Starting at 2nd level when you select this school, 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 gemstone), iron, copper, or silver, transforming it into a different one of those materials. 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.
I feel the usage limitations on a transmuter's Minor Alchemy feature make it much less able to be used in typical play, as compared to a conjuration wizard's Minor Conjuration, an evocation wizard's Sculpt Spells, or even an illusion wizard's Improved Minor Illusion (which is still pretty bad, but better than a 10-minute setup time).
Am I mistaken, or is there an imbalance here?