Everybody has their own methods to remove slag. Here are some of mine. I rarely needed acids.
For small, smooth Au or Ag bars with a little slag here and there, I have had good success using a clean 3/16" - 1/4" round steel rod about 6"-8" long and just gently tapped the slag (with the side of the rod about an inch from the end) to shatter it. Once shattered, to remove it, put it under running water while rubbing it gently with your finger. This works well and won't damage the surface if you're careful. You can also tap the slag while under running water. If the slag is down in a hole, you need some sort of a small probe to carefully break it up. Rinse it out with running water or use the probe under running water.
If you don't care about the surface, both ends of a welder's chipping hammer work well.
For large bars, an air needle scale remover works fast and complete. Of course, it will somewhat alter the surface appearance.
http://www2.northerntool.com/air-tools/item-15763.htm.
For large silver bars with a thick (say, 1/4" or more) layer of slag, I usually used a small, short sledge. Often, it works best to put the bar on edge and beat along the Ag/slag interface. Sometimes, it works best to just lay it flat and beat on the slag or turn it over and beat on the silver side. Sometimes, the slag will all come off in one piece but, often, it won't. Sometimes, it's really stuck and you have to beat the hell out of it. A chipping hammer will remove small amounts of residue.
