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Question asked 2016-12-14 11:19:45 ...
Most recent comment 2016-12-14 18:16:00
Some paints are fast drying but have a high oil content to pigment ratio out of the tube. Is it safe to use these in an underpainting?
Answers and Comments
EditDeleteModerator AnswerIt is generally best to use paints that have a lower oil to pigment ratio in underpainting. Some paints are inherently very fat and while they may dry well (eg prussian blue, raw umber) they should only be used in underpainting when mixed with a leaner paint. This is one of the great virtues of lead white. This does not mean that one can not dilute their underpainting colors with solvent to create a more matte film. It also does not mean that lead white is the ONLY white that can be used for underpainting. Titanium white has been shown to make a less satisfactory oil paint than lead white (this is practically true for all pigments as compared to lead white) and it does make an oil paint with a higher oil to pigment ratio (again true of many pigments) It is, however, suitable for underpainting as long as the artist tries to make sure that all subsequent paints have an even greater oil to pigment ratio. In practice that means that if one uses titanium white in one's underpainting, it should also be used in overpaints. As far as other paints that are naturally fatty, it is generally best to chose leaner, and generally less intense colors for the underpainting and reserve fatter, and often more intense colors, for surface effects.
EditDeleteModerator AnswerTactile qualities of the paint are not always related to the amount of oil required to properly bind a given pigment to make it into a workable. Additionally as Matthew stated, the specific formulation chosen by the manufacturer will impact how much oil a given color will have. The more stabilizers, fillers, and other additives, then the fatter the paint will be.
EditDeleteModerator AnswerIt can get really complicated. Sometimes a very low quality paint will appear leaner and apparently loaded with pigment because of the superabundance of aluminum soaps or castor wax. It is actually very easy to make a paste of drying oil and aluminum stearate and only put in just enough pigment to give the appearance of color. Paints like this are overloaded with oil and make extremely poor dried paint films. Unfortunately, many student grade paints are just this. It should come as no surprise. There is no way that a 100 grams of cadmium red pigment costs (eg 25$) but a 200 ml tube of a paint that states that it is genuine cadmium red could possible cost (eg 12$) all while the artist grade from the same manufacturer costs (eg 28$) for 37mls. Genuine cadmium red pigment is in all three products but the amount incorporated into the paint is vastly different. On the other end of the spectrum, sometimes a very high quality paint with the minimum of stabilizers will appear to be under-pigmented because there is a good deal of oil separation when you open the tube. In practice, this extra oil is either wiped away or reintegrated into the paint if you are intentionally working in later, more fatty layers. It really does pay to understand the nature of the pigments that you are using and the quality of the paints that you buy.
I need to respectively and gently disagree with some of Brian's comments and would like to put forward some different thoughts around this.
Deciding what paints are fat or lean based on oil absorption rates is generally misleading and leads to wildly wrong classifications of colors. The reason is that this is always reported as a weight ratio - how many grams of oil needed to 100 grams of pigment - rather than what one usually is thinking about, which is what percentage of oil vs pigment is in a tube of paint. I wrote an article on just this issue which is worth reading:
Volume, Weight, and Pigment to Oil Ratios
Some of the surprises that this shows is that Prussian Blue, for example, is EXTREMELY lean, while lead white - often thought to be super lean - is only middle of the road. In fact not much above Alizarin Crimson, often thought - mistakenly - to be particularly oily. How can this be? In the case of lead white, it is simply that lead is so dense that its reported oil absorption of 10-15% oil is misleading. Yes, by weight the oil is a very small amount of the mix. By volume, however, it is a very different story. Below I reproduce two of the relevant tables from the article:
Average Percentage of Oil by Weight
|COLOR||Avg. Oil Absorption (wt/100 wt)|
|Color||Avg. Oil Absorption (wt/100 wt)|
|Flake White ||15%|
|Zinc White ||15%|
|Cadmium Red Dark||20%|
|Burnt Sienna ||38%|
|Prussian Blue ||40%|
|Phthalo Green ||50%|
|Phthalo Blue ||70%|
|Alizarin Crimson ||75%|
Average Percentage of Oil by Volume
|COLOR||Avg. Volume of Oil|
|Color||Avg. Volume of Oil|
|Prussian Blue ||45%|
|Zinc White ||47%|
|Flake White ||51%|
|Cadmium Red Dark||52%|
|Phthalo Green ||54%|
|Phthalo Blue ||55%|
|Alizarin Crimson ||55%|
|Burnt Sienna ||61%|
|Lamp Black ||75%|
Other surprises include that Titanium and Zinc White, as well as Ultramarine Blue, edge out Lead in leanness and that at least some common organics - such as the Phthalos - are not particularly high in oil as well.
We think in many ways the 'fat over lean' rule has been a well-intentioned but confusing and likely out lived rule-of-thumb that smooths over many critical issues and doesn't take into account the plethora of exceptions. We agree with folks like George O'Hanlon that the concept is CPVC, or Critical Pigment Volume Concentration, is a more valuable and accurate one to base things on. By using that as a starting point, one can essentially consider any well-made paint as being 'lean' when squuezed out of a tube - since it is at the optimal ratio of binder and pigment - and see 'fatness' as referring to the amount of any medium being added afterwards. This relieves the painter of having to track down oil absorption rates - which are fairly useless for this area anyway - and allows them to concentrate on what we think are the more critical factors: dry time and the inherent film qualities that the pigment provides, such as flexibility.
Anyway, those are a few thoughts to add to the conversation and to help correct what we think is a wide misunderstanding of 'fat over lean' as a handed down adage.
Senior Technical Specialist
Golden Artist Colors
EditDeleteModerator AnswerYou know Sarah, I don't really disagree with what you write. The old fat over lean "rule" doesn't work perfectly and is certainly problematic to use. I have been resistant to completely abandon it despite seeing the value of CPVC. I will officially here declare myself a convert to this way of thinking and hope that the original poster reads your response .
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