This x1000. I am still dumbfounded that the knowledge that tar can be smoked off a titanium nail in a bong isn't widespread by now. The best way to inhale vaped H without exception. Only, watch out for your tolerance. The first few days will be awesome but once your body gets used to taking in 5x the amount of vapor per inhalation, the tolerance will start to skyrocket.
One thing that wasn't mentioned - heroin is highly water soluble. Dry fire the bong, don't fill it with water. You'll lose some percentage (don't know how much, but any is too much with The Preciousness) in the water if you do. The smoke isn't acrid and I acquired a taste to it when I was using H so water is unnecessary anyways. The only drawback is that you'll get H resin caked everywhere in your bong (whereas if it had water at least the bottom would stay clean and only the rigging would get resinated) but that's not a big deal. On the plus side, when you're out of dope and starting to hurt, if you've been using the bong for a while you can actually vaporize the resin in it with the same torch you use to heat your nail.
Now - I'm not sure if what you have is tar but I think it is, being from the midwest which if I understand correctly gets both ECP and Mexican tar, and you said that the stuff you have is a hard chunk/rock that needs to be cut with a razor, which sounds like hard tar (tar comes in every kind of consistency you could imagine from goopy to rock hard and even powder, albeit the powder form of tar quickly reconstitutes into solid tar once it hits the moisture of open air, or when breathed on).
While I am certain that tar can be vaporized in this manner, I'm not sure on the other types of H like ECP (never had it) or Euro dope (which comes, as I hear, in multiple forms, some bases, some salts of diacetylmorphine). I don't know the boiling point of any of these other forms of dope so do some due diligence before you try to smoke it - it might not work (like trying to smoke cocaine without freebasing it first).
PS - To actually answer your question, OP (lol) - I smoked tar for about two years at the rate of at least 1g/day, sometimes 2g's if I had the cash. The worst that ever happened is I started getting tons of lung butter to loogie up, and one batch of badly cut dope made my voice all scratchy after a single hit, but that's all. I don't think it is particularly harmful in comparison to tobacco or cannabis, both of which have high levels of tar in their smoke.