Factorio Forums

Haven't tested on latest versions but didn't see anything about it in the patch notes.

As per viewtopic.php?f=8&t=25615&start=10#p163441, using production modules in a furnace to build both copper and iron can result in free resources.
Example from link:

4 iron ore + 1 copper ore = 4 iron plates + 2 copper plates. x2 copper!
20 iron plates + 1 copper ore = 4 steel + 5.2 copper plates. x5 copper!

Not my find but posting to bug reports since it seems like an unintended exploit.

I don't understand. Giving you extra resources is the entire point of productivity modules – when the bar fills, you get one extra. If you smelt 4 iron ore, then 1 copper ore, it's just enough to fill the productivity bar, so you get your bonus.

Not a bug, unless I'm misunderstanding. In which case, please explain.

Sorry I wasn't clear.

I think the productivity bonus should reset on recipe change. It makes no sense to get a free copper from smelting mostly iron.

Looks like the productivity bonus is going into into 1 of the ores. I dont see this as a worthwhile problem if only iron or copper are concerned because they have the same value kinda. The bonus you received for copper is the same you would have received on iron in total amounts.

But perhaps this may become a problem once assemblers can automatically change recipe. Like start an expensive product, then cancel it early and continue with cheap product. The bonus will be another expensive product. Im not sure if this is the way it happens. Maybe i misunderstood.
But just saying...

Fixed for 0.15.13