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PHB Page 119, Wizard, School of Transmutation, Minor Alchemy:

For each 10 minutes you spend performing the procedure, you can transform up to 1 cubic foot of material. After 1 hour, or until you lose your concentration (as if you were concentrating on a spell), the material reverts to its original substance.

There don't seem to be time limits on how long one can concentrate, as time is not listed in the things that could break concentration on page 203 of the PHB.

Given this, could a wizard of the school of transmutation spend 12 hours converting 72 cubic feet of material?

Could a wizard spend 30 hours converting 180 cubic feet of material?

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up vote 6 down vote accepted

It seems based on a close reading, that there is no volume limit on how much material you can transmute. You are limited by only three factors:

  1. It's a single object. The larger the object the more time you have to spend, so you can in theory spend 30 hours converting a small outhouse of 180 cubic feet.
  2. The object is all made out of only a single material from the allowed list. If there is any metal in that outhouse, you can't transmute it.
  3. You are able to perform the procedure for that long. There might be reason to believe that you can't sustain a single act for more than 24 hours. But perhaps you can.

As long as you are dealing with a single object, made of a single material and you can keep up the procedure, there is no limit to the volume of the object you are transmuting.

[...] you can temporarily alter the physical properties of one nonmagical object, changing it from one substance into another. You perform a special alchemical procedure on one object composed entirely of wood, stone (but not a gem stone), iron, copper, or silver, transforming it into a different one of those materials. [...]
(Page 119 of the Player's Handbook)

Basically you have two possible readings of this description. Either:

  1. You perform a procedure. During the performance of this procedure, every 10 minutes, another 1 cubic foot of material gets transmuted into the new substance; 60 minutes after that happens, it turns back to its original form.
  2. You perform a procedure. The procedure takes 10 minutes per cubic foot of material. When you finish the procedure, which can take anywhere from 10 minutes to several hours, the entire object transmutes into the new substance.

I think that if the first reading was intended, then the description would include something along the lines of "you can transmute the material at a rate of 10 minutes per cubic foot." I also don't think the rules would be limited to a single object then, but would rather encompass 1 cubic foot of material every 10 minutes, be that 1 object such as a box, or 100 copper coins.

Further, the idea that you can have, say, a "wood" box where half of the box is wood and half of the box is stone, doesn't make much sense. A strict reading of the rules would also make this usage impossible, since once you have transmuted the first cubic foot of material, then the box would be made of two materials and you would no longer be able to perform the procedure!

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On further consideration and re-reading, this seems right. The reaction that "But it could be any size then!" seems to be the inspiration for opposing this reading, but! if one considers that the result of those painstaking hours of work will only last for an hour or until you lost your concentration, that's not really abusable unless you get really creative. (And "really creative" is actually the encouraged mode of play in 5e.) – SevenSidedDie Aug 21 '14 at 16:51

The limit is built into the text you quoted:

After 1 hour, or until you lose your concentration

The maximum limit for the duration of the transformation is 1 hour. At 1 hour 10 min from the time you started, one hour after the conversion, the first cubic foot of converted material reverts back to the original material. The concentration check is if something happens that could break your concentration before that hour is up.

Example: You don't have the money to buy something so you take a cubic foot block of copper and convert it to silver to make your purchase. Fifteen minutes later someone comes in the shop while you are making the purchase and starts yelling you are a fraud, you have to do a concentration check to see if you can maintain your conversion, if it fails the block reverts to copper and you are caught.

The maximum volume you can have converted at any one point is 6 cubic feet, since the time it takes to get the seventh cubic foot exceeds the duration of the first cubic foot. "As if you were concentrating on a spell" as with the spell the duration does not exceed the defined limit just because you can concentrate longer.

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I’m not entirely sure whether the duration is supposed to be whichever comes first or last (although most spells end early if you stop concentrating). And I’m not sure whether the hour starts at the beginning of the spell or the completion of the transmutation. But I do think your interpretation is a reasonable one. – Bradd Szonye Aug 20 '14 at 22:59
I'm really surprised here. Can you explain why half the object reverts to its previous material if it's a 12 cubic meters and how this works with the rest of the rule? – GMNoob Aug 21 '14 at 3:51
You can not transmute a block of copper into a block of gold using this spell/ability. – GMNoob Aug 21 '14 at 6:56
@GMNoob Good catch, fixed. – SevenSidedDie Aug 21 '14 at 16:53
Unless the disgruntled customer punches you, you shouldn't need to make a concentration check, as "normal activity, such as moving and attacking, doesn't interfere with concentration". The PH says that the DM may call for a concentration check due to environmental conditions, and gives the example of a wave crashing over you in a storm. By contrast, being yelled at by an irate NPC is all in a normal day's work for an adventurer. – mattdm Aug 23 '14 at 10:45

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