Home Forums Explore Media Oil Painting The Technical Forum The case against fat over lean

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  • #474456
    Pinguino
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        Fat over lean. Really?

        Having read through many pages here at WC and elsewhere, I wonder about the technology behind “fat over lean.”

        Seems that in most cases, the “lean” early layers have solvent. The “fat” upper layers lack solvent, but may or may not have oil added to the paint.

        My puzzlement is regarding the solvent. It seems to me that if paint has some medium added, either an oil/solvent mix or just solvent, then the solvent evaporates long before the paint layer begins to cure. That being the case, in terms of “fatness,” the solvent isn’t there at all. In other words, adding solvent does not make the paint any leaner. Indeed, if an oil/solvent medium is added to the paint, it should be “fatter” instead.

        The only advantage to solvent in early layers would be to thin the paint, allowing thinner layers.

        Is my observation correct, or not?

        I can also visualize that in the early days of paint ground on-site by apprentices while the master paints, there might have been variable amounts of oil added as the painting progressed. Or not. In any case, with modern tubed paints, that’s not how it works.

        #841322
        Raffless
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            Its nothing to do with the solvent. Its the ratio of oil medium to pigment that decides which is fatter/leaner. The solvent evaporates, but what you have left is still a ratio of oil to pigment. If there is no medium added on the underlayer then its at its leanest.

            #841313
            Pinguino
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                Its nothing to do with the solvent. Its the ratio of oil medium to pigment that decides which is fatter/leaner. The solvent evaporates, but what you have left is still a ratio of oil to pigment. If there is no medium added on the underlayer then its at its leanest.

                Yes, that’s my understanding, as expressed in my original post.

                But I’ve see many posts from others, who complain about having a sensitivity to turps or OMS, or insufficient ventilation, in the context of “fat over lean.” Seems that they have (incorrectly) interpreted this mantra to mean that solvent has to be added to the early layers.

                For that matter, I have seen advice indicating that it’s not really necessary to make upper layers fatter by adding any medium, if paint straight from the tube is used throughout. This makes sense (possibly except for thick sculpted layers).

                #841302
                AnnieA
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                    As a person trying to use as little solvent as possible, I can tell you that the issue is not that “fat over lean” [em]requires[/em] solvent at the initial layer(s) per se, but instead the issue is that it’s very hard to lay down an early first layer without solvent.

                    The issue is really “more flexible over less flexible” rather than “fat over lean,” although fat over lean is what’s used to achieve more flexible layers over less flexible ones. The reasoning is that if you put a less flexible layer over a more flexible one, that’s when delamination or cracking is more likely to occur.

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                    #841297
                    WFMartin
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                        There is not now, nor has there ever has been anything in the fat-over-lean principle that is violated by painting either “fat-over-fat”, or “lean-over-lean”. As someone has mentioned, it is the flexible-over-less flexible concept that is important, and that concept is never violated by using the same, exact medium for every layer of the painting.

                        Somewhere along the line, someone will include the content of the paint as a factor in the fat-over-lean principle. Quite realistically, the paint is the only factor that is a constant in the principle. It is only the painting medium that should be considered as a relevant factor in applying the fat-over-lean principle.

                        And, yes, it is the ratio of solvent to oil in the medium that determines the fatness, or leanness of the medium being mixed with the paint. The brittleness, and drying properties are very much affected by the solvent in the mixture, and the fact that it eventually evaporates does not detract from the fact that it initially contributes to the faster-drying, and less-flexible properties of the paint with which it is mixed.

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                        #841303
                        Anonymous

                            Is my observation correct, or not?

                            methinks there is more to it than that.
                            I wouldn’t call it an observation, I would say it is just armchair thinking about some stuff.
                            Oil to pigment ratio won’t matter, and solvent evaporation won’t help or matter, when the paint’s physical rheology is changed, and the film’s durability and adhesive properties are progressively thinned out and diminished, and oil and pigment is dispersed and separated more and more, and the paint film is weakened and weakened to the respective degree that solvent is added. But one can be secure in the knowledge that the good old ratio is still the same, and it is just as fat as it ever was, :confused: just not me though.

                            #841300
                            Don Ketchek
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                                Even though the solvent evaporates, it is there when you apply the paint. So the “layer” of paint that you apply has less oil molecules than if you applied the same “layer” of just oil. The more solvent, the less oil molecules – or another way of thinking of it is that the oil molecules are spread farther apart. Thus less oil in that paint layer – thus leaner.

                                Thus the evaporation is irrelevant. When the solvent evaporates, oil molecules do not suddenly generate out of nothing to fill the void, so to speak. Your layer has less oil in it when you add solvent. The more solvent, the less oil molecules in the same volume of that layer. Thus less oil in that paint layer – thus leaner.

                                Don

                                #841310
                                Gigalot
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                                    I never follow this idea because I had never agree with “fat over lean”. This idea is practically nothing but speculation for me. How fat is acrylic? :confused: :lol: How fat is varnish? :confused: How fat is sunk in layers? Who really needs that to know and why? I like to study deeply oil drying processes and I like to apply my paint according my knowledge. Well drying oil is the best rule for me.

                                    #841329
                                    arnaldino
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                                        1 fat over lean (more oil)
                                        2 thick over thin (less solvent)
                                        3 slow drying over fast drying (different oils, pigments, or siccatives)
                                        1+2+3 = more elastic over less elastic (valid for alkyds as well)

                                        #841324
                                        Richard P
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                                            More elastic over less elastic would mean acrylics over oils, which doesn’t work.

