No Chemicals in the method I used, and such rich color from black and white photo paper. The convention issue of School Arts Magazine (Page 38) has info about "how to" make the camera. (But NOT how to do chemical free photography). This article, by the presenter, Nicole Croy, explained and demonstrated the use of chemical-free photography on Page 21.
Essentially you are using photo paper and over exposing it for many days, burning an image into the photo sensitive paper. It can be exposed for at least 24 hours in bright sunlight, or even up to a year!
You do need a flatbed scanner to "process" the film. For the Chemical-free processing, you scan the photo paper at high resolution. Because I used a small Altoids tin, I scanned my paper at 1200 DPI WITHOUT a preview (because the preview ruins the paper). If your camera is larger, and the film paper big, you might only need to scan at 300 DPI. You get one shot and the image is what you scanned. Then you use Photoshop or any other photo editing program to reverse the image and invert it.
Indoor lighting does not bother the film much, but try to keep accidental exposure to a minimum. The scanning process does force a lot of light onto the image and does essentially ruin the image, so again, you scan it without a preview. Practice a bit before you do the real thing.
The camera must be facing a sun-drenched area. Face South, East, Or West, but never north. This process will also not work indoors, unless perhaps it is facing a very sunny window. When you open the camera (In subtle lighting) you'll see the negative! Then you just scan it. The presenter told stories of how she leaves pinhole cameras all over the place in her neighborhood, train station, back yard, even making mannequin-like figures to capture in her haunting images. In hindsight, she thinks leaving them in public transit areas may be problematic. Think about how others may react to seeing a box duct taped to a light pole at a train station... it's important to consider WHERE you place your pinhole camera, and how it may me perceived for weeks in the same location.
I gasped when I saw the tiny image so crisp. It really worked well with black and white photo paper, even the old stuff. My image is black and white but gets a ghostly hue from the overexposure. As the paper yellows, a contrasting color will show in the inversion. That streak across the sky is not a cloud trail from a plane, but the sun going by seven times during my one week exposure from my home's front window.