The mistake was rinsing them. Some seeds produce goo when damp. Basil does it too. You should avoid getting seeds wet once harvested. As long as they haven't got too soaked try drying them out over a gentle heat source, like the top of a fridge, and then just rub them with your fingers to separate. In future clean seed by sieving, winnowing, blowing or whatever but never, ever, rinse them.

If it's any consolation Lemon Balm self seeds like crazy and even if you don't sow any you'll almost certainly get volunteers.

D'OH! Can you tell I'm new to this? I've read so many articles advising to use the float test to see if seeds are viable so I tried it...with the wrong seeds! I was hoping to use this for a swap, wanted to make sure I was sending out good seeds without too much chaff. I didn't soak them for long, just a couple of minutes. I laid them out to dry then separated them. Hoping I didn't destroy them!

I have volunteers in spades! I planted this lemon balm about 4 years ago in a corner of the garden and was happy to let it take over that spot...then it started creeping into my other beds. I pulled up the invaders and cut the rest back. Since it had already gone to seed I figured I would make the best of them (and hopefully keep them from spreading anywhere else I didn't want them).

Thanks for your help! Live and learn, right? :)