let me answer because i know that too.
good fermentation no strong smell, only little bit good tea smell.
bad fermentation not good smell.
during fermentation change color. fermentation enough darker color.
old sack cloth is good material to cover keeping water and temperature, if no cloth fermentation not good. old sack cloth good to let tea leaves breathe and keeping warm. no use to buy new one, old one is better because no bad new smell and softer to cover tea leaves.
