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The Chemistry of Oil Painting

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What chemical properties give oil paintings their luminous glow and deep darkness?
Why do they crack?
What kind of oil is used?
Is it safe to use the oil painting medium on a fresh dandelion salad?

Our blog banner started out as oil on slate. Tough to photograph, fun to scan. I have no patience and scanned it wet.

As an oil painter for the past 17 years who used to manage at a fine art supply store and notably not a chemist, I’ll do my best to explain. Don’t slip on the floor, and remember to soak your cleaning rags in water before disposing of them in the metal bin.  They can spontaneously combust, you see.

Introduction to what paint is and isn’t

All fine art paints share a few properties that make them different from say, dyes. Paints are essentially pigment particles bound in a sticky, transparent medium, whereas dyes or soluble in liquid. So oil paints are pigment bound in oil, acrylic paints are pigments bound in acrylic polymer medium, and watercolours are pigments bound in a water-soluble medium called gum arabic. Fabric dye and fabric paint are therefore not the same thing.

There can be other agents inside a tube of paint these days, that slow down or speed up drying, that lend texture, or help stubborn pigments bind to the medium.  (Inexpensive paints often have too much binder in them and can cause discolouration over time – check out this post by artist Jonathan Linton on his blog Theory and Practice for some empirical tests.) But at their root, all paints are pigment+medium.

Quick History Lesson

Within Western Art History, oils overtook fresco painting and egg tempera painting in popularity relatively quickly. The Master of Flemaille is sometimes credited with beginning the practice of using oil paint for fine art purposes, though more often the credit is erroneously given to the Van Eyck brothers.  In truth, craftspeople and artisans were already using oil for some time previously. ¹

The superior qualities of oil make it easy to understand why it took over from other mediums.  Fresco, such as Michaelangelo’s Sistine Chapel Ceiling, are essentially pigment bound in plaster. You had to mix just enough of the correct colours for one “go” or “pass” at a section, and estimate how much detail you could achieve with it before it dried before your eyes. So something like the subtle blending of God’s robes or tones and shadows of Adam’s skin in The Creation of Adam had to be estimated in multiple swatches, each with its paint mixed just before application. A difficult task.  Oil paint on the other hand, may not fully dry for weeks: you can play with it, correct its tones and even erase missteps from the canvas and start again on a section. Blending becomes open for experimentation.

Types of Oil

Even in the Renaissance when oils first inspired artists to delve wholeheartedly as a medium, a number of oils were tried as vehicles for pigments. And their properties differ.

  • Linseed Oil – made from flax, linseed is the most popular due to its flexibility and resistance to cracking.  It does have a strong tendency to yellow with age, however.
  • Walnut Oil, Poppy Oil and Safflower Oil – much less likely to yellow, these thin, clear watery oils are much more prone to cracking.

Yellowish, flexible Linseed Oil on the left; Clear, prone-to-cracking Walnut Oil on the right.

With these different properties, how do they come into play when actually painting?  Well one of the Ninja Turtle Old Masters had it right: analysis of Raphael’s The Mond Crucifixion (1502-3) shows that the ground, figures and green robes were painted using linseed, and the blue sky painted with nut oil.² This way, the yellowing of the figures and ground were an acceptable trade-off due to their subject, but the blue sky was considered better off being cracked and bright blue than yellowed and smooth. Painting below:

The Mond Crucifixion by Raphael uses two types of oil on different elements to preserve colour and paint film. And it still works, after 500 years, eh?

Watching Paint “Dry”

Watercolour and acrylic paints have water as part of their medium – they dry by evaporation. But oil paints don’t. They dry by what’s called a siccative quality. That is theyabsorb oxygen from the air. This has the undescriptive definition of

(Chemistry / Elements & Compounds) a substance added to a liquid to promote drying: used in paints and some medicines

[from Late Latin siccātīvus, from Latin siccāre to dry up, from siccus dry]

Essentially, oils have a rate of autooxidation from the air, they absorb oxygen and harden. I’ve often described this as putting Jell-o into an enclosed container and adding tons of pineapple chunks to it: the oil is the Jell-o and the air is the pineapple – you can only add so much to the enclosed bowl and it will stop jiggling. Perhaps I haven’t got this analogy quite right. But now I want Jell-o.

