Many cooks have known the temptation to crank up the heat after a custard has been in the oven for an hour with no sign of setting, or a cream has been stirred and stirred with no sign of thickening. But there’s good reason to resist. The gentler these dishes are heated, the greater the safety margin between thickening and curdling. Turning up the heat is like accelerating on a wet road while you’re looking for an unfamiliar driveway. You get to your destination faster, but you may not be able to brake in time to avoid skidding past it. Chemical reactions like coagulation develop momentum, and don’t stop the second you turn off the heat. If the thickening proceeds too fast, you may not be able to detect and stop it before it overshoots done and hits curdled. A curdled cream can often be salvaged by straining out the lumps, but an overcooked custard is a loss.