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The people in my setting (urban fantasy, starting in 1998 America and moving forward) have a bit of an engineering dilemma they need to work out regarding potionmaking. The magical substances they brew with have a notorious tendency to degrade within an hour after collection unless properly stored. They have specially-made flasks for this purpose, made out of a metal with the ability to hold these magical ingredients in proper stasis as long as the liquid is completely encased in the metal on all sides in an airtight seal, with no air getting in the way of the liquid's complete encasement in the metal.

For the potions they brew, normal flask designs work just fine for this. Fill the bottle carefully all the way to the top and then carefully screw it tight, then drink the whole thing when you're ready to use it.

For storing the ingredients, however, this serves more of a problem, because unless they too are all used at once every time they're opened, the half-empty contents can't properly be kept in stasis unless the insides are "watered down" to make the flask full again, which would be dizzying and frustrating to keep track of when using the leftovers for the next potion. It would be ideal if the ingredients were kept in some manner of container, made out of this aforementioned metal, which was somehow adaptive or adjustable and could provide an airless, airtight seal to the contents inside even when the container isn't at full capacity.

I can't even imagine what such a container would look like, but if such a design exists, naturally the people of my setting would have had the incentive, time and resources to invent it, so I need to make sure it actually doesn't exist before I start having it be an actual obstacle my characters need to deal with. So I thought I'd ask around here:

Is there any way to design a metal container so that even when the container isn't filled to capacity, it can still be sealed with just liquid, and no air, inside of it?

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    $\begingroup$ You mean something like a syringe (just move the piston to adjust capacity) or like a wineskin / waterskin? $\endgroup$
    – AlexP
    Aug 29, 2023 at 14:50
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    $\begingroup$ Why should the metal be rigid? Thin sheets of metal are not rigid at all. Consider for example the metal wrappers for chocolate... $\endgroup$
    – AlexP
    Aug 29, 2023 at 15:14
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    $\begingroup$ @CyrusDrake Does the liquid need to be touching on all sides the metal? Or can there be some vacuum space around it? $\endgroup$ Aug 29, 2023 at 16:46
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    $\begingroup$ @Tortliena Let's assume for now that vacuum space is okay. I'm very curious to read whatever answer you have in mind. $\endgroup$ Aug 29, 2023 at 16:58
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    $\begingroup$ For rigid metal only, imagine a box (or any other shape with a consistent length and width) open at the top. Add a lid that's not attached to the box but which fits snugly in the opening at the top and can slide down until it rests (or "floats") on top of the liquid inside. Awkward to travel with (but not impossible, with some simple accessories to lock/clamp the lid in place), and may require some design elements to allow air to be expelled when settling the lid. But seems to fit the requirements. $\endgroup$
    – aroth
    Aug 30, 2023 at 2:39

18 Answers 18

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  • Drop 'marbles' of this special substance into the flask. Remove $1\: cm^3$, add a sphere $1.24\:cm$ in diameter. (Might that be combined with measuring? Throw a marble in, a measured amount overflows.)
  • Try and construct a sufficiently tight piston, a bit like the comment by AlexP. Basically, the opening is the full diameter of the flask, and you screw it down until it touches the surface. (Problem: how to get the remaining air out?).
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    $\begingroup$ You get the air out exactly how a nurse gets the air out of a syringe before using it to inject something: push the piston until a tiny amount of the contents comes out of the spout. Injecting air into a person's bloodstream is very bad juju. $\endgroup$
    – AlexP
    Aug 29, 2023 at 15:09
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    $\begingroup$ Oh wow, the marble idea is absolutely genius. $\endgroup$ Aug 29, 2023 at 15:12
  • $\begingroup$ A balloon might be more flexible $\endgroup$ Aug 31, 2023 at 20:21
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    $\begingroup$ marbles (glass balls, in fact) are pretty much traditional as well, alchemists and high-end sommeliers used them on a daily basis $\endgroup$
    – fraxinus
    Aug 31, 2023 at 20:49
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They just need a Spout Pouch

We have already solved this problem for situations where we need to reliably contain liquids while still being able to access them, particularly spaceflight. You need to make the metal thin enough that it becomes flexible, while attaching it to a substrate sturdy enough to prevent it tearing.

While we usually use a plastic substrate coated with aluminium using vapour deposition, even a medieval society could have achieved something similar with animal bladders and chemical deposition. We would certainly have had ready access to this tech in 1998, although if your magical-stasis metal was not aluminium they would require some custom production runs.

