oh, compared to some of the tricks we got up to, the spud mortar was pretty restrained. I vaguely remember making a "scaled up banger" from the contents of several packets of conventional ones and the inside of a loo roll, wrapped in gaffa tape for a bit of extra security... the more astute amongst us probably realise that is basically a black powder equivalent of a stick of dynamite. I know it left a crater about a foot deep on the school field when it was let off...
And then there was the time when I was doing the "pyrotechnics" for the school play... Something crap that involved a Djinn / Demon arriving in a flash and puff of smoke. Rehearsals required multiple attempts to get this right, each time re-loading the flashpot with a bit o fuse wire as the igniter and a single teaspoon of flash powder. After the 9th take, where the Demon still hadn't hit his mark, I'd got sloppy, and forgot to remove the flashpowder tin and replace the lid. Yep, spark hit the half-pound of flashpowder - basically a 12 foot high 30 second+ duration blinding magnesium flare erupted blinding everyone on stage, scorching the Demon, charring the proscenium arch over the stage and leaving a big burned spot on the stage - not to mention flooding the entire school audtorium with acrid white smoke that you couldn't see your hand in front of your face, and causing pretty serious panic... So much for my career in Special Effects