Soak it in the FC for a day or so. Take out every few hours and scrub off, then put back in.
Rinse well and wash off. Slap the scale on the bench to dislodge the rest of the tap.
TIP:
If you have to locate the place on the other side of a scale or blade to drive it out try this:
If the hole you want is to be 1/8" use 1/8" bits ( as in the example below), adjust the bit size mentioned below to match the hole you need to make.
Take a piece of hardened 1/8" stock and grind a point on it at around 45 degrees. A broken 1/8" drill bit is perfect. Once roughed in, the best way to get it perfect is put it in the drill press and use a diamond stone to finish the point. Once made, break the point off so it is about 1/2" long. Grind the break clean with a chamfered edge.
Put a good 1/8" drill bit in the press and lock a 1/2" or more thick piece of mild steel in the vise. Drill a hole 1/4" to 3/8" deep in the metal. Place the point piece in the hole so the tip sticks out. If you don't move anything on the press, the drill bit is exactly aligned with the point.
Set the piece you want to drill the knock out hole in on the point and when lined up, drill from the other side. The holes should align nicely.
Once the hole is started, take the metal with the point out of the vise and close the jaws to leave a gap for the tap to drop out of. Continue to slowly drill the hole through the object. Once you hear the drill chatter as it reaches the tap stop drilling. Sometimes the tap fall right out, and other times it takes a tap or two with a piece of hareden rod (old drill bit works). Once out, finish what you were doing when the tap broke.