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    $\begingroup$ in 1969 capri sun is invented so you have time for them to perfect the design for the new metal. $\endgroup$
    – John
    Aug 29, 2023 at 18:50
  • $\begingroup$ This is probably vast overengineering for the task, but when you mentioned space, I expected a mention of propellant tank technology solving the same problem: quora.com/… $\endgroup$
    – ain92
    Sep 1, 2023 at 16:13
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If you want a cheap and reusable solution, liquid ingredients can be stored in syringes. You can make them any size you want (see image below for a really big one sold in Walmart for about ten bucks). You can also make them out of your special metal, though you might end up with opaque syringes. You'd still know how much liquid you have in by seeing how far in the plunger is.

A 150 ml syringe to help you get higher than the Milky Way

Put a cap at the tip of the syringe to preserve stuff. If you have a 1L syringe full of cactus juice and you only need a small dose for a potion of Commune:

  1. Pull cap
  2. Push plunger to take 1 ml out
  3. Put cap on again to preserve the remaining 999 ml of fun.

Do notice that if the city guard enters your shop you better be able to produce your apothecary license, as you can't claim personal possession when you got a closet full of goodies in syringes.

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    $\begingroup$ The plungers themselves would likely have markings on them for measurement; numbers toward the center, with lines out to the edges where the plunger meets the body of the syringe. Whichever line is at the opening of the syringe body, that indicates the remaining volume. $\endgroup$
    – Doktor J
    Aug 30, 2023 at 3:43
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    $\begingroup$ Note the rubber piston seal. A sliding metal-on-metal seal isn't easy at all. $\endgroup$
    – Chris H
    Aug 30, 2023 at 15:01
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    $\begingroup$ The first paragraph of the Q rather implies it needs to be the same fluid throughout. The second permits dilution, but doesn't mention liquid that don't mix (otherwise a bucket with a layer of immiscible liquid on top could work). I wonder if grease would be permissible - ask airport security and I'm sure it's a liquid $\endgroup$
    – Chris H
    Aug 30, 2023 at 15:09
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    $\begingroup$ Anyway the syringes listed on Walmart are from third party sellers, who list them as for scientific use and other applications. Google Lens finds the same photo on eBay ("Nutrient Liquid Food"), as well as AliExpress, they it is apparently for enemas. At this point I'm sure it's badly shooped $\endgroup$
    – Chris H
    Aug 30, 2023 at 15:37
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    $\begingroup$ @ChrisH I don't think the size is that far off. Take, for example, this Amazon customer photo of someone holding a 250ml bottle of shampoo. Considering the walls of the syringe are significantly thicker than the bottle, I'd expect their outer dimensions to be in roughly the same ballpark. Which appears to be the case. $\endgroup$
    – FeRD
    Sep 1, 2023 at 13:48
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The traditional solution with paint was to store it in a bladder tied with string, and squeeze out what you needed. This was replaced by the paint tube. American portrait painter John Goffe Rand, living in London in 1841, patented the zinc collapsible paint tube with a stopper cap.

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  • $\begingroup$ Best answer. This was also how in the past water and other liquids were carried. $\endgroup$ Sep 2, 2023 at 17:21
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I like the Spout Pouch idea, but it sounds like you might be looking for something rigid. If a metal cap threaded into the neck of a metal bottle works, then just use a cylinder that's threaded all the way down, so the the cap can be screwed in as far as it needs to be. Think like the syringe, but instead of plastic with a rubber push, it's metal threaded on the inside, and the cap is like a set screw that can go all the way down.

Not sure I've described this very well, so here, have some quick (read: crappy) renders: Base and Cap Cap Screwed In Cap Screwed Down

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    $\begingroup$ I think your design also needs a valve of some sort to release any air in the chamber, as it stands if it's an airtight seal it would just compress the air into the potion ingredients. $\endgroup$ Aug 29, 2023 at 18:35
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    $\begingroup$ What if there were a cap on the other side to be the proper cap, you opened that, and then you turned the bottom screw until the container was small enough for the liquid to reach the brim? $\endgroup$ Aug 30, 2023 at 2:13
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    $\begingroup$ Common threads are not tight, only special threads like NPT are and with those you can't make a thread which is tight at all positions. $\endgroup$
    – Arsenal
    Aug 30, 2023 at 12:45
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    $\begingroup$ Isn't this just a syringe with extra steps? $\endgroup$
    – Christian
    Aug 30, 2023 at 13:47
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    $\begingroup$ Thanks for the comments! I like the idea of a second 'cap' on the other side that's used for access, while the one shown is used to set the size of the container. Yes, this is a syringe with extra steps, but OP specifically asks for metal container, and the syringe relies on the rubber stopper to slide down the tube. Correct that threads don't normally create a hermetic seal, my answer assumes that OP has dealt with that somehow since they can 'screw it tight' for the potion container. $\endgroup$
    – AeroSigma
    Aug 30, 2023 at 19:43
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Does the potion mix with oil? If not, then you can carry your potion in whatever bottle you happen to have, use as much of it as you like, and top up the bottle with oil which would prevent air from coming into contact with the potion.

