Hmm... Time to take inventory of the slimes real quick... Lamp Oil Slime, O- Slime, Yellow Paint Slime, Pur-... Red Paint Slime? Blue Paint Slime? You two didn't...! Great... So I need a new RPS and a new BPS. That's going to be annoying to make. Note to self, ask some adventurers to grab me two new slimes from the wild.

Let's check the Food Slimes now... Honey Slime? Check. Grape Jelly Slime? Check. Chocolate Slime? Oh, you split and made a second one... Awesome! Hey, little guy, you're about to make me some serious money! Maple Syrup Slime? Check. Butter Slime? Check. Barbecue Slime...? Barbecue Slime...?! Ugh, not another one... Add that to the number of slimes I need to remake... That's really going to hurt the wallet since we're out of brown sugar... Maybe I can get an adventurer to help figure out how I can keep the slimes from mixing and escaping?

Well, adventurers? This is a formal Quest being put out to the entirety of the WB.SE Adventurer's Guild. In the past month, I've lost over a dozen slimes. Either they have eaten each other, mixed with each other, or escaped their enclosures/pens altogether and have either run off or been stolen. How can I prevent the loss of any more slimes? I use them for various purposes including selling what they produce, much like a beekeeper.

Info about the slimes:

• No, I can't just wait for my slimes to reproduce. That would take too long and make the new slime pretty much useless for my purposes for too long. (The whole thing would take 2 weeks at best, 3 months at worst.) It's easier to just get an already close-to-neutral pH slime out of the wild, cycle water through it until it has gotten as clean as possible, and then change the newly made Aqueous Slime into whatever I need by feeding it appropriately. (At most, 1 month.) Otherwise, I use too much water cleaning the bred slimes. (At this point, though, I'm pretty sure a couple people from the Guild already have prepared "wild" slimes to sell to me because of this... but it's still unnecessary spending, though.) The slimes I breed are better off selling for money. (Not to mention this Purple Paint Slime abomination...)


• No, I can't just tame the slimes with food or toys to keep them here... They're too mindless. They eat what they find unless they get hungry. Only then do they hunt. Since they seemingly can't taste, they don't care if they get their food from me or elsewhere... Just wherever they end up hopping, sliding, and/or oozing across first. (Depends on the slime.)


• Slimes won't split to be smaller than $8π,in^3$ OR $131π,cm^3$
  


• A slime can slip through smaller cracks based on their kind. A paint slime can slip through the crack between a door and door frame, but the lamp oil slime can slide pretty much anywhere water can. Additionally, a sticky material allows slimes some climbing ability, so paint slimes can just crawl up walls if the ceiling is exposed.


• If two slimes meet, either one will eat the other (becoming contaminated and unusable, so imagine if lamp oil got mixed into your milk) or the two slimes will mix (creating abominations like the purple paint slime).

While you already know, adventurers, here are some of the things not to forget about this world we live in:

• Skilled mages are limited to the point of being effectively nonexistent. The best I could ask a mage to do is cast an Ice spell constantly to keep the slimes in the enclosure, but the money it would take is impossible to accrue even if I sold the most valuable slimes I have. (Ignoring that one such gold mine ran away today it seems!) Beyond that, even the most talented of mages has enough mana to cast the spells necessary once ever few hours. In the summer, the ice would melt in minutes. Finding enough capable mages would be actually impossible. So, no requiring mages (or magical items), please.


• Craftsmen likewise aren't generally helpful. Back home, there was concrete I could use, but in this world nobody knows how to make it. It looks like the extent of materials available is wood, glass, stone, rudimentary metals, clay, and so forth. While a craftsman could probably make something, I'm concerned the cracks between pieces will still afford the slimes a chance to ooze out and escape. If your solution requires a craftsman, the fewer pieces and types of materials necessary the better.


• The world is very medieval in a lot of ways. Right, that won't mean anything to you... Basically, our technology is really limited. So don't suggest something that would require sophisticated technology. There was someone a town over who was burned for being a devil-worshiper because he came up with a highly rudimentary sewage system for his town. (He was reincarnated as a dragon, so it's fine now... but I was already dragged into this world from another. I don't want to risk reincarnation on top of that.)

So, basically, here is what you have to work with:

• I have plenty enough land available to me. (about 2.5 acres/10,000 m$^2$)


• I feed the slimes myself once a day so they don't get restless nor grow too out-of-hand in size. This means I need some kind of opening or slot that I can put their food into in order to keep them fed. At the same time, this means the slime could potentially escape that way, which I want to avoid.


• I need to be able to access the pen in order to get the products I'll need for the day.


• My slimes are pretty docile unless I forget to feed them for too long. I have no intention of being away for a week, so it SHOULD be fine, but just in case, assume an agitated "large" slime has the physical strength of a medium-sized dog like a Border Collie. (Actually large slimes are worse, but I have no intention of letting mine get that big.)


• Their acidity is about equivalent to Lemon Juice at worst. I make sure to keep them from being too concentrated, but assume a Lemon Juice Slime is just as concentrated as the lemon juice bottles you can buy in a store. (I used to have one before it mixed with a Sugar Water Slime... and now I have a Lemonade Slime... best mistake I ever made.) I try to avoid having anything that's TOO dangerous, so no HCL or HF, but still, I do have Slimes that make food preparation so much easier.


• Their strength and acidity may not seem that bad on their own, but consider those factors together and the durability of any materials used.

The best answer is one that uses the fewest types of materials, the fewest number of fastened-together pieces, holds together well, and gives me the best access to my slimes while they still can't get out. Basically, the best while cheapest pen wins. I'll even pay 10 Silver Coins (100 Reputation) to the winner (once I can place a bounty on this)!

