Factorio Forums

I was trying to make a furnace that can receive a air filter as optional ingredient to change the recipe into one that don't make pollution (the other ingredient is a fluid) I successfully made 2 recipes with and without the filter but when I try to insert the filter in the furnace I receive a msg saing the filter cannot be smelted if I make it working as an assembler or remove the fluid ingredient I can have the 2 recipes without problem, but I can't make it with an optional filter, I'm wondering if it can be changed