Saturday, October 30, 2010

Why Not Polish Your Glasses with Paper Towels?

I just got a new pair of glass. Yes, they are highly fashionable -- probably too fashionable for me. The tech at the optician's said, "Never clean them with paper tissue." So why?

Paper tissue is made form trees; trees are made of cellulose. But wait, soft cloths are made from cotton which is made of cellulose too. What is the difference?   I know that a toothpick could not scratch a drinking glass; it just does NOT seem possible that cellulose could scratch glass.

Well there are two factors, first my lenses are polycarbonate, which is much softer than regular lens glass, and second, lens glass is softer than a drinking glass.

Second and more important is that paper is not what it seems. Paper is routinely filled with minerals to make it cheaper -- maybe a little stronger and whiter, but mostly cheaper. So you think you are wiping pure cellulose on your lens, but no!  It's really talc, and clay and titanium dioxide and other cheap filler that the paper maker dumped in. Also wood pulp isn't all cellulose these days either: sometimes there is ground bark which has lignin, sand and the mysterious "foreign matter".

There seems to be a lot of controversy about using Windex to clean glasses. It seems people are worried about Windex washing the coating off of glasses. This is crazy. Windex is about the weakest detergent that there is. It is pretty watery. If a lens maker is using a coating that washes off in Windex -- you have big trouble. No reputable maker would do that. Anti-Windex hype is probably from competitive cleaning solution makers. I don't believe that Windex is a problem.

The cleaning-cloth that people talk about is "microfiber."  Microfiber is finely spun synthetic polyester or polyester nylon blend. (Other polymers are used in engineering applications.) The fibers are 10 microns or less by definition It is quite small as the figure shows. It is used because it weaves into a thin fabric with good conformability. Almost all of it is used for garments including so-called ultra-suede, but also in underwear.  In cleaning, a thin fiber would make finer scratches if it did scratch your lens, it is safer.

Some fibers are made from mixed polymers split after drawing, and split fibers can be 2 microns. As you can see in the picture at left, some splitting is incomplete. I assume the nylon is in the center and the polyester is on the outside. The differential shrinkage causes some splitting, and that gives more surface area for wiping, but only a little more since the splitting is imperfect.

I read a camera site's discussion on lens cleaning and they were all over the map. Some always use microfiber, and some claims it scratches lens. Some use T-shirts and some never do. Some use lens paper, and some only tear the lens paper and use the torn edges. Lens cleaning seems like more of a superstition than a craft.

In summary, I am going to stop using regular tissue and paper towel, and buy some lens paper. I am going to continue to use Windex.