Smelting is the process of separating iron from slag; slag being the waste product left behind after extracting the iron. The process requires multiple steps. First you need to collect Iron boulders that need to be crushed to produce crushed Iron. Crushed Iron needs to be roasted to produce Roasted Iron. Roasted Iron needs to be hammered into Roasted Iron Powder. This is combined with charcoal to produce Smelting Mix. Smelting mix is what you put into your smelter to produce Iron blooms and slag.
Smelting requires you to heat the iron mix to a temperature of at least 1350 C. This requires a large charcoal block which produces heat of 1800 C. When the Smelting mix reaches this temperature, it turns to Iron & Slag Blocks. This is the first stage of smelting. When heated long enough, these blocks will produce an iron bloom. During the smelting process, you'll see hot slag drop from the smelting blocks. You need a drainage hole for it to fall into. Most of this slag magma will evaporate leaving nothing behind. Some of it will form slag blocks however which can clog the drainage hole stopping the smelting process. You need to make sure to clear out the slag blocks to avoid this problem.
More drainage holes reduces how many slag blocks form. Having a large distance for the molten slag to drop produces some nice effects.