My smelter "button" currently reads:
They appear to cost 200 minerals. The +/- buttons enable themselves when I have >200 minerals. When I hover over the "+" button, a "+all" appears below it. If I click the "+" button, nothing happens, as far as I can tell. (It continues to read "Smelter (0|0)".) The "+all" looked like maybe it needed a kitten, so I free'd up a kitten, but to no avail.
I don't know how to produce iron, and I don't think I've successfully produced iron ever. (This is holding me back from some of the workshop upgrades, such as iron hoes & iron axes.)
I've researched up to Philosophy, Machinery and Steel. (i.e., those techs are not yet researched, but everything before them is.)
How do you build a smelter?