As oils harden, there’s an interesting problem: oxygen is absorbed through the paint surface, meaning if the paint is very thick, you can see a different “drying” rate on the paint’s film than on the first layer’s applied on the canvas. The surface could be hard and the oils underneath still squishy like yummy lemon Jell-o.  (Warning: oils processed as art supplies are not cleared for human consumption.)

Fat Over Lean

One of the main appealing properties of oil painting are the glazes.  By adding a small amount of pigment to the relatively clear oil medium, you can very subtly tint an image.  This is called glazing. Most Renaissance Old Masters (think the Ninja Turtles and their peeps – Artemisia Gentileschi not April O’Neil) used a toned underpainting and then built up several of these thin glazes of colour on top to create astonishingly realistic figures and scenes. The translucence of the paint film allows for sophisticated ranges of flesh tones. But then we hit the problem of the upper layers of oil glazes drying before the lower (first) ones do – and this is where cracking comes from.

Okay, another analogy: imagine the top (newest) layer of oil is stretching as it dries out hardens, and it stretches to the max.  Its surface is expanding because it is absorbing oxygen (not evaporating water). Now, they oxygen eventually begins to hit the layer below.  And it stretches and expands to the max. But they layer above is already dry, how can it expand any more with the one below pulling it!? >crack<

Like a big cookie on a pan. Slide an uncooked cookie under a cooked cookie, a bigger one and stretch and heat up that dough: as the bottom cookie dries and expands its surface, it will crack the smaller cookie it is now stretching on its surface. >crack< Nomnomnom.

To get around this, painters developed the Fat Over Lean rule. With each layer of glaze, add an increased amount of oil paint to the layer. (Less pigment, more oil.) This way, the rate of oxygen begin absorbed by an oily (fat) top layer will be slower than the hidden lower, less oily (lean) layers, and hopefully they will saturate with oxygen and harden at approximately the same time.

This leads to other tricks and techniques too.  If you use too little oil in an early glaze, it can obliterate the drawing or painting underneath that you want to show through all the thin transparent glazes. It also can make the paint too pasty and thick, which is unworkable for fine detail. So, in the early, lower glazes, sometimes solvents such as turpentines are added.  The loosen the paint, disprese the pigment particles, and then kindly evaporate in a big hurry leaving the old that’s left to be covered by another turp+pigment+oil layer that has a little less turp and a little more oil. And so on.

To answer the question above about the dandelion salad, oils themselves are not harmful (though not processed to be safe for food). An open container of say, safflower oil on the table will do no more harm to breathe in than some extra-virgin olive oil with Balsamic vinegar and a few chili flakes on your table for bread.  It’s the solvents you have to be especially wary of. Even some of the odorless ones have harmful vapors, although it’s possible nowadays to buy non-toxic alternatives.  I’d be happy to recommend some I’ve tried if anyone has email requests (this is not an infomercial).

Patron Saint of Pigments

In Renaissance Italy, the patron saint of painters was St Luke – who was also the patron saint of doctors. Painters didn’t have a Guild of their own, they belonged to the same as doctors. Why? Besides the mythology of the saint himself, it was for the practical reason of painters and doctors both frequenting apothecaries for medicinal and artistic ingredients.

The pigments in oil glazes add another property and challenge to the artist who up until about 150 years ago, had to mix each batch of paint by hand. The pigment particles are not all the same size, and do not all disperse at the same rate within the oil medium. What this means is some colours will have more oil, and others less. Yeah you see it coming: the glazes following the fat over lean rule are best applied in certain orders to reduce cracking upon hardening.

