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  • Fast Drying, Fat PaintsApproveRejectUn-ApproveSubscribeUn-Unsubscribe
    Question asked 2016-12-14 11:19:45 ... Most recent comment 2016-12-14 18:16:00
    Oil Paint Pigments
    Question
    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.
    Baade, Brian
    2016-12-14 18:34:26
  • ApproveRejectUn-ApproveUser CommentVery clear answer. Thanks
    2016-12-14 21:11:46
  • ApproveRejectUn-ApproveUser CommentThe only follow up question I have, is whether or not the look and feel of the paint out of the tube is a reliable indicator of oil content. For instance, the raw umber and burnt sienna I have from Gamblin look very lean, dry and stiff out of the tube, but both are categorized as a higher oil content than mars black, which is a bit oilier in appearance and feel, but lower in oil content according to their chart. Is there an explanation for this other than variations/hiccups in production? Can some pigments appear lean and matte but have a high oil content?
    2016-12-14 21:34:57
  • ApproveRejectUn-ApproveUser CommentAppearance from the tube is not a reliable indicator of leanness in oil paint. Paint made with a lean pigment can sometimes shed a significant amount of oil because the raw material takes up less vehicle. In the case of scholastic grade paints, colors with a relatively low pigment load can look very dry and waxy if the manufacturer has used a lot of stabilizers to compensate, even if the pigment is relatively fat. Matthew Kinsey, Utrecht Art Supplies
    2016-12-14 22:27:59
  • 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.
    Kristin deGhetaldi (CAS)
    2016-12-14 22:47:52
  • ApproveRejectUn-ApproveUser CommentGreat to know. Thanks
    2016-12-14 23:37:08
  • 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.
    Baade, Brian
    2016-12-15 00:00:32
  • ApproveRejectUn-ApproveUser CommentThe only thing I don't understand about that is how the superabundance of aluminum stearate makes the paint fat. I understand it decreases the pigment load and makes for a weaker color, but don't those fillers still count as part of the solid particles that make up the PVC? Rublev's paste mediums were described to me by a rep as being "lean" mediums, or at least, not fat, because they are simply extender pigments in oil, and thus had a high PVC. What is the difference between aluminum stearate and calcium carbonate as a paint additive?
    2016-12-15 01:41:06
  • ApproveRejectUn-ApproveUser CommentAluminum stearate is a metallic soap which causes oil to gel while preserving neutral color and translucency. The gel effect can be achieved with a relatively small proportion of stabilizer. Similar compounds sometimes form naturally in small amounts when paint is made using cold-pressed oil (but not alkali-refined oil). It doesn't support film integrity like pigment, which is more analogous to an aggregate in concrete. Calcium carbonate, used as a filler, absorbs a lot of oil and imparts some stringiness to the paint body. It's also a weak white which increases opacity and dilutes color.
    2016-12-15 08:41:02
  • EditDeleteModerator Answer

    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 A​bsorption (wt/100 wt)
    ColorAvg. Oil Absorption (wt/100 wt)
    Flake White 15%
    Zinc White 15%
    Titanium White20%
    Cadmium Red Dark20%
    Mars Black20%
    Violet Oxide20%
    Raw Sienna35%
    Ultramarine Blue38%
    Burnt Sienna 38%
    Prussian Blue 40%
    Phthalo Green 50%
    Raw Umber55%
    Phthalo Blue 70%
    Alizarin Crimson 75%
    Lamp Black160%

     

     

     

    ​Average Percentage of Oil by Volume ​​

    COLOR​Avg. Volume of Oil​
    ColorAvg. Volume of Oil
    Prussian Blue 45%
    Titanium White46%
    Zinc White 47%
    Ultramarine Blue48%
    Flake White 51%
    Violet Oxide52%
    Mars Black52%
    Cadmium Red Dark52%
    Phthalo Green 54%
    Phthalo Blue 55%
    Alizarin Crimson 55%
    Raw Sienna55%
    Burnt Sienna 61%
    Raw Umber66%
    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.

    Sarah Sands
    Senior Technical Specialist
    Golden Artist Colors



    Sands, Sarah
    2016-12-15 15:54:47
  • 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 .
    Baade, Brian
    2016-12-15 16:17:05
  • ApproveRejectUn-ApproveUser CommentThanks everyone for the responses. This has been very helpful. To anyone interested, Sarah cleared up a concern I had about adhesion to high oil content paints in the comment section of the article she wrote (http://www.justpaint.org/volume-weight-and-pigment-to-oil-ratios/).
    2016-12-15 18:33:41
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