Answer by Tortliena - inactive for Is it possible to design a bottle that can always be "full"?

Wine bottle air pumps

^{This is only a coincidence that I'm French and that I'm talking about wine. Yes, yes! "Seulement" a coincidence :p.}

Be the newest trend by getting inspiration from wine bottles. Indeed, to keep your wine fresh after opening its bottle, you need to keep the air out. To do that people use stoppers which also act as small air pumps.

^{An air extracting stopper. Push/pull repeatedly the top to remove air. From Wikimedia Commons}

So all you have to do is replace the glass bottle with a thick¹, metallic one and plug in a stopper to it. Take the air out with the appropriate pump and you get a nice, airless, fully metal-enclosed container to store your high-quality ingredients of the 1964 vintage. Here's a few things to improve the origin wine stopper further with metal in mind :

• Wine stoppers are generally kinda flexible so they can be plugged more easily. Change that with a stopper which can be screwed into the bottle².
• If you choose a classic hand pump, there's usually a flexible part made of latex or rubber :
  • You can replace it with a metallic sheet.
  • Alternatively, you can close access to this part with a metallic strip which can be moved from the outside with a small, lockable rod going through the bottle.
  • Motorized pumps could skip this problem entirely as they can easily be all iron.

As you can see, there can be quite a bit of variants of this concept. From the outside, it will look like neatly arranged bottles with metallic caps. It's easily storable and labellable, relatively cheap and reusable, easy to use and to clean and allow wide yet precise control of liquid flow. Finally, it will be the talk of the potion brewers council with its classy bartender's shaker look. What else do you need?

^{From wikimedia commons}

^{1 : Thick because you don't want your bottle to crumble onto itself as pressure lowers.
2 : Another answer pushed the concept to the limit, litterally : Make the stopper the same size as your bottle, so you can push it alllll the way down. That could help if you wish the liquid to have the most contact with the metal.}