As an example, let’s say you’re painting a red rose, with all it’s subtle shadows and highlights. To get ideal results in your glazes, you may want to apply the glazes in this order: manganese blue, cadmium red, quinacradone red, alizarin crimson. Mostly this will not matter to modern oil painters, but it can still have an effect even today. Most true alizarin crimsons will have up to twice as much oil content as a lead-based white.³

Bouncing light

What’s the point in all these complicated glazes?  Just to mix colour?  Not just – they add luminosity to the painting. You see, when light enters the hardened oil paint film, it passes through several distinct layers of mostly transparent paint.  And sometimes, before being reflected back out to the surface, it bounces off of one of the colourful pigments, and back down to the layers below, and then out.  Sometimes it will bounce on the boundaries of the separate glazes before bouncing out to meet your eye. And this is what gives oil paintings their glow and their deep deep blacks.  The dancing behaviour of the light in the complicated multiple layers and their colour pigments.

Here you can see the light beams (blue) bouncing multiple times through the oil layers, off oil membranes and off the colourful pigments. This bouncing gives oil paintings their luminous glow.


The New Oil

Consider this little afterword the start of another conversation for another day.

Oil painting gave artists the tools necessary to create images that can be corrected easily due to their long drying times and that seem to glow due to their layers. As an oil painter myself, these are highly prized qualities. And the last several years, we’re seeing another technology that prizes these same qualities of easy correction and luminosity. Digital painting has exploded in popularity with programs like ArtRage (used to create the simple image above), Photoshop, Corel Painter, and the shareware Gimp. Ctrl-z is the new solvent, and pixels the new luminous colours. And I don’t think it’s an accident. What would pioneers like The Master of Flemaille or the Jan Van Eyck have done with current technology?

If they’re like me, they’d want to experiment with the ease of the new tools but still stick their fingers in the sticky paint, smell the soft odor of the oil, and play with their pigments.


- -

I hope you’ve enjoyed my contribution to Chemistry Day here on the Scientific American Blog Network!

I’m not a chemist – I could be wrong. Feel free to offer corrections and tips of your own in the comments.

1. History of Art, Fourth Edition. H.W. Jansen, revised and expanded by Anthony F. Jansen, 1991 Harry Abrams Inc. p.425-426. (Link leads to newer edition)

2. The Artists Handbook. Ray Smith, 2000 Alfred A Knopf. p.180

3. The Artists Handbook. Ray Smith, 2000 Alfred A Knopf. p.182

Glendon Mellow About the Author: Glendon Mellow is a fine artist, illustrator and tattoo designer working in oil and digital media based in Toronto, Canada. He tweets @FlyingTrilobite and is on Instagram. You can see Glendon's work-in-progress at The Flying Trilobite blog and portfolio at Follow on Twitter @symbiartic.

The views expressed are those of the author and are not necessarily those of Scientific American.

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Comments 25 Comments

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  1. 1. Banksy 8:53 am 08/2/2011

    Great, informative piece! And I’m glad I’m not the only one to think of certain Old Masters as Ninja Turtles.

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  2. 2. Alex Wild 10:02 am 08/2/2011

    I know literally nothing about oil painting. This is a great introduction!

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  3. 3. jasongoldman 1:33 pm 08/2/2011

    Awesome, awesome post! My favorite of the day.

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  4. 4. Mary Dreyer 8:37 pm 08/2/2011

    Is there any point in glazing with acrylic paints? Any issues with cracking, uneven drying, etc?

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  5. 5. Glendon Mellow 5:30 am 08/3/2011

    Thanks Banksy, Thanks Alex, Thanks Jason!

    Mary you can buy special glazing mediums for acrylic paint which work hard at mimicking oils. Acrylics have their own set of advantages and disadvantages though.

    When acrylic paints dry, they polymerize, and the molecules form long chains. It’s why you can sometimes peel large globs of acrylic paint that have dried on the floor or table away in one big piece. A lot of people though, thinking that because the acrylic is a water-based paint, will dilute heavily with water to achieve a glazing effect. what this means is that the paint films are not in long chains, (the water having washed all the acrylic molecules and pigments away into little islands of their own) and the danger becomes one of flaking and dusting.

