Chalks vs. Weathering Pigments for Copper oxidation

[jmg]

I'm wondering what exactly the difference is (if any) between using chalk for weathering and using the powders, such as MIG Weathering Pigments and MMP's similar pigments.

Also, I'm wondering a little bit about "sealing" such weathering effects; I've read some places that it's not necessary, in other places that it should be done, and generally that, if you do seal it, you'll lose most of the effect, so you should "over-weather" before applying sealant. Does using pigments instead of chalk diminish this loss? Is there any way to seal the chalk/pigment that minimizes this loss?

Any other tips for using powders for weathering? The specific effect I'm going for is the oxidation of copper, so I plan on applying it dry with an old brush. (For the record, I have read the article in the FAQ section about using chalk for weathering, but I'm looking for a bit more detail.)

Thanks!