Burn the contamination out with a hand held propane torch. Keep it moving and don't get one spot to hot and you will start seeing the contamination starting to sweat out of the pores. When it stops sweating you are done.

I have rehabbed some badly abused molds using this method.
First clean off all the lose metal stuck to the all the faces of the mold with a bronze brush. I also have a piece of bronze about 1/2 inch with that i sharpened like a wood chisel that I use to slide along flat serfaces to take off big spots Do not ues this in the cavity. Next soak it in Hoppies. Take it out once a day and while it down and clean any spots of lead with a bronze brush. Use a jewelers loop or a strong magnifying glass to make sure all the lead has been removed in the pourers of the metal. if not totally clean put them back in the Hoppies and soak again until you get the mold totally clean. Then degreased and clean with break cleaner.

40 Rod

JoeS the propane grill lighter probably did not provide enough heat to burn out all of the existing contamination. Did it start and stop sweating the contamination out of the pores. If done right you can see it sweating and coming out of the pores and when it the sweating is finally burned off its done and ready to use. This also needs to be the very last process as you would now have JB in the pores and it too also has to be removed. When using a hand held propane torch leave the blocks on the handle. Hold the blocks open and keep the flame about four inches away from the blocks and just keep it moving across both faces until its done. I know there might be a fear of warping the blocks but if you keep the flame moving all will be just fine. As you may or may not know I make my own bullet moulds and during this process the cavity does get contaminated and the above is how I clean all of them before use. This was handed down to me by Barry Darr who also did the same when he was also making bullet moulds back in the day.