An Iron Ingot is the metal behind every second-tier tool, weapon and piece of armor, the iron fittings of the Town and a good part of the later buildings. You smelt it in the Furnace II: 3 Iron Ore and 30 Fuel make 3 ingots (4 with the Fireman perk), once you have the Iron Forge technology (35 red) and Smithing mastery 5. The smaller Furnace I cannot do it.
No trader sells Iron Ingots, so all of them are smelted by you or your zombies.
The Smithy burns Coal instead of Fuel. At a Smithy I one Iron Ore and one Coal make one ingot, the same ore per ingot as the furnace, and the recipe needs only Smithing mastery 2. It comes with the Smithy: Hammer technology.
Jack’s first order is a crate of them.Supply: Iron takes 6 ingots at the woodworking workbench, or 8 on the assembly line.
Studying one costs 2 Science and gives 10 red and 20 blue.