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Everyone takes Potions in Harry Potter . . . but why does everyone make a giant pot of potion each time? The final result is a small vial that is nowhere near the size of the cauldron. Besides the disproportionate potion vs. vial amount, making such a large amount seems like a waste of materials.

Is there an explanation for why the students make so much?

Note: Not asking about professional potion brewers (as @Adamant points out in comments) who make a lot for reasons such as selling.

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    I think Rowling & co were simply invoking a pre-existing trope. I think if you look at other depictions of potion-brewing which pre-date Harry Potter, you'll find the same sort of thing - a generously-sized cauldron, almost brimming over with brew, from which only a tiny portion, sometimes just a few drops are drawn off to be administered.
    – Anthony X
    Mar 21 at 1:11
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    And of course people who brewed actual mundane potions sometimes did so in large batches because that was often a source of income, so they sold potions to many people, and because some ingredients were seasonal, perishable, or limited, so it was easier to store the potions than the raw ingredients. Also, sometimes they only had the one pot, and they had to have one big enough for soup!
    – Adamant
    Mar 21 at 1:17
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    Just guessing, but maybe the active part of the potion is usually in an oily layer floating at the top, and wizards haven't invented sep funnels. en.wikipedia.org/wiki/Separatory_funnel
    – PM 2Ring
    Mar 21 at 6:58
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    When I was in grade school, a lab partner and I botched an experiment mid way through and ended up doubling the components. This had the unintended side effect of making our calculations twice as accurate as everyone else's. Perhaps to get potions right you must be very accurate with the proportions and small batches lead to larger errors. Off by one drop when 10 are required vs off by one drop when 100 are required because you are baking a batch an order of magnitude larger.
    – JonSG
    Mar 21 at 13:37
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    This is 100% speculation, but perhaps there's a "reduction" step that's undepicted. It's not uncommon in cooking, for example, to simmer to reduce a liquid by a significant fraction to thicken it, or to concentrate the flavor. Perhaps a similar mechanism reduces the potion to intensify the effect, or make it more convenient to administer (if a whole cauldron results in a dose or two, that's a highly inconvenient volume to both transport and/or consume when needed). Mar 21 at 16:43

7 Answers 7

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It's quite possible that some recipes don't divide well. Have you ever tried to halve a recipe that calls for 1 egg? It can be done, but it's a bit annoying and cannot be done without physically modifying the ingredient. Quite a number of potion ingredients are things that may be not so easily divisible like eggs, eyes, or entire creatures or specific body parts. Anything that is not a homogeneous substance like a liquid or powder may be damaged by splitting or not work as well in the potion - imagine trying to halve the recipe for a hard-boiled egg!

For these reasons, recipes that call for 1 of something indivisible may not usually be made in smaller quantities. Since some recipes aren't easily divisible, it may not be common practice to divide any recipes, which may typically taught in standard cauldron quantities.

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    This useful analogy between magic potions and kitchen recipes suggests an additional possible reason: heating to a precise temperature a small quantity of liquid (think milk or custard) is more difficult than working with a larger batch, because the temperature grows more quickly.
    – lfurini
    Mar 21 at 18:52
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    It's also possible that it cooks down a lot. If I need a half a cup of cooked spinach, I'm starting with a lot more than that in raw leaves. Mar 21 at 21:11
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    This. I would imagine that certain ingredients would be indivisible for magical reasons, in the sense that they are only effective if added intact, and prematurely "tearing a piece off" would undermine or invalidate their efficacy.
    – Blackhawk
    Mar 21 at 21:53
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    Even if the recipe needed only homogeneous substances in powder or liquid form but needed it in extremely small quantities, it would be very difficult to measure to a sufficient accuracy if you only brew a small dose (especially by children, especially without modern tools). And in case of magical potions, if you are not accurate enough with your measurements, the magic could backfire (which we often see in several occasions). Potion-making is explicitly depicted as very dangerous in the HP universe if you make even the smallest mistake.
    – vsz
    Mar 22 at 5:17
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    @lfurini So it's double the trouble to burn fire to make cauldrons bubble?
    – Machavity
    Mar 22 at 17:25
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Hogwarts shopping list in Harry Potter and the Philosopher's Stone (chapter 5) states that students are required to have "1 Cauldron (pewter, standard size 2)". I'm unaware of different requirements in subsequent years. Since the cauldron is not provided by the school, it stands to reason that the students should only have to buy one and use it for the vast majority of the Potions programme.

As for what "size 2" means, a small Google search seems to indicate that there is a real-world system of measurement where "size 2" stands for 1.25 US gallons = 4.7 litres. I haven't found a reputable source for this (in fact, the guys in the link claim that 1.25 US gal = 18 (fl) oz instead of 160 fl oz). If somebody can find a source for such a standard I'd be grateful.

Be it as it may, the figure of 4.7 litres is consistent with the depiction of the cauldrons used by students in the films, both in Chamber of Secrets and in Half-Blood Prince.

If indeed size 2 indicates a capacity of 4.5-5 litres, then it isn't all that impressive: items of cookware of that size are quite common in houses. It would be impressive for a chemistry laboratory, but I think we can argue that brewing Potions looks a bit more like cooking or old-school alchemy than modern chemistry.

As a final remark, you don't necessarily need to fill the cauldron. You can just, say, use half of it.

