The period leading up to first crack is one of the most important phases in the roast. The gas changes you make at this time are critical to prevent a crash and a flick later in the roast.
The goal of this stage of the roast is to prevent the bean temperature rate of rise (RoR) from flattening or even rising in the run-up to first crack. The RoR should continue to gradually decrease. If it plateaus or increases, the roast is much more likely to crash when first crack begins.
To prevent the RoR from flatlining when you are roasting washed coffees, you should decrease the gas setting around 45 seconds before the start of first crack. The gas setting should be low enough to keep the RoR decreasing but not so low that the roast loses momentum and the bean temperature stops increasing before the end of the roast. Lowering the gas setting any later risks a worse potential crash in the RoR.
About 45 seconds after the start of first crack, begin gradually decreasing the gas settings again. A good starting point is to cut the gas by about half once the development time ratio (DTR) reaches 12%, cut it in half again at 14% and in half again at 16%. This ensures that the gas setting by the end of the roast is low enough to prevent a flick, which often begins at 16–17% DTR.
Naturally processed coffees are much less likely to crash, so your approach to gas settings for these coffees doesn’t need to be as rigid. It’s possible to continue lowering the gas settings in the early stages of first crack without causing a crash.