Maybe I will gross some people...
I clean the hard cake on the pipe submerging it some seconds in a glass with diet coke (diet, because I don't want the sugar to get inside the pores). Sometimes, the reaction between the cake's carbon and the coke is spectacular...
After that, it is really easy to scratch the cake out.
Then, I put a tampon (yes, a female hygienic tampon) in the bowl, and put the bowl in the oven from 30 minutes to an hour at a little less than 100ºC. I do not use alcohol, or salt, and advice against it. The reason: I used a precision balance several times, and weighted the bowls before and after the soaking process. Even when dry and after cleaning, the bowls had added mass after the process. That means that the pores had absorbed fluid and/or salt, even if just a tiny little. I advocate for a "dry sauna" where the pipe can sweat the tar and oils out. I am really happy with this system.