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never test a boiler of any type using air, it is very dangerous. always use water to pressurise with.the reason is if your vessel fails when using air, it will fail in an explosive fashion.but if the vessel fails using water then the pressure will instantly drop off to atmospheric pressure at no risc to life.
Cleaning up really baked on Roasting tins is easy. Just place outside under a damp bush for a couple of days. The garden slugs will clean all the stubborn residue off for you. The tin will be as clean as new, but make sure that it is washed in very hot soapy water before you use it next.