Hi, Richard. Air-tight can cause problems with happy, active starter; our crocks are designed to let the carbon dioxide produced get out. The reason we call for a scant cup is to better match the weight of the flour and water (weight is always a better way to measure, but not everyone has a scale). To get a scant cup, sprinkle the flour into a dry measure and when you sweep off the top you can leave a bit of an indent so it's not quite filled to the top when you're done. Susan

• Reply