Most “heavy metals” or “toxic metals” as Lead, Mercury, Nickel, Chrome (some add Silver, Iron and Copper in this category) are bad because they are hard to eliminate by a body in good heath. Imagine when your liver and immune system are not doing their job! http://en.wikipedia.org/wiki/Heavy_metals
Note : In colloidal form, most elements will be eliminated in 1 or 2 days without problems.
A zapper, working with small amounts of current, will see its action greatly reduced if electrodes are poor conductors. This happens when the material is a poor conductor (Iron or conductive rubber or most hands free wrist straps, are not as good as copper) and when area is small, like small disks (even made out of silver or gold). This is why feet plates gives a better session than copper handholds.
Copper is almost as good as silver, conductivity wise, and is cheap because it is extensively used by electricians and plumbers. To avoid contamination as much as possible by other compounds, copper should be pure to 99.5% or better. Best kind I know is C11000.
Most copper tubes are made out of drinkable water pipes ¾” Dia, usually pure copper 99.5% or better.
Some people with heavy metals problems are very electrosensitive, and react badly to even copper.
Paper wet towel are always recommended because
1_ They put a good barrier between your skin and the electrodes, even when made out of copper. Control is the key word here. Control of what migrates into your body.
2_ They increase the area of contact, and moisten the skin for a better conductivity. Salty water increases conductivity. Pure distillated water is almost an isolant. Tap water is “polluted” enough to be a good conductor (!?!).
3_ they are cheap and disposable. Take white towels (no ink) and put new ones on each session. It they change color (greenish = copper chloride), change them too
Note: Chrome is bad. Brass has mainly copper and zinc, but other metals as tin or lead makes part of the alloy too, sometimes.
Silver in colloidal form is an excellent germicide. I have no data as zapper electrodes.
Whatever the metal, use always paper wet (salty or not) towel