I'd like to make something take a long while to smelt, specifically the full duration of a block of coal, and I'd preferably have it do so in the vanilla furnace. Anyone know how it can be done? I haven't seen a variation of .addSmelting() that accounts for this.

The cooking time per item in the vanilla furnace is hardcoded to 200 ticks, you cannot change it.

 

Bah, really? Well... maybe I'll make a custom furnace for it then. I was hoping to avoid that. Thanks anyways.

I wonder if you could work metadata into the ore block (and furnace stacks) so that several smeltings would be required (increasing the metadata each time). Only raw ore (meta zero) would spawn in the world, and melting it would produce ore with meta=1. Smelting ore with meta=9 would produce your finished output material. You'd need 10 smelting recipes to convert each level of stack into its output.

 

An automated factory would need to cascade 10 furnaces with 10 hoppers or else work out some way to recycle through one. The puzzle actually sounds like something fun to solve...

I'll note that the only reason I did that was because the ore was easy to find and crap late game.  The player shouldn't want to set up the industry to smelt the stuff into usable metal.  It was only there to provide a metal resource in small quantities.