I'm curious why brass is the metal of choice for a probe?
I've only probed using what I had handy -- the steel wire of a utility company yard flag -- I know that's way too hard and may scratch a coin. Also use a screwdriver for coin popping -- again, hard enough to scratch so not a great choice.
Need something better, so I started looking at the physical properties of metals, and pure copper, silver and gold are all around 2.5-3.0 on the Mohs hardness (scratchability) scale. Brass is 3.0-4.0, which on Mohs, means it is at least twice as hard as the pure metals. Zincolns (who cares, right?) are coated in 100% pure copper, "copper" cents are 95% copper/5% zinc and that alloy that is *slightly* harder. The 75% copper/25% nickel alloy used in Nickels and outer layer of clad is *slightly* harder than the 95% copper alloy. 90% silver (10% copper) alloy may be *slightly* harder still, and I think 40% silver (60% copper) is a bit harder yet.
I wonder if the minting process makes the coins harder than the raw alloys? Many sources say a "copper penny" is Mohs 3, which is the upper end of the copper range, so not hardened very much, if at all. Mohs covers a huge range from talc=1 to diamonds=10, so I doubt the hardest coin we dig is much over 3. And the softest maybe a bit less than 3.
So why do we use a brass probe in the 3.0-4.0 hardness range?
Seems like something less hard, like tin wire (1.5 hardness,) or pure copper, or some alloy with a hardness less than 3 would be a better choice. Or a stiff steel core with a softer metal as an outer layer. I think someone mentioned a bamboo skewer worked ok, too, and that should have a safe hardness around 2, but maybe less tactile feedback than metal-to-metal contact.
I don't probe much, but do coin pop. I'm going to cover my screwdriver with vinyl (electrical tape or plasti-dip, or something similar) which has a hardness around 1.
I'm also curious if anyone with a brass probe has intentionally tried to scratch coins with it? There's nothing like good old fashioned destructive evaluation...