Train your Armorer and Furnace Operators, and duplicate metal bars at the same time!

The steps are as follows:

1. Forge leggings at a Metalsmith's Forge
   
2. Set those leggings to be melted down (click the melt button on the item's menu)
   
3. Set a work order to "melt a metal object"
   
4. Repeat steps 1-3 one more time.

If you have other items already set to be melted, they may be melted down during step 3 instead of the leggings, so keep that in mind. You cannot specify which items to melt down when ordering.


It's that simple.

Leggings are not the only item that can be exploited in this way, however, they are one of the simplest, and they can help you train your dwarves in a valuable skill (Armorsmithing).

Melting objects in Dwarf Fortress is currently bugged.[dwarffortresswiki.org] Some items may melt back 1 bar exactly. Some items may melt back more than 1 bar (but less than 2). Some items may even melt back less than 1 bar (but more than 0).

Leggings return 1.5 bars every time they are melted down. (Source[www.dwarffortresswiki.org])

Therefore, you must melt in multiples of 2 for the extra bar to appear. Fractional bars are implicitly kept at the smelter until they make a full bar. (Source[dwarffortresswiki.org])

You can use this to your advantage. Just beware that it costs 1 coal to forge a bar into leggings at a Metalsmith's Forge, and 1 coal to smelt the object down at a Smelter. However, that's cheaper than making steel bars the normal way. Also, if you use a magma smelter and forge, then this exploit costs nothing.

For more useful information, or to see what other items can be used for this exploit, please refer to the Dwarf Fortress Wiki's Article[dwarffortresswiki.org] on melting items.