When I first read this, I was reminded of a scene out of Fawlty Towers, where Basil and the Major are discussing the presence of a lapdog in the restaurant.

Major - Filthy Creatures, I say.

Basil - Indeed. It's a shame one can't store them in air-tight containers.

Major - Well, he wouldn't be able to breathe then, would he Fawlty?

Basil - He could try, Major. He could try.

While there's nothing in your missive that says that the slimes have to breathe, the fact that they need to eat gives it away and therefore I can assume that the following criteria apply;

1) You want air and food to be able to get in

2) You want nothing to be able to get out

3) Cleaning, especially with water, can be done at a specific point in time, one at a time, prior to sale.

If all this is correct, what you want is a Check Valve. You already ship wine and ale around in your territory via casks I'm assuming, so you have coopers that know how to create watertight (or slimetight) barrels, and given that you undoubtedly have brewing you'll have people around who know how to make a check valve as well.

Put simply, a check valve is a special type of valve that lets air and water in, but doesn't let anything out. It's in essence a one-way valve. This means that your slimes can be fed on a liquid diet of some form, and can breathe through the check valve as well, but for all other intents and purposes they are enclosed and can't escape.

Edit to address comments it would appear after a more careful inspection of the source material that check valves are a more recent invention, early 1900s. That said, they are amazingly simple, and more importantly easy to hide as they just look like an additional fitting in your pipes. Still, this news does invalidate some of the answer, although if you put all your slimes in oversize barrels, they can probably retain enough air between feeding times in any event so I still think there's some merit in this approach. (end edit)

So; build a really big barn, and store the barrels in them. Run some pipes through for the liquid food, and put a second check valve in there for air. If anyone asks, you're not doing anything really weird in terms of plumbing or 'science' that will get you burned at the stake; you're running a brewing company. Not only will no-one be suspicious, they'll actively protect you.

NB; It's probably best at this point to have a few barrels dedicated to actual brewing, so that you actually output a product and make your warehouse look legit. But, I digress.

Your slimes will need their barrels cleaned out or changed periodically, but if you have time and manpower, you simply do this one at a time. They can't interact with each other and even though this may seem inefficient, it's got to be less time than you're currently spending trying to keep them apart.

So; high quality oak barrels, professionally coopered, 2x check valves per barrel; one for air, one for liquid food, and a regular cleaning / replacement regime for the barrels in which you store the slime.

And the bonus; you can sell off this idea to poultry farmers and invent the battery hen model about 500 years ahead of this world's timeline.

Bundt cake pen.

Frisky slimes gotta move! If it hits a dead end it will start looking for cracks or go up the wall. But if there is a path of least resistance the slime will take it. It will head off down the pan. Yes, it will circle around, but these are slimes. It will not know it has circled around. It will keep going, moving fast, feeling good, getting that slime energy out in its bundt cake hamster wheel of a pen.

This was inspired in part by the phenomenon of the ant mill in which ants will circle around and around until they die of hunger. But you will be throwing slime food over the wall of the pen now and again, so that will not happen to your slimes.

Glass jugs

Make them big enough to accommodate your slimes; the slime should fill the bottom few inches and most of the jug should be empty space to prevent escapes.

A slime that climbs up the wall can't pass the "upside-down" section to get to the entrance. Since it has not bones or structure, it won't be able to hold itself up the wall; so it will just slide down into a puddle on the bottom. So, it doesn't matter if they are corked or un-corked. You can cork or not cork them, as you see fit.

Lemon juice won't etch glass; a well made class has nothing for a slime to get a hold of and won't leak.

Plus, clear glass jugs make it easy to display slimes for sale. Customers can even safely lean over the top to get a whiff. Do you sniff barbeque slimes for quality?

A Pithos

The term in English is applied to such containers used among the
civilizations that bordered the Mediterranean Sea in the Neolithic,
the Bronze Age and the succeeding Iron Age. Pithoi had been used for
bulk storage, primarily for fluids and grains; they were comparable to
the drums, barrels and casks of recent times...

The external shape and materials were approximately the same: a
ceramic jar about as high as a man, a base for standing, sides nearly
straight or generously curved, large mouth with a lid, sealed for
shipping. Jars of this size could not be handled by individuals,
especially when full. Various numbers of handles, or lugs, or some
combination, gave a purchase for some sort of harness used in lifting
the jar with a crane. (ref)

Or use the smaller version, the amphora.

What you want is a ceramic pot designed to be watertight/oiltight. Nothing can escape it from the rim down. Use plain clay if you want some oxygen exchange. Or use beewax or a glaze to keep air in/out.

While they both have openings that are narrowed, you can choose a design that allows you to let down a bucket, or reach your hand in. Your version does not have to be a traditional pithos or amphora; you can design it to your needs.

Multiple versions of these and similar containers existed long before Medieval times, so there should be plenty to choose from. The terms pithos and amphora do refer to very specific shapes and sizes, but don't let yourself be constrained by that. Any shape and size you can imagine probably already has a name. If not, you can have it made for you anyway.

To keep them from toppling over (if those slimes get frisky), you can set them partially buried in the ground or use a stand/support. Some have rounded bottoms and some stand on their own.

Now, you need a lid. One possibility is a heavy clay disc with a hole in the middle. The hole allows you to pull it on and off without any trouble (use a specialty hook to help you if needed), permits air to circulate (twice as fast as wood), and allows you to dump food inside. Stick a cork in it when you're not using it. A stone lid may be harder to find (as making it fit exactly is a chore) but it may be heavy enough on its own.

The clay lids are flat so you can stack other heavy things on top, if you're worried about the slimes pushing them off from the inside.

For transport, seal the lid in place with wax. Your jar will be 100% watertight. Soften the wax with heat and scrape it off to remove the lid.