I'd suggest adding .5ml of water or so, then adding drops of white vinegar until it dissolves completely. like scag said the citric acid is the best route, but if you remember to dilute the white vinegar it won't do any serious damage if you don't use it excessively. I wouldn't add heat, it wont kill all the bacteria and some people think it will cause insoluble cuts to dissolve.
if you decide to try the vitamin c, which is probably a safer option than vinegar, I would add 1 part vit c to 2 parts crack. depending on the quality of the crack you might have to play around to get the proportions right. be careful with dose too, when I used to I.V. crack I would always push it to the limit. when your smoking crack if you start to feel too high you can chill out and put down the pipe, when your shooting if you do too much it's too late by the time you notice.
