There is an interesting and frustrating problem with at least level 2 smelters. I'm unsure if this affects other artisan thralls.
Here is the expected scenario: I put stone in my furnace, and I put consolidate in my furnace. Say it's enough stone for 100 bricks and enough consolidate for 50 hardened bricks. For argument sake, it takes 10 seconds to make a brick and 20 seconds to make a hardened brick. Assume there is plenty of coal. Okay, with no smelter, the bricks should take, for this example's sake, 1000 seconds, and then the hardened bricks should take another 1000 seconds. You end up with 50 of each type of brick in 2000 seconds, or a little over 33 minutes.
Enter the smelter level 2. Now, I don't know exactly how much faster she speeds things up, but for this example, let's pretend a level 2 smelter doubles the cook speed, or halves the cook time, if you prefer. So, we hook her up to our furnace, and she cuts the time it takes for the whole thing to 1000 seconds. Well, no. She gets lazy after the bricks are done, and takes a solitaire break. The bricks took only 500 seconds, but the hardened bricks still took 1000 seconds. If I catch her slacking off, I can reassign her back to the furnace – remove her from the furnace, then immediately place her back at the furnace – to fix it.
What happened? Well, there appears to be some issue when a furnace switches production. Once the material to craft changes, the furnace ignores the smelter. If you replace the smelter, you will actually see the progress of the current brick change at a noticeable rate. This seems to happen with any change in production. If I have started my hardened bricks, then load the furnace with stone, the furnace will finish the current hardened brick, then move on to making regular bricks. Regular bricks seem to have a higher priority, so this effect should always happen.
As soon as the furnace shifts gears, the smelter stops working. Replace her, and the bricks go back to speed mode. Then, once the bricks have finished, and the furnace goes back to hardened bricks, replace her again for more speed mode. This works on at least iron, steel, stone, and hardened stone. I assume it works for glass, and whatever else you can cook in the furnace.
As an aside, if you have a dedicated furnace that always makes the same material, I don't think you need to worry about this issue. It only happens when the furnace changes materials after a smelter is already attached to the furnace.