Shine vs. Sheen - why polyester doesn't look like silk

The difference between the sheen of silk and the shininess of polyester is like the difference between white gold and chrome: one softly smiles at you, the other wolf-whistles to get your attention. 

When I see a polyester charmeuse, I'm often tempted to use the wrong side to cut down on shininess, but even the shiniest silks, like silk satins or charmeuses, still have a gentler look than polyester versions of the same.  How come? 

Raw silk in the Suzhou No.1 Silk Mill in Suzhou (Jiangsu), China. Picture by Armin Kübelbeck [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], from Wikimedia Commons. Cropped by me; click picture to see it whole in context.

LIGHT!

The tactile texture of a fabric is how our fingers react to it, but the visible texture of a fabric is how light reacts to it.  When light hits something smooth, it bounces off in a straight line and our eyes say "oh, shiny".  When light hits something with bumps and angles, the light bounces in various directions as it hits different planes.  Then our eyes say "oh, matte".  Let's start with a look at natural fibers:

Magnified fibers of silk wool and cotton. Popular Science Monthly Volume 39, 1891. Public domain, via Wikimedia Commons.

A -- SILK
Silk, a protein-based fiber, has a relatively smooth surface.  This picture shows fine striations, but that is not accurate.  This page has more detail.

B -- WOOL
Wool, also a protein-based fiber, has microscopic scales on it.  This is common to most hair: human hair has scales, too. 

C -- COTTON
Cotton is a cellulose fiber (because it's from a plant, not an animal).  It's ribbon-flat with twists and curves. 

Based on the surface qualities of the fibers, we can easily guess (and a quick trip to our closets should confirm) that silk has the greatest potential to look smooth and shiny, followed by cotton, while wool never does.  No matter how straight you pull wool fibers, or how you weave them or how you iron them, they will still have scales that cause the light to bounce off chaotically. 

What about semi-synthetics?  Rayon, when it was invented, was marketed as faux silk.  But rayon has striations along its length, so it's ideal to imitate matte silks like crepe de chine or sueded/sandwashed silks.  It can also make a very nice satin, but for some reason polyester satins are much more common nowadays. 

Which brings us to polyester, the shiny fiber.  In its microscopic smoothness, it's the closest to silk.  Yet, as I said in the introduction, there's still a difference in the quality of the shine.  And that difference comes from the cross section:

See, silk is a long smooth triangle, while polyester is a long, smooth tube.  Polyester is also regular in diameter all along its length, while silk may taper at fiber ends.  With polyester, there's no escape from the smoothness!  It's too perfect!  With silk, there are corners to turn, and the light can be diverted.  Somehow, the addition of only three rounded corners is enough to create a softer sheen. 

Another important difference is mentioned in the that Textile School site I linked to above.  The description of the silk fiber is as follows (emphasis mine): "Smooth surface, Structureless, triangular shaped transparent rod".  Polyester fibers are not transparent.  What does this mean for light?  Light can pass through silk fibers, giving the silk fabric a translucent appeal (trans-lucent = letting light pass) and a tendency to pick up on neighboring colors.  Polyester doesn't do this.

PERFECTION AND VALUE

It's interesting how the irregularities of silk fibers now inform our sense of luxury.  Through most of human history, perfection was difficult to achieve and consequently valued.  Silk was valuable because it was the smoothest and shiniest fabric available.  I can easily imagine that if an ancient Roman or Elizabethan saw polyester charmeuse, they'd think it was the best thing ever, worth far more than silk!  But post the industrial revolution, perfection is routine.  Machines can make things perfectly round, perfectly smooth, so we are not excited by perfection any more.  Though silk is no longer the shiniest option, we are drawn to it for its softer luster.  We value what technology cannot (yet) replicate.  This same dynamic explains why we now value silk dupioni with its slubs, when Victorians would have scoffed at its flaws! 

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SOURCE FOR FIBER DESCRIPTIONS

Kadolph, Sarah J. with Anna L. Langford.  Textiles, 9th Edition.  Prentice Hall, Upper Saddle River, NJ.  2002. 
          Figure 3-3, pp. 20. 
"Microscopic Appearance of Fibers."  Textile School.  Posted 12 March, 2018.  © 2018 - Truent Systems LLP. 
          Accessed 9/6/2018: https://www.textileschool.com/330/microscopic-appearance-of-fibres/

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(Incidentally, you may have noticed that I haven't posted much of my own sewing recently.  That's because I'm struggling with time management and haven't been getting projects done.  I have a few deadlines coming up, though, and deadlines usually get me moving!)