Paint: "Darker When Dry"?

Question

A great question I enjoy bringing up is why are things, in general, "darker when wet". This applies to porous and granular materials like wetted stone, paper, sand etc. It also comes up in painting where "oiling out", applying a thin layer of medium over a dry painting, darkens most colors and makes them more saturated. There are many good explanations of "darker when wet" as, essentially, refraction index matching so that light has a longer stochastic path length though the absorptive medium (a "deeper" U turn).

Some paints are actually darker when dry, the effect doesn't seem to be as strong but it is certainly true. Acrylics generally dry darker: https://www.artistsnetwork.com/art-mediums/acrylic/acrylic-painting/

Some watercolors: https://www.handprint.com/HP/WCL/cds.html

And this seems to happen in oils too. I am in the middle of a discussion right now on how this, according to some accounts, happens more in mixtures of titanium white. Note pure titanium white seems to be the same value wet or dry; values of mixtures seem to shift more with it.

I can explain acrylics in a handwavy way, since the polymer emulsion probably scatters more than the film => drying reduces scattering and makes colors darker. I can't explain watercolor shifts or oil. In those cases when the medium (water or oil) leaves the paintlayer, the light should hit the particles more directly and there should be a greater difference in refractive index => more scattering => lighter color.

Apart from just jazz hands of non-idealities, does anyone have a good explanation as to how paint might dry darker? It would seem that dry and darker paint would somehow be making the light spend more time in the absorptive medium...I can't explain it in a way that would be consistent with why things are darker when wet. Is there another factor that isn't being accounted for in terms of refractive index changes across mediums?

Update: I'm virtually certain the answer is a well known paint film defect known as "flooding". It is seen as uniform darkening due to vertical separation of a (binary) pigment mixture. It is dependent on various factors including "wetability" of pigment particle surface, specific gravity but most importantly pigment particle size. Titanium white and carbon black (which are very fine, even though it looks like some sources report agglomerate sizes). I presume convective transport (volatile components in the case of oil paint) drive the separation. More here: https://books.google.com/books?id=T6XzCAAAQBAJ&pg=PA77&lpg=PA77&dq=paintfilm%2Btitanium%2Bwhite%2Bfloating&source=bl&ots=rvPQyHeJE5&sig=ACfU3U15E2jDFGgdfh4YxJwD6-SgK_9G9g&hl=en&sa=X&ved=2ahUKEwjzn9OoxMTvAhVS-qwKHQHTDh04ChDoATACegQIExAD#v=onepage&q&f=false

Answer 1

I realize your question is old, but I found it interesting and didn't see much interaction.

"Is there another factor that isn't being accounted for in terms of refractive index changes across mediums?"

I can't give a total physics perspective, but I can give perspective as an aspiring physicist who likes to paint.

1. The water/alcohol used as a solvent evaporates. The paint color technically never changes, and just seems lighter because the wet solvent-paint is a more reflective surface. Basically: When the paint is wet it has more solvents/water. This will give it different optical properties compared to dried paint and solvent/water. Like you said, dry paint=less scatter.

2. Acrylic paints in particular have a medium or binder in them. It can change the consistency and make it glossy or matte. When using these additives (some paints come with them already in them), the medium tends to be a clearish-white color but dries completely clear. When you add white to a color it gets lighter, but as it dries clear it's back to its original darker color.

3. Oil paint will also use binders/mediums. As the name suggests, this medium is oil (often linseed or walnut I believe?) As oil paint dries, the oil oxidizes and polymerizes, making the paint solidify. the refractive index of the oil medium and the pigments leading to a similar light scatter/absorption effect as stated above.

4. Watercolors are again a pigment and a binder. In this case, the binder is water. The water evaporates, and the pigment particles come closer together, potentially making it darker as more light is absorbed. (usually watercolor doesn't dry darker but I have seen it, especially if you try to contain your concentration to one spot and not spread it thin.)

5. There are a few other influences, such as the texture of the surface painted on, the brushwork and thickness, how much pigment the brush absorbs, etc. Different paint brands will also have different pigment particle sizes, binders, etc. Paints that create a more compact and uniform structure of the pigment particles when dry will increase light absorption and make it darker.

Hope this helps a bit :)

Answer 2

When something is wet, it has a layer of water that cause TIR (total internal reflection) and some rays get trapped in the water layer And this cause the object looks like a little darker.