Mineral-based and/or Inert Substrate Only?

Remember, lithophytes don't grow just on the surface of rocks, though some roots certainly cling to them... roots tend to grow into crevices that have organic matter - leaf detritus, etc. They also get moisture in those cracks. So they don't grow in a purely inorganic environment.

 

There is a big difference between natural rock outcroppings and LECA. Rocks tend to be essentially non porous, which is why lithophytic plants sink their roots into the cracks and fissures where organic matter and moisture accumulate. The relative lack of porosity and accompanying insolubility means the rocks will dry fast, will not accumulate residues, and probably don’t do much in the way of providing nutrient ions.

LECA is porous in its own right, and while it might be chemically inert, it’s ability to absorb and hold solutions make it more of an “active participant” in the plant’s environment than are rocks in nature. Unfortunately, that porosity also leads to the gradual buildup of residues, especially if allowed to undergo significant drying between waterings, so it will become toxic over time, and require replacement.

 

I'm attempting growing on terra-cotta pots. They are, unlike rocks, very porous such that plants can be hydrated trough the surface.
Ceramics can hold and spread moisture, which is great, but it tends to evaporate water like crazy.

I found a German selling pots speccially designed for orchid mounting. I did not find many experiences about this though. (Buy Orchds online now - Roellke Orchideen)

They off-course have to be porous, so unglazed. Non toxic. Nutrients given inside the pots appear to tend to accumulate on the outside, which resulted in people recommending spraying nutrients on, and watering without additions.

My university gives access to a ceramics studio, so I'm making my own pots. First versions are being tested with a base-layer of various mosses. Moss is not attached yet.

Pots drain ~1-3cm of water per day. They feel damp and cold, but not really wet. Thoughts on this are welcome.

 

Generally when mounting, especially Cattleyas and other dominant epiphytes that need to dry out between waterings, I don't put any moss or other organic between the mount and the roots. (If I need extra moisture, I may put a bit OVER the roots, but most often don't). Whether cork, wood, or terracotta, the same principle holds. If there is moss between the roots and the mount, they tend to go into the moss rather than onto the mount. The critical factor in any sort of mount is to do it when new root growth is just starting. Those new roots will grab a substrate. Older roots hydrate the plant but don't attach to anything. But to keep the pot (mount) moist, you could put sphagnum or similar material inside the pot to hold water longer, while mounting the plant to the outside.

 

I followed the approach on the mentioned webseller from germany.

2. Once the clay is wet, please use moss spurs (can be found in all kind of wet places outside)
3. If the moss is established you can start mounting your orchids.

The water seems to hold pretty well in the pots, so I thought that the moss was more neccesary to create a more organic structure and mini-biome.

The pots are damp, but not wet, even though they get a lot of ventilation in a dry room. Your mentions of not-drying are indeed potentials for trouble. I'm hoping that the slight dampness with high ventilation will be enough to avoid rot.

 

I made a dedicated thread for growth on ceramic jugs, for the ones that are interested.
Cultivation on exterior of porous ceramics