Repeat the procedure at one-minute heating intervals until the weight change is less than one gram. The total heating time can be the basis for drying samples of similar moisture. A few simple steps can help decrease the inaccuracies associated with microwave drying. Use shallow sample containers that can be intermittently stirred or shaken as well as rotated. Beware of small flakes of metal in the samples.
Metal causes sparks in the microwave oven that can char parts of the sample.
In any case, discard samples that look or smell like they have been charred. Because microwave ovens heat unevenly, several short repeated heating steps are less likely to burn samples than a single uninterrupted heating step. When a high degree of accuracy is necessary, there are good practices that apply to all gravimetric methods. The weight of sample containers should be accurately determined before filling. When using paper containers, dry the container first to drive off its moisture.