Sourdough starter: what finally worked

Third attempt. The first two died quietly within a week, the third one is now six weeks old and doubles reliably. The difference was mostly boring.

What changed

I switched from all-purpose flour to about 30% whole rye, started weighing everything instead of using cups, and moved the jar off the windowsill to a shelf that stays around 24°C.

Feeding is 1:2:2 by weight (starter : flour : water) once a day. When the kitchen is warmer, 1:3:3 keeps it from peaking too early.

Signs it is ready

It doubles within 4–6 hours, smells sour rather than like nail polish, and a spoonful floats in water. I ignored the float test for a while; it turned out to be a decent check.

First loaf was flat but tasty. Second one actually had an ear.