minecraft-feed-the-beastminecraft-java-editionminecraft-thaumcraft
From the Thaumonomicon: "By capturing a Blaze and harnessing its energy within a furnace crafted from obsidian and netherbrick you have created a highly efficient and fuel free furnace. So efficient in fact, that occasionally it will produce bonus material in the form of nuggets or other items[…]"
What are these bonus items and how high is the chance of producing them?
No, furnaces do not start fires
The fire effect from one is purely visual.
An electric furnace is far superior to an iron furnace, due to its ability to use energy efficiently.
Imagine you need 10 energy to complete a task. Coal uses 20 energy in this example because it will be used up entirely. However say you use a battery it will take 10/20 of its energy and you will be able to use it again producing twice as much for the same amount of energy.
This means that instead of wasting energy regardless of what you are smelting you are using only the required amount.
Recipe
2 Redstone Dust, 1 Electronic Circuit, 1 Iron Furnace.
The following sources both show how to use it.
Sources
http://technicpack.wikia.com/wiki/Electric_Furnace
http://wiki.industrial-craft.net/index.php?title=Electro_Furnace
Best Answer
When smelting Ores (not dusts or processed materials) You will receive a nugget for that ore. This also works on meat (cooking chicken will give chicken nuggets)
I'm not sure on the chance but I believe its around 25%, but when you add all of the arcane bellows to it that you can(one on each hole on the side), it not only becomes faster and uses less aura, it also increases the reward chance to something like 90%.
One thing I should mention is although the description says its "free" it's far from it (in Thaumcraft 3 anyway. in Thaumcraft 4 it is free, as explained in the comment). it uses aura from the atmosphere in order to smelt, so if you are using it as a primary furnace and smelting a lot. I suggest getting some aura generation items around otherwise that area will become a painful area.