    Using a glazing medium for acrylic can mitigate the effects of this, and it will help preserve the polymerization. To mimic oils though, the drying times a lot longer, similar to oil. Acrylics tend to crack only if the paint is under a lot of stress, or if you do things like speed up drying with a hair dryer.

    Anyone with experience in glazing acrylics found the above problems or a solution?

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  6. 6. SLW-L 7:23 pm 08/4/2011

    Formerly an oil painter, I’ve been using acrylics for several years now. Personally, I haven’t had trouble with cracking or flaking even when diluting heavily with water. I like using Gloss Medium instead of water for some layers (maybe that’s what’s prevented flaking issues?). I rarely work wet into wet, and build my paintings using many glazes and layers. So yes, depending on the look you’re going for, I think there’s a point to glazing in acrylics. With lots of layers and gloss medium, I’ve achieved some really nice glowing dimensionality. I should note that I’m not trying to mimic the look of oil paintings with my acrylics though – it has a unique look of its own.

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  7. 7. SLW-L 7:24 pm 08/4/2011

    Great post, Glendon.

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  8. 8. Glendon Mellow 10:31 pm 08/4/2011

    Thanks Sharon, that sounds like great information on acrylics, which I rarely use.

    I love the really extra-hard gloss gels. Like you’re burying the painting behind molten glass.

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  9. 9. RussellDickerson 10:32 am 08/5/2011

    This is a great article Glendon, and I agree, a great introduction to oils. I also think it’s a great start to understanding how paints interact with light, something that not all artists consider when they are painting. Well done!

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  10. 10. DidgeEight 12:35 pm 08/5/2011

    Great article!

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  11. 11. dbclemons 2:34 pm 08/5/2011

    There are special versions of acrylic paints available from some brands that allow them to be more heavily diluted than regular paints. Golden or Da Vinci brand “Fluid” acrylics are 2 examples. There are also some acrylic “gouache” paints and even inks that are designed to be more heavily diluted.

    To pull this back to an oil topic, one thing you haven’t mentioned (although, not really a chemistry issue) is the use of oils on top of dried acrylics. Acrylic primers are very common these days, and some artists also finish their acrylic paintings with oil paint glazes. This is mainly due to how much better oils are at blending than acrylics. The more porous the surface is the better the oils will adhere. The long term effects of this practice are not certain.

    It’s a pity that art paints don’t come with instructions. As such, sharing information like this is benefitial. Keep up the good work.

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  12. 12. Glendon Mellow 11:18 pm 08/5/2011

    Thanks DidgeEight!

    Thanks for mentioning the fluid acrylics, dbclemons. I’ve used them a little in the past and found them to be more similar to watercolour, though I was kind of goofing around with them.

    These days, virtually all commercial gesso (canvas primer) is made with acrylic, instead of the traditional calcium carbonate and rabbit skin glue.

    The problem with the old rabbit skin glue gesso was that it absorbed humidity from the air, and could greatly accelerate cracking in paintings outside of climate-controlled museums and galleries.

    As you say, the long term effects of the acrylic gesso are largely unknown, though most paint companies “age” artworks in tests using gases and UV light to check the long term properties.

    I have heard from one artist that a possible long term effect of acrylic gesso with oil on top, is that the turps cannot “bite” into the gesso enough, and over time the oil paint layers will sag on the surface, causing wrinkles. But I’ve only ever heard that from the one source. Anyone have a reference that escapes my Google-fu?

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  13. 13. dbclemons 9:28 am 08/6/2011

    Two resources I would suggest to investigate using oils on top of acrylics would be and
    Even though Golden makes acrylic gesso and you might think they would have a bias, their testing is very sound and they themselves mention that there is further study required. They have a “Just Paint” newsletter where this topic comes up several times. They also now distribute the Willamsburg line of oil paints. AMIEN is a good general resource for many issues related to the use of art materials.
    Much of what I’ve read on problems with oils on acrylics seems anecdotal; however, from my own experience I don’t like using an acrylic ground. Often there are uneven spots in the surface of different brands that are too slick to accept oil paint. The surface is also engineered to be porous by using solids (typically marble dust) which makes it rough and therefore hard on brushes. Oil primer would be my preferred ground, amoung others.