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This is a problem that exists in the real world and was solved for some medications and... toothpaste.

Modern toothpaste tubes are made of plastic and go back to the original shape after squeezing, sucking air in. I am talking about the old metal kind that you'd have to squeeze from the bottom and doesn't recover the original shape.

Pros:

  • A toothpaste tube is made of metal.
  • Can be filled from the bottom and sealed.
  • Air does not flow back in and the exposure to air while pouring is very limited.
  • The tech level is appropriate for 1998.
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    $\begingroup$ I think toothpaste tubes are the best solution, as they come closest to the original constraint of metal blocking the magic of the substance in all directions. $\endgroup$
    – arp
    Sep 1, 2023 at 2:42
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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 removing stopper
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?

Bartender's shaker set
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.

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    $\begingroup$ The effectiveness here is limited by how good of a vacuum you can get - no matter what apparatus you use, there will always be some air left inside, which can cause problems depending on how reactive the liquid is. One review of a popular brand of wine vacuum called it "completely ineffective after a couple of days". I don't think this has much chance to preserve liquids that normally degrade in the span of just 1 hour, which is at least an order of magnitude faster than wine. $\endgroup$ Aug 31, 2023 at 13:47
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    $\begingroup$ @NuclearHoagie Yes, that's something to consider. Using motorized pumps can help a lot with that, but I think it's much cooler to have mad brewtenders juggling and pumping their bottles mid-air :p. $\endgroup$ Aug 31, 2023 at 14:09
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Very low tech: Pour a layer of paraffin wax over the top of the liquid ingredients. The wax will harden into an airtight seal with no gas between it and the liquid. To use, push on one side of the seal to dislodge it from the sides of the container, remove the wax "puck," pour off the liquid you need, then reseal with a new layer of hot wax.

Advantages: Super low tech. Use any container that is compatible with the ingredients. No machining or other precision manufacturing needed. Irregularly or roughly shaped containers will work fine. A sealed container can be tipped or inverted without harm.

Disadvantages: Super low tech (i.e. not as cool a solution). Slower to use than e.g. a syringe. Need to have hot wax ready before use, to avoid having the contents exposed to air for too long. Ingredients need to be tolerant of heat of melted wax (approx. 50-60 degrees C).

This is how jams and jellies are sealed when canning them at home, except that jams and jellies are not typically resealed between uses.

You can also use beeswax for this. It is even more low-tech. The melting point of beeswax is about 63 degrees C, so the same caution applies about making sure the ingredients can take the heat.

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Heavier than water

A frame challenge, though I've already upvoted many excellent straight answers here.

Sometimes fluids and/or gasses don't mix. This phenomenon is used in interesting ways. From using it to create different useful products from oil, like gas and gasoline, to special coffees and cocktails where different fluids do not mix, giving interesting layers to the drinks. There are many uses.

Potions can use this as well. Instead of watering down the potion, the potions might not so easily mix with certain fluids. Be they water, special oils or other. A layer on top of the potion can protect it against oxygen or other contaminants. If it isn't jostled, a small layer can be enough. If it is put through more rough handling on the road it can simply be filled, removing the contaminants out of the container.

The bottle would have designs to allow drinking of the bottom fluid first, which glass blowers already have ingenious glasses for. Otherwise it could be a special potion fluid, which will sit on top and can safely be drank.

The special potion fluid could also be tiny first, but small amounts turn into large amounts of gas, forcing all contaminants out as it expands. As you open the bottle the gas escapes, while you drink safely from the potion.

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Add an inert gas

Another option: take a bottle of compressed inert gas that is heavier than air, e.g. Argon. After using the ingredient, release a puff of argon into the container, that will displace the air. This method is also used for preserving wine after opening.

