Evil, I have no idea what the health risk of shooting tar are. It's just not available in the area. So I couldn't tell you at all if there is a chance for bacteria and stuff to reside in it. But I'm pretty sure that heating it up until it's just about to boil should basically take care of that.
If you're unsure about it not dissolving enough and are afraid of large particles, you can always go through the steps of preparing it using 15-20 more units than usual, then instead of injecting it into your vein, inject what you've filtered into another spoon and use another cotton to filter it twice.
Again, I've never used tar, so I don't know what the potency is like, but I've gotten really dark rocks of dope called "raw" that will turn the water black. And it is BY FAR amazingly potent. So I doubt double filtering it will reduce any bang for your buck.
And just so I don't have to start a new thread since we're already on the topic, does anyone of any methods to get rid of any bruising related to injecting? I've got two where I think I started pulling out the needle a little bit while injecting and some just got under my skin and left the bruises. Basically I'm looking for anything to quicken the recovery AND I'm also curious of any types of make up I might be able to use to cover them up.