I purchased some Staedtler french curves last week to help in a logo I'm drawing. These curves are basically flat pieces of injection molded pastic in the proper shapes. If you've ever used a plastic protractor, it's the same kind of thing. If you've ever used a plastic protractor, you have probably also discovered that they break.

In fact, one problem that my friend Kevin had in high school all the time was that his protractors broke with amazing regularity and always in the same spot. At some point, probably at a science museum, I learned that by placing plastic between polarizing filters, you could identify the areas of high stress inside the plastic. I did this with an intact protractor and found a line of apparently very high stress. It coincided not only with the location at which the protractors consistently broke, but also with two scores across the tool--top and bottom.

Fact or Conspiracy Theory?
I hypothesize that the manufacturers of plastic drawing aids deliberately stress the plastic by scoring the instrument. When I took my Staedtler french curves out of the package, the first thing I did was inspect them for scores, which I found pretty quickly. Examining the tools with polarized light showed exactly the same type of stress at these points.

Wonder what I'm talking about? See the pictures below.

I suspect this is a dirty little trade secret, and I'm happy to expose it. Staedtler, do you care to respond?

Here are the photos. I placed "X" marks at the edges of the stress areas. Notice how clearly those stressed lines stand out when viewed with polarized light.