20 minutes at 20°C/68.0°F
Caffenol as per Reinhold's recipe. Most critical element is 'washing soda': You need monohydrate for it to work, not the stuff they sell as washing soda in the UK, normally decahydrate (full of water). I had to make my own anhydrous soda (baking soda at gas mark 6 for 1 hr - voila!).