Another procedure involves placing a sample in a crucible, covering it to prevent loss, and mounting it with a ringstand directly in the flame of a bunsen burner, the idea being that whatever can be oxidized, will be, and that the remaining material is chemically inert. As a general rule of thumb, it is assumed that these remains cannot be metabolized, should the sample be consumed, and will either pass through the system, or stick to fatty tissues. It's a good place to begin a qualitative analysis in a biological or environmental context.
At a Science Olympiad meeting, we set up one of the above tests to see if the store brand cheeseballs that we'd been eating were actually digestible. After ten to fifteen minutes over a full flame, the cheeseballs produced a thick black soot, coated in a tarlike liquid. Further flaming could not dispose of either, and it was concluded that at least 90% of cheeseball mass cannot be broken down by any of the body's systems, and that each of us now has several ounces of unidentified crud stuck to his liver.