I created a two small terrariums a few weeks ago, one about a gallon and one about a half-gallon size in beautiful vintage egg-shaped glass globes. My intention was to create some miniature sculptures to go inside of them for an upcoming art exhibition. I laid down sterile gravel, then charcoal, and then a couple inches of potting soil, and planted baby's tears and golden moss. Then I lightly misted the plants and closed the lids. I assume they were fairly airtight.

Here's where the mistake came in. I went away for four days --- and when I returned, the plants were DEAD. I mean really dead, slightly translucent and brown. I think there may have been a mold that killed them, as there were some very fine filaments draped over the dead growth.

Does anyone know where I went wrong? I was careful not to overwater, and as I understood it, it's OK to have those sorts of plants in an airtight container. Should I have left the tops off? How can I prevent this from happening again?

I'm going to replant them, so any advice would be VERY appreciated. Is it ok for me to just clean out the top layer and use the same soil?

Thanks!