a new wrinkle

"What - did you grow up in a barn?", the Rocket Scientist sighed, rolling his eyes in-between picking up my soda bottle from his table and inserting a coaster underneath it.

You learn a lot about people from their offices.  My office is a barely controlled state of chaos, which pretty much mirrors the rest of my life.  The Rocket Scientist is the only faculty member I've ever known who keeps coasters in his office (and requires their use).  I'll let you figure out what a coaster fetish tells you about RS - I have my own theories, but (ignoring for the moment the fact that we work for a public university and all our furniture is laminate) there actually are really good reasons for one to use coasters.

The cool liquid in glass condenses water from the air onto the glass.  The water rolls down the glass onto the wood table and produces a white ring that doesn't wipe off.  Removing that ghastly mark of shame requires esoteric cleaning approaches, like a warm iron applied to a towel over the damaged area or rubbing with toothpaste. 

But these fixes usually work only when the damage is confined to the top layer of the finish. Most real wood furniture is stained - pigment is absorbed into the wood fibers and the solvent (the stuff in which the pigment is suspended) evaporates. The furniture is then coated with something to protect the finish.  Back in the day, they used penetrating oil, which is absorbed into the very top layers of the wood, and/or coated the whole thing with a paste wax (sort of like like the plastic that covers certain types of cheeses).  The final layer is a barrier between the wood and the elements, but the finish can also affect the appearance of the furniture.

Light behaves really nicely when it hits a smooth, flat surface, which is called specular reflection.  (Remember "the angle of incidence equals the angle of reflection"?) Specular reflection gives you a nice clean reflection. The smoother the surface, the more specular the reflection, which is why you want a blemish-free top layer.  The smoother the finish, the more mirror-like the reflection.  For example, the Hubble Space Telescope's mirror has a surface smooth to within about half a millionth of an inch.  The mirror you carry in your purse is nowhere near that precise, but it still has to be pretty smooth to ensure that you don't feel like you're at a circus fun house.

A rough coating produces distortion and diffuse reflection. The light hits the rough surface and goes every which way, mucking up the reflection. ('Mucking' being a scientific term.) If you put the same paint on a smooth wall and a textured wall, it can look like two totally different colors because the surfaces reflect differently. Gloss paint tends to produce more specular reflection and matte paint more diffuse reflection. Diffuse reflections are less focused.  The old camera trick of filming through a piece of gauze works because the gauze diffuses the light and softens the image.

Polyurethane is not nearly as fancy as penetrating oil or paste wax, but you can put one or two coats on a piece of furniture and be done with it (compared to the ten or twelve you might need for something like tung oil).  Polyurethane is clear, so you don't have to worry about what your protectant is going to do to the stain color you so carefully chose.  When you look at the furniture, the polyurethane has little impact on what you see.  Light travels into your perfectly smooth furniture coating, refracts a little at the interface between the polyurethane and the air (as shown at right), hits the actual wood, and then comes out again , refracting back an equal amount at the second poly-air interface. Theta-i equals theta-r and everything is clear.

A scratch or other imperfection in the top coating makes the reflection more diffuse.  Instead of passing through the polyurethane as if it weren't there, light can reflect from the surfaces created by the imperfections in the top layer. When water is absorbed into the surface layer that protects the wood, it creates imperfections in what ought to be a transparent layer.  Those imperfections are responsible for the white ring.

When you put a warm iron onto a cloth over the wood, you transfer heat to the piece of furniture.  That does two things.  First, if there's any moisture left, the heat will help evaporate it.  Second, you soften the topcoat  enough that it smooths itself out.  When polyurethane is heated, it flows and evens out any roughness or irregularities.  If you've ever soldered something, you know that when you heat the solder, it liquefies and forms a nice smooth surface. If you don't want to take an iron to your grandmother's heirloom dining room table, buffing with an abrasive (like white toothpaste) also helps to even out the surface and decrease all those disorganized reflections.  I've also seen suggestions to rub with alcohol, which I think is a mild solvent for a lot of surface finishes, but if you use too much or rub too hard, you'll remove too much of the finish and that area will become a different color (especially if the finish is old and it's become darker).  If you've actually damaged the wood, none of these will work and you're going to have to remove the finish, fix the wood and then re-finish it.  The same goes for scratches - as long as the scratches are confined to the surface layer.

