You can also use the microwave or stove. In the microwave go 30 seconds at a time and let it cool in between. The stove works best I think. Especially a flat top stove. Use a plate. Pour your bong water out into a filter to catch all the gross stuff in your bong. Put the stove on low. Keep an eye on it. You absolutely do not want the water to boil, just evaporate. It usually takes about 30 min or so.
It's important to note that you will not get what you are used to seeing. After scraping, you are left with more of a powder. It may still be a little wet so you can leave it overnight to dry out a little more. It will not take as much heat to smoke so you might want to use a bic. It will also taste nasty and leave a thick black residue behind.
Good luck