The second batch turned out very much like the first but looking at the nail i tossed in with the cones now, I suspect it didn't have much iron in it. Just enough to rust a bit. Not enough to extract the iron. For the future, a splash of vinegar at the start would have helped.
Speaking about chemical reaction, it's still interacting with the baking soda, so I will need to let off the gass each day until it's finished.
Anyway, it writes okay with a glass pen. Not great, but that could be a reflection of my cheap glass pen with the chipped tip.