Xylene Versus Acetone

Acetone is what's called a "polar organic solvent". That is, it has a positive and negative charge on it. Because of it's polarity, it is completely miscible with water. It has a ability to dissolve polar AND non-polar (no charge) surface residues. However, it is pretty limited in the non-polar compounds it can dissolve.

Xylene is a completely non-polar solvent. It is non-miscible with water, that is water will form a separate layer when mixed with xylene. It's a much better choice for most surface residues because they are typically non-polar. However, since it is so non-polar, it also has more of an ability to remove a natural patina....especially when over used. This is why many collectors stick with only using acetone.

In general, I've always found xylene to be a much better solvent for copper. It's rare that acetone can help a copper coin.