Since 1.3.2 you can put lava bucket of lava in cobblestone furnances and you will gain the bucket back. So a stone furnances can smelt 100 units whereas a more modern iron furnances can smelt only 12,5 units.Here you can see it even in the wiki
http://wiki.industrial-craft.net/index.php?title=Electro_Furnace.

Increase energy output of a cubic metre lava
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September 3, 2012 at 1:54 PM -
Closed
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1. Using the lava without consuming buckets was implemented into vanilly minecraft MONTHS ago.
2. Lava+Furnace is considered OP (at least for me) and thus I've implemented IC furnaces to 'nerf lava'