I just read the OP's post, so forgive me if I'm repeating anything that's already been said.
First of all, it's not ideal to cook dope in a spoon (but fuck, what junky hasn't used a spoon-I used to not only use a spoon, but I'd use spoons that appealed to me aesthetically as well, ridiculous I know), make sure it's as clean as possible. It's better (if you're going to use heat), to use one of those little metal cookers that they give out at needle exchanges and come in little baggies. IF you don't have access to that though, sterilize the spoon to the best of your ability.
Second, to everyone here, we all got to get this straight, yes it's better not to cook heroin, but people will always cook dope, no matter what. For one thing, people that have access only to black tar heroin, almost always cook their dope a little bit, because depending on the dope, it can take a long fucking time to break down and lets face it, we're IV drug users, we aren't known for our patience. Another thing is that while there is danger of making insolubles temporarily soluble, cooking dope can also help kill blood born diseases and some viruses. I saw a study once which was interesting (I think it was a pamphlet I got from a needle exchange in Berkely) that showed that there were way less instances of blood born diseases, and incidents of HIV/HEP C exposure on the west coast than on the East Coast, because black tar users cook there dope more than people using ECP.
Now one thing that someone should not do, is purposely heat dope with the intention of dissolving sediment that's left floating in the solution. What you are doing here, is just making sure that you shoot up non soluble cut which will solidify again in your blood stream and can cause all types of problems. Heroin is very soluble in water, and if it's in powder form, or base form, water (and a little citric acid for those who get the base) and slight/no heat should immediately dissolve your dope after a few stirs of the plunger.
But really when it comes to tar heroin, it can take up to 20 minutes to cold cook that type of dope sometimes, and I've found that this isn't dependent on quality, but rather cut. I've also found that when you heat tar, most of the cut seems to stick to the sides of the cooker or settles at the bottom of the spoon.