Hi Grace, when a starter develops a hard crust it's usually because it's being exposed to too much airflow. The reason we say to "cover loosely" isn't because the starter needs airflow in order to ferment properly (it doesn't), but because fermentation gases can build up in a tightly lidded container and cause the lid to pop off. To protect your starter from drying out it's fine to cover the container tightly with plastic wrap, which will flex. You can even use a lid, as long as you open the container now and then to vent the gases. 

Barb

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