So what do the artisans and craftspeople who make fine furniture have in common with pitchman Billy Mays and his latest product that fixes scratches on cars?  He keys a car, then takes a little tube that applies a clear liquid, runs it over the scratch and voila! - the scratch is gone.  It works for any color car because cars are finished very similarly to furniture.  The base color is applied and then a number of clearcoats are put over the top.  As long as you don't scratch deeply enough to hit the color layer, the scratches you see are just because the clearcoat isn't smooth anymore.  The tool he uses applies a combination of a resin that is compatible with the clearcoat and a solvent.  The solvent melts a little of the clearcoat and the resin fills in the material that was removed by the scratching process.  The little foam applicator helps smooth out the surface.  That's why you don't need different product for different colors - you're only fixing the coating. 

And while we're talking about finishes and reflection, I'm still musing over the amazing things a professional can do with makeup - which I got to learn about from my experience with The Science of Speed video series.  A wrinkle in your skin is (from the point of view of a physicist, at least) really not very different than a scratch in a car's finish.  Over the couple sets of filming sessions, I had so much attention to my face that I became acutely aware of wrinkles I didn't know I had. The warm iron idea clearly is not going to work here and if we apply alcohol anywhere to fix that problem, I'm pretty sure internally is the only way to go.  This introduced me to the miracles of foundation.

Foundation has two purposes: hiding variations in color (like blemishes and age spots) and hiding the canonical "fine lines and wrinkles".  Some foundation is opaque cream or liquid that contains pigment particles - usually platelets - that cover your skin.  The platelets are opaque, so what you see is not your skin, it's the pigment from the foundation.  This type of foundation makes the skin very uniform in color because you're essentially covering up the skin with paint. 

Making the foundation more translucent requires decreasing the amount of pigment, but less pigment means less color correction. To make it worse, the pigments - which are often metal oxides of nano or micro scale dimensions - can actually collect in those 'fine lines', so some makeup actually makes lines look more pronounced if you don't touch it up every five minutes.

In contrast to covering up the skin entirely, some foundation is designed to make you more 'luminous'.  This makeup uses spherical microspheres (or nanospheres) made of silicas, polyethylene, or polymethylmethacrylate ("PMMA") that are very good at scattering light.  The idea is that the overall appearance of the skin is somewhat blurred by the makeup.  It's called the 'soft-focus effect'.  The problem is that these little particles work because they are transparent enough that light passes through them before reflecting.  That means you're seeing the actual skin, although you're seeing it through a blurry optical filter.  So if you have a pimple, someone looking at you sees a slightly blurry pimple because the particles aren't opaque enough to cover it.

You can guess what the next generation of foundation contains: opaque pigments combined with light-diffusing particles.  That can be done by using the traditional metal oxide opaque pigments like aluminum oxide and iron oxides and adding some less-opaque particles to produce the soft-focus effect. To the right, I've shown on top a flat pigment particle coated with tiny light-diffusing spheres. 

Flat pigments have the advantage of lying flat on the surface of the face;  however, spherical particles tend to be sensed as smoother and more desirable by consumers.  Some formulations either put the diffusing particles on the outside of a larger pigment particle (lower left), or use core-shell particles (lower right on the picture) with the opaque pigment on the inside and the diffusive part on the outside.  You can even make multiple layers for your opaque pigment and design it so that it reflects more green or blue or red, depending on the type of skin you're trying to correct, or mix different types of beads.  If you've ever wondered why cosmetics ads always use the phrase "reduce the appearance of fine lines and wrinkles", that's because those products don't actually do anything to fix your wrinkles - they just make them less noticeable.

The moral?  For furniture and cars, beauty may be only skin deep, but when it comes to people, beauty sometimes isn't even skin deep.

Comments

[Captain Skellett]

Nice post! I can't believe you managed to make reflection physics interesting, it was the biggest snooze-fest in High School!

[Jennifer Ouellette]

I am a coaster fanatic. Thanks for giving some scientific underpinning to my personal fetish. :)

[Michael Riley]

I had no idea that makeup ever got this complicated :) It's good to see science seep its way down into everyday life, you always know it's the more I know about the little things around me and how they work, the happier I am.

[Mark]

The result of a wet glass on a marble table is another problem altogether.

[Antique cabinets]

I have always enjoy your post, the other i learned how does our eyes perceive colors, that was interesting, i even explained that to my friends, by the way my condoleances to Billy mays, he was a great salesman.