I wanted to have a gas cylinder that would hold up to about 10 litres of gas (oxygen, mainly) at 20 atmospheres of pressure. Or, if that's impossible, 7 atmospheres would suffice.
The setting is pre-modern, so late medieval tech at best. I'd like the lowest tech solutions possible, but anything from the 16th century would be fine.
These don't need to be high quality or particularly safe gas cylinders (early boilers were plenty dangerous), but they need to basically work. They don't need to be precise or scientific in the exact pressures and measurements, either, but could work in the longstanding tradition of playing it by ear, where journeymen learn traditions from previous masters. They would also need to be able to exchange gas between canisters, using smaller ones to fill bigger ones.
Generating Pressure and Gas
I actually have the details worked out for how they can generate pressurized gasses. The question I have is how they contain it at those pressures. I can give additional details if necessary, but you can likely guess, if you've seen my other questions of late.
If they could get suitably pure copper, it has more than enough tensile strength for the task... but I don't know enough about the development history and purification of copper. Bronze is stronger, so it may be better. Since copper and bronze can be cast much more easily, they may be much easier to form into canisters.
Early boilers were made out of many small, copper, riveted plates, since copper withstands heat better than bronze. But the heat combined with the riveted construction made it limited in the pressures it could withstand (something like 500 psi was commonly handled, all the same). When they developed larger plates, the boilers got a lot safer.
This suggests several advantages for our canisters. Since they only need to contain a few litres of gas/liquid, instead of huge amounts of a boiler, they can be made possibly out of a single mould hammered into shape, instead of many small plates. Since it isn't a boiler and doesn't need to handle high temperatures, we can use bronze instead of copper. And since the heat from boilers is said to have weakened the copper considerably, we could assume our bronze canister can handle much greater pressures.
Notably, thickness makes an exponential difference to strength, so presumably what they lack in pure copper and flawless bronze alloys, they could make up for with thicker, heavier canisters? Of course, some merchants will use thinner, cheaper, less safe canisters, as is good for a story/setting.
Valves and Pressure Regulators
I had trouble finding information on the historical development of these, aside from very broad strokes. I don't need highly tuned pressure regulation, but there will need to be valves that allow for gross adjustments of pressure. It is also necessary that it's possible to refill one canister with another.
So, within the constraints of the setting, would it be possible to produce suitable valves?