Crust on starter
I've been using a starter off and on for a few years now and just this week, twice, I've seen it develop a hard crust on the top as opposed to a watery liquid that one might normally see.
The only thing I can pinpoint that was different was that one time it was in a rather small glass container, in the fridge where normally it would be one much larger (8 oz vs about 16). I always cover with just a coffee filter and rubber band. It had been about a week when I checked it and found the entire top layer was a hard crust.
The second time was when I was actively reviving it to use and was after a 3rd feeding. I had it in the oven with the light on overnight since it's getting chilly now. It was just lightly covered with a cloth to keep out any dust. The top had a hard look and feel to it though it had obviously grown well since the previous night's feeding and the part just below it was fine. I just went ahead and used it as is since Saturday is the only day I have enough time to do it but the resulting loaf didn't rise very well at all.
Why would it be developing the crust and is it unusable?