Why you should never buy or make a brass, bronze or copper slingshot.
Bronze and brass are both alloys of copper, bronze is mainly copper with tin and brass is mainly copper with zinc. And for those of us that like our catapults copper is a problem, or more precisely putting copper and rubber next to each other is a problem.
I first noticed the effect after going back to a catapult I had banded a few weeks earlier where the bands broke at the forks on the first pull. I expected to find a nick or cut that had caused the band to tear. But instead the tie and the band adjacent to the brass fork was sticky and degraded. At the time, I thought oil or petrol (more specifically Zippo lighter fluid, as I’d been messing around with an old lighter at the time)
However, when I noticed the effect on another catapult, a bronze one this time that had been in a storage box for a few months, I considered it a bit further. Luckily my wife is a chemist and suggested it may have something to do with the sulfur in the rubber producing copper sulphate and thus degrading the rubber.
On the internet, I found a couple of studies going back to the early twentieth century where the phenomenon had been noted in rubber washer on copper pipe and in rubberized fabric where the cloth had been died using a copper based dye. Links are below and if you want to go into the science a bit deeper there is plenty to get your teeth into.
But avoid lashing your bands directly to a brass, copper or bronze frame and if you can’t resist the gold then use tabs. Or if you ate after a heavy catapult without the copper go for stainless steel.
Ind. Eng. Chem., 1917, 9 (12), pp 1092–1093
Publication Date: December 1917