I just started using some of the black walnut ink I made last November. I have four jars from that batch. The first one I opened was oddly thick. I could still use a paint brush with it, but it was too viscous to use with a dip pen.
The second jar I opened was normal, and worked wonderfully with a dip pen.
I wonder what happened with the first jar? (I haven't checked the other jars yet.) It was pretty weird; not crusty at the top like it had dried out, just uniformly thickened. That jar did have a small film of mold growing in it, but I've had mold in jars of ink before and not had it affect the ink flow. I wonder if the gum Arabic I added did something strange? But if so, why didn't it do that to the second jar?
I might be able to re-thin it with water, and even if I can't, it's not a great loss, and like I said, I can still use it with a brush.
I had the idea of obtaining a large number of small bottles and making ink to sell, but if I do that, I guess I better establish some quality controls!