Please hold your hate mail and hear me out….
I like bottle caps because of what they teach. A normal signal–such as dime only teaches you the bare basics. You hear a tone, see a meter reading and dig. It would be great if that was all there was to success with a detector in any conditions—but it’s not. Bottle caps teach you the whole detector. To ID a cap you need to understand a lot more. Caps are composed of multiple metals—so you learn to recognize alloyed targets. These may have broken or with a tone ID machine—multiple tones. These metals may include brass, aluminum, and steel—each with different response characteristics. Where you have gold or silver mixed in with other targets that are alloyed such as screwcaps, tin, or “can slaw”–being able to spot narrow meter responses can help you to zero in on the best, most consistent signals.
Steel in particular corrodes over time and caps will respond differently depending on how rusted they are. This makes them much harder to identify by meter–so a range of skills are needed. Rust, is more like the ground than are “pure” non-ferrous metals and to be able to examine your signals for this similarity or difference is a critical skill. When you switch to All Metal or Pinpoint mode—these rusted caps will be a wide, drawn out signal. This simple iron / not iron confirming test is the most important skill you can have with any detector—especially if you want to run high Gain where there is a lot of iron falses sounding off. When you want to learn the operating characteristics of a new machine—good targets can only teach you so much. If you want to learn how that detector responds to alloys and separates non-ferrous targets from iron—get your self some caps—they’re not hard to find! A good cap test kit should include several–some rusted and some not.