Of course this assumes that a gas like argon won't degrade the magical ingredients.

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Smaller, Single Serving Ingredient Bottles

If you have a standardized potion bottle, then make standardized ingredient bottles in the proper sizes.

If you combine 1 cup ingredient A, 1 cup ingredient B, and 2 cups ingredient C to get a 1 quart potion, then you'd just have lots of 1 cup ingredient containers, each one filled to the top and sealed exactly the same way the potion is filled and sealed.

Scaling

If you decide want to make 4 potions in a batch on a regular basis, then you up the size of the ingredient containers to 1 quart, and measure them out into the four different 1 quart potions.

Size the ingredient bottles to whatever your standard batch is -- different producers might have different batch sizes, so there's probably a variety of bottle sizes.

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How about an airless pump? I have had foundation come in such bottles to prevent oxidation.

It is basically like a regular pump bottle, but there is a disc inside that rises up as the liquid is dispensed. I'm not sure if links are allowed, but this amazon page has pictures / a video that explain better than I can. Otherwise, just google "airless pump".

Product: https://www.amazon.com/Sterile-Airless-Pump-Bottle-1oz/dp/B01CWY33U0?th=1 Image: https://m.media-amazon.com/images/I/61fNSbEaNHL.AC_SX522.jpg

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    $\begingroup$ If you can find a free to use picture, you can put it in directly, and that would be recommended. Links aren't disallowed, but they are discouraged to prevent problems from link rot. Good answer though $\endgroup$
    – No Name
    Aug 30, 2023 at 22:55
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I feel like this is less a question about how to design an air-proof bottle and more a question about how to prevent the spoilage of a substance. We might need to think outside the metal container to get an answer. The first question I would ask is what in the air is causing the substances to degrade. It seems likely that the answer is oxygen, as it is reactive and makes up 21% of the Earth’s atmosphere. Water vapor is another possibility, but one that is disproven due to the substances being used in potions which likely contain water. Nitrogen is unlikely as it is not very reactive but does make up 78% of the atmosphere. The same goes for CO2, although it does react to water and form carbonic acid, but CO2 makes up less than 0.1% of the atmosphere.

To my mind, that means the method of preventing spoilage needs to be the removal of oxygen from contact with the substances. Before getting into solutions, we need to determine what is happening when air (or oxygen) encounters the magical substances. If we assume that the substances follow the laws of physics (matter cannot be created nor destroyed, merely change states) the oxygen would bind to the atoms/molecules of the substances and change them into a mundane/magically neutral form. A tightly sealed container nearly full of magical substance would thus have some portion of its contents spoiled, but the spoilage would stop once the available oxygen was bound into the new form. The spoilage is obviously not strictly from evaporation, as a container with vacuum space would not prevent evaporation and evaporation stops when external atmospheric pressure equalizes with emission pressure.

If oxygen exposure is the culprit, it can be removed a variety of ways, especially with 1998+ technology. The oxygen can be replaced with an alternative, such as nitrogen or a noble gas, prior to the container being sealed. Dropping a pellet of dry ice into the container would eject most, if not all, of the oxygen inside a container as it evaporated. An oxygen absorbing agent can be used to remove trapped gases. Something akin to ascorbic acid coating the underside of the lid which binds the free oxygen and prevents it from spoiling the substances. Vacuum sealing can also be accomplished using both rigid walled and flexible walled containers. The vacuum might not be perfect, but it would limit the amount of oxygen available. If combined with a flexible plastic or rubber coating, the substances would be well protected. This is used in food preparation to prevent spoilage. Put the substances in a metal foil sleave, remove the air, seal, and you are good to go. A well-designed system could even be used to re-seal opened packages after removing a portion of the contents.

It might also work to change the way the substances react to air to avoid needing such a container. If the substances are liquid, perhaps they form a “skin” when exposed to air. The hardened skin is of no use, but the liquid underneath is still viable. Or, as mentioned above, they degrade until the available oxygen is used up. Perhaps an additive can be used to extend the shelf life of the substances, but it causes an unpleasant aftertaste or makes the substance less effective and thus resulting potions are in less demand and sold cheaper. This would introduce an element of class division, as the wealthy could afford the better tasting/more effective potions, while the poor would use the cheaper mass-produced potions. Homebrew potions might become a thriving industry if people are able to create and use the substances before they degrade to make small batches.