                                            #841311
                                            Gigalot
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                                                More elastic over less elastic would mean acrylics over oils, which doesn’t work.

                                                That doesn’t work, but if you want that much, it will work fine. :lol: The only problem for me is final varnishing of acrylic layers because acrylic can’t attract varnish well. Therefore, I tried to cover acrylic with regular (not wmo) oil paint as I can varnish it without having trouble.

                                                #841304
                                                Anonymous

                                                    meanwhile, this is the oil paint forum and the topic has veered into the acrylic realm again.
                                                    The oil painting guidelines regarding fat over lean are specifically relevant to oil paint.
                                                    Acrylics, watercolors, crayons, pastels and other media may require other considerations when combined with oil paint.
                                                    The consideration with painting acrylic over top of oil paint includes factors like adhesion.
                                                    quote from Golden:

                                                    Overpainting artist oils with artist acrylics is definitely bad practice. The surface of hardened oil paint does not have “tooth.” It is too hard and often too shiny to afford good adhesion by acrylics.

                                                    while some may do it, and it appears to work fine, it is not always going to work fine, can lead to delamination at some point, and it is not a best practice.

                                                    arnaldino’s post is correct.

                                                    #841314
                                                    Pinguino
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                                                        Back to oil…

                                                        @AnnieA: You say, “it’s very hard to lay down an early first layer without solvent.” I assume that depends on method of painting. For example, I paint only on flat, pre-gessoed board. The brands of paint I use, although artist grade, are not among the stiff high-load premium brands. I have stiff synthetic brushes, among others. So I can put down a thin firt layer without solvent. I use solvent only for toning; in this case, it is more like a color wash, done outside (where I do’t care about solvent) then put aside to cure for a long time. If anything that tone layer is under-bound, but that doesn’t matter.

                                                        I can see that someone using a large canvas, and stiffer paints, might need solvent in the early layers, to prevent unintentional impasto effects.

                                                        @WFMartin: You say, “the fact that it (solvent) eventually evaporates does not detract from the fact that it initially contributes to the faster-drying, and less-flexible properties of the paint with which it is mixed.” Now, that’s something I dispute! Or rather, it’s not that I have evidence to the contrary; it’s that I cannot think of any reason why solvent (evaporated or not) would make paint dry faster. I can only see that the post-solvent fresh paint is less fluid than the with-solvent paint, but nevertheless the post-solvent fresh paint is by no means close to drying at that point. The remaining oil will flow together to fill the voids left by the lost solvent.

                                                        @DAK723: You say, “oil molecules do not suddenly generate out of nothing to fill the void, so to speak.” Indeed they do not come from nowhere. By existing oil molecules flow together to fill the voids, since the oil is still in liquid state at the time the solvent evaporates. So the voids are filled by inflow. This does mean that the residual paint film is thinner (in vertical dimension) and thus subject to whatever advantages and disadvantages come from a thin paint film. But if the with-solvent paint film is originally thick enough, then the post-solvent film should have adequate thickness.

                                                        In other words, I surmise that a with-solvent film painted at 0.08mm thickness, which quickly proceeds to a post-solvent film at 0.05mm thickness, should perform (drying and elasticity) just the same as a film that never had solvent and was originally painted at 0.05mm thickness.

                                                        #841319
                                                        contumacious
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                                                            I can’t imagine trying to calculate how much solvent to add so that everything ends up with the same lean or fat status while actually painting. Sounds like a recipe for a failed paint layer to me.

                                                            When you add Solvent, the ratios will change if you are talking about the same final volume, say 10 ML which is really the only way I can imagine looking at this.

                                                            If you create three precise, specific volumes of paint and apply them to the exact same surface area of canvas, the mixes with solvent in them will have less oil and thus be more lean than anything without solvent added, and will be more prone to cracking if applied over fatter layers. Painting this way, is more realistic than trying to lay down a 0.08mm solvent leaned layer over a 0.05mm solvent free Fat layer. If anything the trend would be to apply LESS of the solvent mix than the fat mix since it will be much thinner and will tend to level out to a thinner layer.

                                                            Something has to give way in volume when you add the solvent. It will be either a reduction in the amount of oil in the paint or the medium. Let’s assume that both the paint and the medium contain drying oil and has no ingredients that will evaporate. So once it is dry everything that you put on the canvas other than the solvent is still there.

                                                            Volume 1 is 10 ML of Paint A straight out of the tube
                                                            Volume 2 is 10 ML made from Paint A + Medium B
                                                            Volume 3 is 10 ML made from Paint A + Solvent C
                                                            Volume 4 is 10 ML made from Paint A + Medium B + Solvent C

                                                            Regardless of the ratios used, what remains on the canvas from 10 ML of Volumes 1 and 2 is always going to be more fat than what remains on the canvas from Volumes 3 and 4, both before and after the solvent evaporates.

                                                            #841315
                                                            Pinguino
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                                                                … Regardless of the ratios used, what remains on the canvas from 10 ML of Volumes 1 and 2 is always going to be more fat than what remains on the canvas from Volumes 3 and 4, both before and after the solvent evaporates.

                                                                Now, that’s what I dispute! Seems to me that 2 or 4 should be more fat (oily) than 1 or 3. But the depth of the layer (after solvent evaporation, if any) should be less for 3 or 4, than for 1 or 2.

                                                                No doubt the fatness (oiliness) and layer depth are both factors that affect such things as drying time and elasticity.

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