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    Just as arbitrary, but here's an independent(?) source that indicates a 4.75-liter pot might be size #2. There must be some obscure ISO standard that covers this :)
    – chepner
    Mar 21 at 20:10
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    @chepner Thank you. I mentioned the "real world standard" as a curiosity, but in hindsight it might just be a red herring: for instance, even if there were one, if it isn't standard terminology in the UK it might not be what JK Rowling meant. My claim on the cauldrons being comparatively small rests chiefly on the depictions in films.
    – Gae. S.
    Mar 22 at 3:14
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    JK wasn't much of one for detailed research, it's not what she was going for, so I think you're right that it's a red herring. I think it was a throwaway line rather than a specific reference. Had she researched, she'd probably have realised that pewter isn't a great material for a cauldron. Mar 22 at 9:22
  • I found a secondary source for the same size 2 pot you found which describes the size as '5 quarts'. The 18oz on the original source seems to be a terrible typo. However, the size 2 without legs is 7 quarts, so the sizes seem to be entirely internal to both that manufacturer and each particular style
    – RisingZan
    Mar 22 at 11:50
  • Ok, thanks everybody. Time for some heavy editing.
    – Gae. S.
    Mar 22 at 16:03
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As alluded to in comments, I suspect that the reduced error from scale may also play into it. Some of the ingredients added to the potions are fairly small amounts, and we see in the books that small differences sometimes seem to have large effects. If you need to add five drops of liquid to a gallon of potion material in the student recipe, scaling that down to, say, 8 oz, means that one accidental extra drop makes much more of a difference than one off from five.

As to the waste, wizards never seem to think too much about that. There are huge tables of food created out of thin air for each of the meals. Snape routinely disappears the contents of the cauldron, an effect noted to return the material into nothingness. Either something in the nature of magic means that there's some degree of recycling going on, at least to preserve a conservation of mass and energy, or the wizards just don't care, possibly because they don't understand the potential implications of their actions.

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    Food cannot be conjured out of thin air (Gamp's Law of Elemental Transfiguration ...), I seem to remember meals at Hogwarts are just teleported from the kitchens to the tables. Apart from this, I agree with the rest of the answer.
    – lfurini
    Mar 21 at 18:34
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    Food can be duplicated out of "thin air", though. You have a sandwich and you can duplicate it multiple times.
    – jo1storm
    Mar 22 at 8:16
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    I think Snape's "disappearing the contents of the cauldron" might be more akin to him Disapparating it somewhere rather than destroying it, similar to how Wizards would Disapparate their own stool in the halls of Hogwarts before they introduced plumbing.
    – Nzall
    Mar 22 at 10:47
  • @jo1storm but then u already had a sandwich. how often will you be able to copy it until it is spoiled? i guess you can create ingredients out of nothing or transmute something into ingredients. i'm not entirely sure why this rule is needed in the world, tho. other universes have evokers who call matter into existence, or use a form of energy-conservation where you can't create something without sacrificing something else. but in the wizarding world, i guess you can't evoke food. maybe to always have an option for poisoning or the like
    – clockw0rk
    Mar 22 at 15:22
  • @clockw0rk yes, but that was my point. The rules don't make sense. I guess the house elves would make a single perfect baked chicken and duplicate it a thousand times then serve it upstairs? Or they would make a single perfect bread loaf and do the same? Except nobody mentions that. And it also begs the question of why didn't wizards solve the world hunger already if that is the case.
    – jo1storm
    Mar 23 at 9:17
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In some few cases the potion will be the result of reducing through boiling for a long time. This is somewhat like maple syrup. You boil it for a long time to concentrate the non-water ingredients, as well as producing a small amount of oxidation to produce the color and taste. You must boil 40 gallons of sap down to 1 gallon of maple syrup.

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Magic in Harry Potter is quite often rather symbolic in how it works. I think that, when it comes to potions, this means that the size of the batch matters; like, if you're adding 7 pinches of dried nettles, maybe it's important that it is specifically 7 pinches as the potion invokes the connection between power and nr 7. This would make potions akin to simple rituals, and neatly explains why you have to brew a specific amount each time.

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As rightly pointed out by others, Rowling seems to have invoked a pre-existing trope, and all we can do is look for post hoc rationalisations.

One additional possibility that comes to mind is that boiling large quantities of potions is the skill that wizards will normally need as adults and hence the skill that they want to be learning as students. It stands to reason that an adult making a potion is most likely either a) working as a professional brewer or b) preparing a large batch of a potion they need regularly - otherwise it's probably more efficient to just pop to the Diagon Ally and buy a vial. In analogy with cooking, preparing a tiny cauldron of a potion is probably similar but not identical skill to preparing a regular size cauldron, and quite possibly a more difficult one in some ways (and possibly easier in ways). Thus, if you're going to be making large portions as an adult, you might as well make large portions as a kid. Admittedly it's a bit more wasteful than preparing small portions, but students wouldn't be preparing expensive potions to begin with, and any quantity of the potions that they make is going to be wasted anyways.

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Some other good answers here, but I can add another reason for this: sometimes, in cooking, you separate out parts of a mixture. For example, I have a cousin who makes her own yogurt, and part of the process involves separating the whey from the remainder. Or, if I'm cooking bacon, I may save the bacon grease for later use.

It reminds me of my high school chemistry class, where we learned some reactions will form a precipitate. It may be the small amount of precipitate is what you need, and you use a larger container in order to skim off just a few ounces of the final desired mixture.

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