If the substances are heavier and do not mix with water (think of oil and water, but the water on top), you could have a container with a spigot at the bottom and a layer of water on top. The spigot allows for the removal of the magical substance as needed, without risking the remaining liquid. Eventually, the container would have a small amount of magical liquid sitting below the spigot opening, but it could be combined with another container with little loss and excess water skimmed off the top.

The workers producing the substances might work in a chemically “clean” room, wearing breathing masks while working in a pure nitrogen atmosphere. If nitrogen was being forced into a room from above, and vents carried any escaping oxygen and CO2 away in floor vents, the substances would only be exposed to nitrogen. A lid with an inflatable bladder could then be used to pump out the substance as needed without introducing external oxygen into the mix. Think of a chemical spray tank with a manual pump on top. The bladder pushes the substance out a hose while preventing oxygen from contaminating the remaining stock.

The solution to the problem depends heavily on the story and if these substances “need” such complicated containers for the story to be told properly. If there is not a story need, focusing too heavily on a solution can be a waste of time. Handwave a solution and move on with the story. If there is a story need, then looking at the long history of food preservation might help. Push comes to shove; you could always just set the magical substance creation in space and import sealed containers which were never exposed to an atmosphere. Or you could just have vacuum pump technology advance rapidly and become mass-produced. Everyone has a vacuum pump to remove the air from substance containers and prevent their loss. A national standard could specify connector types and container requirements.

Whatever option you choose, it should fit into your story and make sense. There is no point in inventing new technology if readers will look at it and wonder why your world did not just use an obvious real-world solution to the same problem.

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If you want to use a non-flexible container, another option is to have the bottom be a piston that's spring-loaded. The spring's force is then tuned to the density of the ingredient and the size of the piston (denser ingredients, and larger piston surface areas would get stronger springs), or they could all use the same spring, with the piston surface area "tuned" to the ingredient density to make it work out. Either way, as you remove the ingredient, the piston would automatically push the bottom up, so the top of the ingredient is at the top of the container. This solution has the advantages of being possible with 20th century technology, and taking a fair amount of work to get it right, which I gather would be desirable in your world, to explain why it's not a thing in our (the readers') world.

Disadvantages of this approach include the need to keep the container exactly vertical when it's open, and the tendency to overflow slightly at higher altitudes (thought this could also be a cute plot point), and not working at all in space or on another planet, though of course a container could be made for any one planet.

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Collapsible metal tubes — like the first toothpaste tubes sold in the 1800's.

According to this site (emphasis as in the original):

The first toothpaste tubes, appearing on the scene in the 1890s, originally were made entirely of metal. In the 1940s, because of metal shortages during World War II, they were constructed of a mix of plastic and metal. A hundred years after their invention, toothpaste tubes became completely manufactured of plastic as we know them today...

A Connecticut dentist named Dr. Washington Sheffield introduced collapsible metal toothpaste tubes in 1892... Sheffield got the idea for the tube from his son, who had traveled to Paris and saw artists using oil paint from metal tubes. The first toothpaste tubes were made of tin and lead, and remained basically the same until a metal shortage during World War II....

Metal tubes do have some advantages over plastic tubes. They stay coiled up from the bottom better, and because of this, they are less likely to draw air back inside because the paste stays put at the top opening. Metal tubes are still commonly used for oil paint and certain gels containing pharmaceutical products and supplements, to keep air from getting in at the top. Air can dry out the product, or it can cause oxidation that may result in some degradation.

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The juice pouches my children sometimes drink from are like this.

It is a simple foil pouch.

enter image description here

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There’s a balloon in my flask!

☑ Ingredients are contained within airtight magical metal

☑ Ingredients never touch air when the contents are poured out

Air displacement flask (Flask with partial fill)

Full flask (Flask filled up with fluid)

It is the air that contaminates the ingredients, and not being fully encased in magical metal. While you could try to put solid beads or marbles into the container for displacement, why not make the displacement a softer material: a balloon?

The flask is fully inflated when the balloon is flat. There is no air in the flask or the balloon. If you need 2.56 ounces of fluid out, you inflate 2.56 ounces of air into the balloon, which pushes 2.56 ounces of fluid out the spout. The air never touches the fluid, only the balloon does. Find whatever material does not spoil the fluid (like the marble), and that is the material for your balloon.

Filling the flask is just as easy.

When you remove air from the balloon inside the flask, the spout will suck in a fluid at the mouth.

Build the air cap out of your magical metal

This way the fluid is completely enclosed when you are not filling or pouring.

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