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  14. 14. Glendon Mellow 12:39 pm 08/7/2011

    Thank you Russell! (#9) – Sorry, your comment was held in moderation for some weird reason.

    The interaction of pigments versus pixels is interesting me more and more.

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  15. 15. Glendon Mellow 12:41 pm 08/7/2011

    dbclemons, I agree Golden acrylics are top-notch professionals about their information, and their sales reps know the chemistry of their product and how to use them.

    On the oil side, I heartily recommend Gamblin Oils & mediums and Lefranc & Bourgeois Oil mediums. They also offer tremendously technical information to really help artists instead of no-info colourful labels.

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  16. 16. Don Reba 4:40 am 08/8/2011

    Just as a very minor point: Gimp is not shareware, it is free and open-source.

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  17. 17. Glendon Mellow 9:07 am 08/8/2011

    Ah: thanks for making the distinction, Don.

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  18. 18. Glendon Mellow 6:27 pm 08/29/2011

    Some discussion has continued between myself and a fellow oil painter at Lines and Colors about this article and what ‘fat over lean’ actually means in painting: check it out!

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  19. 19. JonathanMagus 5:54 am 08/30/2011

    What is being said here is not entirely correct and neither is it in the separate discussion on ‘fat over lean’ that is referred to, though the essential points being raised in both cases are heading in the right direction.

    There is more to understand. No distinction is being made between ‘fat’ and ‘lean’ media for example: Sun thickened, boiled, or stand oil is ‘fat’ and more suited to glazing, as are cooked resin-oil media. In comparison the oil that the pigment is usually ground with, to make paint, is ‘lean’. The longer chain molecules in part-polymerised ‘fat’ oils and media, makes them much more durable. Diluents such as turpentine were not part of the earliest oil painting techniques and the painterly understanding of ‘fat over lean’ probably originates from the methods of the early Flemish Masters that perfected its use. Yes, the painting medium may ‘sink in’ to lower absorbent layers. However, gesso was usually sealed with a priming to reduce that effect and the painter’s choice of medium may also make it less likely, as in the case of Rubens.

    The reduced yellowing of poppy, walnut (the more likely oil to find in whites and blues during the Renaissance and a more thorough drying one, so less prone to cracking) and safflower oil is temporary and after 100 years, or so, its discolouration catches up with that of linseed. Indeed it is correct that cracking has often been caused by a more thorough drying layer being placed on top of one containing these oils. However, that is more a phenomenon of the 19th century onwards and reflects a decline in technical knowledge among painters (which begins to be a noticeable factor in the 18th century in fact). Comparatively, oil paintings that pre-date the 18th century are generally in quite good condition for their age, with the possible exception of some early works on canvas where a skim of a glue based gesso is still being used as a first layer of the ground and the odd occasion where a vulnerable pigment has failed (such as the blue leaves in a Dutch still life where the yellow in the mixed green has faded).

    Much of the damage encountered in later works in oil has a lot to do with the practical expertise of the painters (or lack of it) – or their pushing of the media in inherently unstable directions; and arguably to some extent by this combined with the choice of ingredients and manufacturing methods employed in commercially made artists’ materials.

    The aging properties of oil paint are known. Different issues have emerged in the case of acrylics: the absorption of dirt by their after-tack is an example of this. The logical projection is that oil paint used over an acrylic ground, or dead-colouring, will eventually suffer, because the flexibility of the layers will become significantly different over time. That is a common approach these days though, so it will be interesting to see what eventually happens.

    Incidentally, the use of transparent glazes in historic methods is often over stated. They are best used selectively with colours that lend themselves to that process. Oil colour yellows and darkens as it ages. The best of the ‘Old Masters’ compensated for that by how they painted to begin with.

