I grew some good lettuce this year, cos (white seed) and crutterbunch (dark seed). I like the crutterbunch better and it is sometimes tough to find the seed in the store so I made sure to cut all the cos before it flowered and let the other go to seed. No other lettuces local to me within a mile at least. As individual flower heads ripened and showed fluff I picked them carefully and saved some good black/brown seed. It was easy to seive and blow off chaff and from these I saved pretty much 100% good dark seed so I have lots for next year.
I got fed up with this slow process, decided to upscale the production and waited for the flowering head to mature much more and then just cut the whole head with lots of stalk and brought it in the house to dry. After much threshing and drying the winnowing process produced the mix as in the picture: As you can see there is a lot of immature seed mixed in with the fully mature dark seed and a bit of chaff. Quite a different result from the painstaking careful selection of fully ripe.
I'm now faced with how to separate the pepper from the salt. I tried setting up a blower but it did not work, salt fell in the same place as the pepper. Experimenting with water separation (heavy seed sinks, immature seed floats) only works partly and was very messy. I could have waited longer for the heads to mature fully but was trying to be careful not to have lettuce all over the garden next year. For my own use I could also just sow the white along with the dark in confidence that the white will not germinate. But how to separate the white from the dark?