-Truesmiths

"I'm a Truesmith, technically a form of Silver Magic. I don't need a forge to work metal, although it helps, and I can add materials I have to make alloys, or remove them if they are present. I can temper or distemper without heat, sharpen or dull, rust or rustproof. And I can pretty much do whatever I like to magnets. I can't transmute metals from one element to another, or create metal that isn't there, or make it disappear."

Jeremy Lannigan's explanation to Charlie

The Truesmith has an inborn ability to perceive and alter metals. This is inherent, but must be developed with practice. A practiced Truesmith over the age of 30 can do amazing things, within limits. But Truesmithing is work. That's why Jeremy needs not only a lot of pure carbon (commercially available as lampblack), but several day's time to turn an iron safe into steel. Most of the Truesmithing done in the story so far are small, fiddly jobs, not likely to exhaust anyone. Tweaking Spiral's sickle was more work, but he has a long night to do it.

A greater limitation is that there has to be a way to achieve the desired result with the materials available. If you want to make bronze, you must be able to obtain a useful ratio of copper to tin and they must be sufficient to create the amount of bronze required. With time and skill, copper could be extracted from other alloys, and tin stripped from old fashioned "tin" (really tinned steel) cans, and re alloyed to make bronze, but you must have the elements of bronze to make bronze.

Also, Truesmithing is no use as a design tool. If you don't understand differential tempering, you cannot make a first class sword blade. If you don't know how a lock works you cannot build a lock. Jeremy can whip up ingenious devices out of odds and ends by Truesmithing, but they work because he is ingenious and experienced with small mechanisms, not by magic.

Which brings up the next point: Truesmithed items are not magical. If you want a fine sword that will look good, balance well, hold an edge, and not break under the stress of combat, by all means find a Truesmith who knows about swords. But if you want Excalibur, talk to Merlin and the Lady of the Lake, Jeremy Lannigan can't help you. Final point: while Truesmithed items are not magical, and therefore don't show a trace on those damned Magehunter's lenses, active Truesmithing is, and shows as a clear, bright, silver glow. Jeremy is reluctant to work in exposed places for this reason.