I use nick a date works rather well and I do have some great dates that were certified and am grateful I saved them as I have made a few bucks to reinvest and the collectors are well aware of the acid I used been doing it a great many years
When a planchet is struck, the pressure from the two dies coming together causes the metal to 'flow' from low to high (high to low on the die): the high areas of the die strike the planchet first, and as the die enters the planchet, subsequent low areas of the die cause the final impression to form. With that thought in mind, sense the date on the coin is raised, the metal would be less dense? So yes, a different density of the metal reacts to the acid differently...or so I understand.
Iceman- the reason the dates on your buffs continued to be eaten away is that you didn't neutralize the acid (lemon juice or vinegar). Over the years, the moisture in the air kept reactivating the acid. In general, acid recovered date nickels (how's that for polite term?) clearly are damaged but also have some level of market acceptability - as a hole filler for those dates we will never find in circulation... better living through chemistry. Strangely - as long as you're honest when the time comes to pass them on - I find I don't really object that much to the practice. It's not like you're removing paint from the Mona Lisa to see what's underneath.
Would using straight lemon juice or vinegar have the same effect as using a 10 dollar bottle of the stuff?
I've never tried any of those but I recommend a mineral acid. They're cheap and quick. As a chemist, I've used concentrated nitric acid, it only take a couple of minutes for the date to show.
So what is the safest way to attempt to retrieve a date from a buffalo that we can find in house-hold goods ?
I'm gonna say vinegar and check on it so it doesn't go too far. And rinse it very well when you're done.
No, Nic-A-Date works only on nickel - it's hard enough for the very slight differences to be meaningful. Sadly, there's no known fix for dateless silver, except VERY careful analysis of die markers. If you saw the end of the trail collection of PO1 Morgan's at the PCGS booth in 2014... one of the coins took TWO YEARS of research to prove it's date.