While scanning, you may notice a lack of contrast in pencil sketch scans, even in color mode. To solve that, you can use the mogrify, an [ImageMagick] command that allows batch processing. Note that this will overwrite the existing files:
mogrify -colorspace Gray -gamma 0.65 -white-threshold 85% -level 10%,90% *.pngIf you want to be safe and not overwrite your scans, you can specify an output folder using the -path argument:
mogrify -path output -colorspace Gray -gamma 0.65 -white-threshold 85% -level 10%,90% *.png