if those don't work try a popsicle stick or in an extreme case a razor blade.
tin will stick to brass like crazy since it is what you use to solder copper wires together with.
just get the mold hot enough to melt the tin and it will come right off.
This has been a road I have been down and it's not a fun one. Prevention is absolutely the best way here. When the brass mold is new heat cycling it helps tremendously. With MP molds I find it takes a lot of heat cycling to get that protective color on the mold. With mine I have heat cycled on my hot plate over a week to get it where I feel its ready to use with lead. During the course of a day I will heat it for couple hours let it cool and heat it again. I have a Waring hotplate not a Walmart cheapie and it holds a uniform temp pretty well and doesn't burn my garage down either. I don't recommend leaving it unsupervised either way. Anyways I lay the mold on old skill saw blade cavities facing up and do the heat cycling thing through out the day. With a infared thermometer the brass reads like 275 and the sprue plate reads just a touch under 300. MP molds are absolutely stunning to look at when you first open the package but man will they attract little deposits of alloy right on the edge of the cavities that don't wipe off. I have tried beeswax, bullet lube, heat, pin point torch with little success. I have found that when the blinding shine of the brass goes away the mold is ready to accept lead. During casting I use a piece of burlap to wipe cavity faces for errant lead spattering. Over at cast boolits a guy named Malpaso has a couple threads on this subject. My first brass molds where so bad that I had to use a techinque used by brewers to remove lead from surface of brass in some of their processes. I could never bring myself to bring anything harder than brass next to molds. I know my limitations, my momma wouldn't let me play with a crowbar in my sandbox, she knew the crowbar didn't have a snowball's chance. I suggest you do some reading and think on it for a moment. It's fixable, and more importantly preventable, it will just cause some uncomfortable sensations in your gut working to get it off without damaging your property. Good luck Sir.
Has anyone ever tried floating the blocks face-down in a saucepan that had about 3/4" of Cerrosafe alloy melted in it? My thought is that since solid lead is soluble in liquid mercury at room temperature, maybe solid lead would dissolve in a liquid bismuth/lead/tin/cadmium solution, and the low melt temperature would ensure no damage to the blocks.....maybe.
Makes sense to me.
Cerrosafe contains lead and tin already so we know they are soluble in the alloy.
I bet that would work just fine. Maybe a little beeswax as a reducing agent to get the oxides to go away too.