    There is much more on this and the tangential subjects in the comment thread, in my book ‘The Materials and Techniques of Painting’, which was published in the USA in 1989 by Watson-Guptill and by Thames and Hudson in the UK. There should still be a few copies of it around and in libraries.

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  20. 20. Glendon Mellow 8:17 am 08/30/2011

    Thanks Jonathan! There is indeed more to understand. My post is only a brief overview and certainly whole books have been written on the subject of oil painting techniques and practices. Thanks for raising some of the more important points such as the different types of oil and the absorption into the supports: I agree with the problem of absorption in that case.

    Can you point out anything in my post above that “is not entirely correct”? I think it agrees with your more thorough explanation even in its brevity.

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  21. 21. sarahe 10:37 am 08/30/2011

    Hey Glendon. Missed this post when it first came out, but glad I just ran into it. Will never look at oil paints without thinking of jello. (For better or for worse! :)

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  22. 22. Glendon Mellow 2:02 pm 08/30/2011

    Thanks Sarah! “The Mona Lisa ‘n’ Lime”.


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  23. 23. JonathanMagus 6:12 am 08/31/2011

    Hi Glendon

    It would be churlish of me to pick on every detail of your article that could be commented on because you make it clear that you are not a chemist, or an art historian—and that the piece is a light-hearted introduction to the subject. And, it is good to see these considerations being raised among painters. I welcome the encouragement to explore the full capabilities of oil painting that your feature offers. What concerns me most is that in order to make good use of this knowledge there are certain fundamentals that a painter needs to fully comprehend.

    One factor I would wish to clarify, which is not unique to your account, is the tendency to overstate the historical use of glazes, whilst not fully recognising the selectivity of their application in reliable techniques. Another is not differentiating between a true glaze; which is transparent and suited only to certain pigments (ultramarine, alizarin crimson and quinacridone pink yes—cadmium red, no, for example); and the effect of a thinly applied translucent layer, which is not a glaze—though allows some light and lower colour to pass through it. A paint layer with differential thicknesses actually.

    Another is the nature of the oil in the medium for glazing and ‘fat over lean’ painting, which is a point I touched on before. This should have ‘body’ (think of this as substance or physical presence) of its own that compensates for the lack of ‘body’ in the paint when the pigment is thinly dispersed in it. Ordinary oil is of no use for this and just adding more of it to an upper layer will cause problems not prevent them. A treated oil that is part-polymerised, has more plasticity, a tendency to flow and possesses greater adhesive properties.

    Though I may have described it differently your account of cracking is broadly correct. A dry and brittle layer on top of a still wet and flexible one; so one is free to move and the other is not; is the usual cause. However, the root cause of this is more often the technical competence of the painter than an inherent failure of the materials. The blue sky in the Raphael you reference, if I remember correctly, is not in fact cracked.

    Another factor in the equation that most modern painters are not aware of, is that oil colour from the tube, which is a 19th century invention, is nowhere near as malleable, or responsive to the touch, as the original studio-made paints were. So, adjusting the handling qualities of modern oil colours to suit the preferences of the artist may lead to the introduction of imbalanced mixtures of oil that then affect the longevity of the painting. Interestingly even the Impressionists were still using suppliers that hand-ground.

    I empathise with you point about digital painting incidentally. My background embraces digital media as well as the art of painting. There is still something particular about the physical rather than the virtual though, do you not think?

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  24. 24. Glendon Mellow 8:20 am 08/31/2011

    Thanks again for fleshing out more particulars to oil painting technique, Jonathan!

    I do love digital as well as traditional and you’re right I think, there something about the textures of traditional painting, even when the image has a smooth surface, that attracts the eye.

    I also find that for non-artists, there’s a bit of a reaction to the ‘stunt’ of being able to do traditional, whereas with digital sometimes people shrug a bit, like the image popped out of a can.

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  25. 25. Fotoclipping 12:40 am 02/3/2014

    You have done great job. I like this. Keep sharing with us.

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