Since this is just practice, I'd suggest you experiment with a few techniques. In all cases, I would start with the warm water and soap bath to get off the dirt. Then, you can try these (and I'm sure others have home remedies to share):
1. White vinegar (soak overnight) and scrub - you can dilute, if you think it will be too strong.
2. One part blackstap molasses to nine parts water - soak and clean with a brush or steel wool, if needed - I've used this off of another board a couple of times with relatively good results.
3. WD40 - wipe, and remove WD40 with paint thiner - probably most expensive method listed.
Etc...
I don't know about the crumbly one, so maybe that one should be reserved for just the water. What you use should really depend on the underlying metal. If you think you have any silver or gold or anything of potential value (like rare coins), I'd have a different answer. These are just techniques to play around with and see what works best for your pieces.