As someone who digs stuff up for a living, I must admit I'm somewhat sceptical that such uniform discolouration of a bi-metallic object can occur simply from being buried, especially with no evidence of the blue/green corrosion product that occurs with all copper-rich metals subjected to damp conditions (as most earth is). I will concede, however, that I haven't seen enough 'buried' badges to make an objective assessment, & would be particularly interested in seeing single metal (both w/m & g/m) buried badges to see if there's any variance in the way the different types of metal tarnish & corrode.
With the badge in post #19, I'm particularly unconvinced of the 'earth' argument, as (as Andy says) the rear of the badge & particularly the slider are not subject the the same discolouration - the back IS dirty & seems to be covered with polish overspill, but if you rub it with a moist finger, it comes off, revealing the metal beneath; on the front, no matter how hard you rub, the brown does not come off (although I dare say it will polish off).
The first badge I posted (posts #14 & #16), again is very dirty & neglected, but the deep brown coating on the front is very uniform in thickness (or thin-ness, in fact), & most critically there doesn't seem to be any obscuration or degradation of the surface detailing - e.g., the seeding on the tablet, scroll & segmented edging of the 'hood' - as I would expect to see if it had been buried for even a short length of time. Also, if you look at the edges of the spots where the brown has flaked off, you can see it IS a coating of some sort, not simply tarnishing of the metal.
The big question of course, is why the badge was coated in the first place, & to that I have no answer at all. Likewise, whether it was done 100 years ago or only 20, is impossible to determine (at least with the tools I have to hand). I will note, however, that I have a couple of other badges that show evidence of some sort of red-brown lacquer being applied to their fronts (RAF & RASC, as attached), so someone's been slathering brown stuff on badges for some reason or other
I'd like it make it clear that in saying all of this, I'm not trying to push a particular position or imply provenance or attribution, & I'm certainly not trying to inflate the value of any particular item (to be honest, I couldn't care less about that - these are all badges obtained VERY cheaply for my own enjoyment & in memory of my Dad or other family members), it's of purely academic interest to me now.
Finally, by way of comparison, I've attached a couple of images of copper-alloy items that definitely HAVE been buried, & while one was in ground for considerably longer than the other (c. 1,900 years for the brooch compared to 100-200 years for the token), you'll note that both exhibit very similar patterns of corrosion & concretion, both of which are absent from the badges under discussion.
Kevin