Oliver,
In most cases you won't damage the coin with a nylon brush.
As in the case of one of yours it might look that way if you remove dirt to reveal a coin that was damaged prior to you getting it.
There is an exception though and that is when you have a very powdery chalky surface.
In spots that can be
bronze disease but it is also common with coins found in some soils - especially heavily fertilized soils. It seems common with uncleaned coins found in British fields and I know from here that it is common in those found in Austrian farm fields.
In my opinion the worst of these are ones where much of the coin is covered with a very thin layer of hard dirt - which can sometimes preserve the surface details in a
very fine state - but where this dirt is sitting on the powdery
patina. Cleaning result in a very
poor and
pitted coin or even a slug as the nice details are really only preserved in the dirt which just pops off.
This is why I always recommend examining the coins dry, before you soak them the first time (as even soaking in DW can harm these coins) or at least drying them out thoroughly and examining them. This is where poking gently with a toothpick
comes in. You will never get hard dirt off with a toothpick but you can get soft or wet dirt off and more importantly you can test the surface integrity.
All I have ever found to do with these powdery surface coins is to a) leave them
as is and put them in your
collection, b) give them a gently "polishing" between thumb and finger (sometimes results in a sort of stabilization) or c) give them a gentle waxing.
But unless you have one of these powdery surfaces you should do no harm with gentle brushing. The safest bet is always to
scale up - soft nylon, then stiff nylon, silver wire and then soft brass wire brush
Shawn