Baking soda will get the job done in a pinch, but that popping is moisture turning to steam. Proper flux is made to handle the heat without all that mess. You can find cheap, real flux that works way better for just a few bucks.

Disagree with max963, thin joints can mean they skimped on the mortar and that's a real worry.

Yeah, the 90s runs were no joke. My buddy tried to move his whole X-Men collection once, just one long box. It was all the big crossover stuff, like every single "X-Cutioner's Song" tie-in and all those "X-Force" issues with the shiny covers. He picked it up wrong and the bottom just gave out, comics everywhere. @jordan464, it was like a flood of polybagged copies and hologram cards, a total nightmare to sort. Took him a whole weekend to get them back in order.