I just accidently clicked in the wrong spot and stumbled upon the following "hack":

1.) Place a stone brick furnace
2.) Fill with coal as fuel
3.) Replace it with a clay brick furnace
Et voilà! Now you can use the coal as fuel in the clay brick furnace. :shock:

But I think it's a bug in factorio ...

Nice mod you developed there. I can see the love for detail.
So I might be wrong, but I think there's something wrong with the brass plates. Probably an early recipe for brass plates is missing. :roll:

When I have a look into the tech tree the only recipe to get brass plates is in the "Leaded ...