I read the same thing but the moss between the layers quickly seemed to develop mold (or some equivalent). Overwatering it I can open and dry out some... but layers beneath, how do you get rid of that infection? Is the moss a must:?

I used to make terrariums at a florist and used to put a layer of pea gravel between the charcoal and soil. I didn't use moss because of water retention and mold. You can use sand as a separation layer too for a variation in design.

Charcoal layer is unnecessary and useless in any real sense. A drainage layer is not generally necessary either unless you are including a water feature in the terr. Just make sure whatever media you go with is rather light and "airy".

Charcoal is commonly used in terrariums to keep the soil fresh, reducing stagnation from bacterial growth. The drainage layer is to keep water away from the roots and goes in first, generally gravel, then charcoal, then soil. It is old school for some, like me, who created closed top, self sustaining environments. And the layers can look more appealing than plopping a blob of soil in a glass with some plants. I have made some interesting projects, and learned from many mistakes (and broken super sized brandy snifters). It boils to down to personal preference I guess. A great source for terrarium plants is Glasshouse Works at www.glasshouseworks.com, and chopsticks are great tools for those tough to plant containers. I started with baby tears in a bottle. And was rewarded with baby slugs...

All a drainage layer does -- whether by using layers of gravel in a terr or putting broken shards of pottery/stones in a flower pot is create a perched water table. It is not beneficial to the plants. The belief that it is, is simply one more "urban" legend. The plants are far better off with an appropriate mix and water regime that allows for soil to be lightly moist but well aerated. The same principle applies whether growing in a pot or in a terr. For far more detailed explanation of perched water tables, go to the Houseplants Forum and type "perched water table" in the Search function. To get started, I've added a link at the bottom of this post. (Quite a bit of good info in the entire post, but it is a lengthy read.)

While activated charcoal is used with the intent to "keep the soil fresh, reducing stagnation from bacterial growth.", its effectiveness is minimal and very short-lived. The usefulness of such is yet another legend. Having the charcoal mixed into the soil -- like any large particles whether they be gravel, perlite, or what have you -- does aid in keep the soil "light" or from compacting. There have been studies done to investigate whether charcoal could provide useful sources of carbon, but the results, as I recall, indicated that its usefulness in that arena was also minimal.

The only real use for such layering is visual appeal which is fine as far as that goes but is by no means necessary nor terribly functional in any other sense.

Mollusks like snails or slugs can be a bane in any terrarium. Had an infestation of bush snails in my 90gal terr once. Tried numerous methods of battling the slimeballs but finally had to resort to tearing the terr apart, sterilizing all media, removing as much media as possible from all plants (and in some cases removing some of the plants or completely derooting them), and setting it up anew. Several years later, all remains well.

Here is a link that might be useful: perched water table

I'm a bonafide layerer then. It's worked perfectly for me through my years of trial and error and I'm sticking with it, and the textures layering lends to my creations.

I totally agree with Paul, though one minor correction: drainage layers prevent perched water tables. Perched just means it's sitting on the pot bottom, and drainage layers act to remove soil moisture.

The problem with creating a terrarium that requires drainage is they are sealed by nature, so you're drainage is limited by the volume in your rock voids. Once it fills up, to keep the soil dry you have to empty it... which is a hassle. That's why Terrariums work better for say ferns than say succulents... it's hard to keep soil dry if there aren't holes in the bottom of the container, there's some evaporation, but it just becomes soil moisture.

When you create a drainage layer, putting charcoal (activated carbon) between rocks and soil just prevents soil from entering the voids of the rock, and helps with smell. It will not filter microorganisms, bacteria, or most contaminants. It will filter volatiles (from the soil vapor), chlorine from tap water, and some heavy metals... until the carbon becomes "spent". By removing chlorine, it actually allows more bacteria to grow in the bottom of the container. A biofilm is also created on the charcoal, where bacteria will grow. It's probably less conducive to biofilm growth than pebbles or dried sphagnum or peat moss though. Bacteria is not necessarily a bad thing though. I've found that dried moss works to keep soil out of drainage, but gets a little nasty over time.

Mixing charcoal into your soil will increase your void space and prevent compaction, but I don't think it's really beneficial. You're creating layers just to create a filter. A water treatment filter is designed with the heaviest (biggest) stuff on bottom and smallest (lightest) stuff on top. In water treatment, this is just the natural tendency of materials to layer, but it also helps keep unwanted stuff at the top while maintaining good drainage. Your yard might be the same way: topsoil, sand, gravelly sand, confining layer. If you want your soil dry, try to design to keep your groundwater table well below your root line.