Grind size
It matters more than almost anything else you control. Grind size sets how fast water can take flavour out of coffee, so it decides whether a given brewing time gives you a good cup or a bad one.
Every brewing method has a rate at which water passes through coffee. The grind has to match it.
- Water moves through coarse grounds quickly, so the method needs to hold them in contact for longer.
- Water moves through fine grounds slowly, so contact time is naturally longer even when the water is only passing through once.
The South Indian filter
The filter works by letting hot water pass slowly, under gravity alone, through a bed of fine coffee held between two perforated plates. Because the water is not being pushed and is not being stirred, the grind does the work of slowing it down. Too coarse and the water rushes through and the decoction is weak. Too fine and it can choke and barely drip at all.
This is why filter coffee grind is not the same as the grind you would use for a French press or an espresso machine, and why coffee is normally ground for the method it is going to be used in.
Curious what this tastes like in the cup? See the coffees.