Friday, December 28, 2007

Acrylic Mirror Failures Learning Opportunities

The acrylic mirror samples arrived. I got 3 mirrors about the size of a credit card, but thicker, like a piece of glass.

OBJECTIVE: Create a "trough mirror" that focuses onto a line.

KEY FACT #1: Acrylic mirror softens and bends at around 230-250°F.

KEY FACT #2: A parabola is a mathematically perfect focusing shape, but (a small section of) a circle is plenty fine for my needs.


I got a big, glass jar with smooth sides. I put it lying down inside the oven. I balanced the mirror on top, shiny side pointing down (and with the protective plastic sheet still on it). I had to tape the mirror in place.

Starting at around 215°F, I slowly heated the oven up until the mirror ends started drooping. After probably an hour of watching it slooooooowly bend, I just reached in with an oven mitt and press-formed it to the glass.

RESULT: Meh. The shape is great and the focusing is accurate but the mirror got all foggy. It looks more like polished metal now. Also the spots where the tape was touching are distorted.


My theory was that I should have the mirror pointing up, which might make the cloudiness go away and would make using tape unnecessary.

This time I used a regular soup pan tipped on its side. I put the mirror inside so it could form to the inside curve of the pan. Left the protective plastic on. Set the oven to 250°F (that's what I had worked up to from the first attempt) and waited. And waited. And waited.

Once again, I eventually just reached in and press formed it. When I removed the sheet....still cloudy.


I watched the video again. Oh, I'm supposed to remove the protective plastic. Replay the second attempt, but this time remove the thing first.

After waiting the requisite Long Time, I could see the mirror was already foggy even before I press forming it. Aha! Not the plastic or the form!


I have small samples, so gravity isn't enough to bend them until they are very soft from being in the oven for 45 minutes or more. In that amount of time, some chemical degradation (or something) is causing the cloudiness. If I rig up some way to put a weight on top of the sample, maybe I could speed that up. Or I could just reach in there earlier and do it by hand.

But now I'm out of samples. I can bend and rebend the cloudy ones just to test out some weighting system and/or get my timing right. But I'll only be able to check if the cloudiness disappears if I buy more mirror. Which I can do, but I hate the shipping charges. Ah well.


  1. What an incredibly boring post. I can hardly be arsed to read it myself. I took a few photos, I should have included those.

    But at least it serves the purpose of documenting this for myself. By the time I order some more mirrors, I'll have forgotten what to do with them.

  2. You really are desperate.

    I planned another post about 3 three Handmade By A Hippie gifts I gave at Christmas (none of them were macrame at least) but I was defeated by Mrs DU. You'll just have to make do, I guess.

  3. Is the acrylic possibly oxidizing (or developing an extremely fine network of cracks? - you've got a dissecting microscope to check this in, right?) in the heat? Or perhaps a coating (maybe left by the protective plastic) is oxidizing?

    Perhaps wiping the samples down thoroughly with an alcohol-soaked lens-cleaning cloth would help. Or maybe sticking your oven in a chamber flooded with argon or nitrogen...

  4. Good thoughts, which lead me to look closer into something I was only half-thinking about.

    If I look through at an angle (no, no microscope) I can tell that the acrylic part seems fine. The problem is the mirror backing. This is especially clear on the one I used tape on, since I taped the mirrored side and it's most distorted there.

    I wonder if acrylic mirrors have different backing styles and if mine is more sensitive to heat than others. Maybe I should just try some different mirror before I get too crazy on the forming methods.