general interest.
I've been following your thread with interest on the cracking of oil paint since being directed here by Margo5 (thanks Margo!) from the Technical Q and A forum. You certainly show a great understanding of the chemistry and resulting behaviors of oils. And thank you for the reminder about Mayer's book - a great resource for the artist. I appreciate it.
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In an oil painting class this year I was introduced to the medium LIQUIN. The instructor has an excellent background and guided me in the use of this medium which alot of my art friends use also when painting in oils..do not use any other medium when you start a painting with Liquin..Use more in laying in the first colors, It will dry overnight, then you use more and more pigment at you build up the layers , also great for glazing!!
Make absolutely certain that when using Liquin, you have a good airflow around you, open windows, a fan blowing the fumes away from you. Many have become ill using it in tight quarters.
If you wish to drown, do not torture yourself with shallow water. (Bulgarian Proverb)