Charcoal is often used as an additive to orchid mixes as it helps with air flow in the medium and does not biodegrade quickly. Certain genera (when watered often and/or when grown in high humidity environments) can be grown in pure charcoal. We have had success with Vandaceous orchids, Angraecoids and even Psychopsis in pure charcoal!

Mixing recommendations:

#3 - medium charcoal. Best for medium or medium coarse bark when mixing (power/power plus orchiata)
#4 - small charcoal. Best for small bark mixes (classic orchiata)
#6 - extra fine. Best for mixing with very fine bark (precision orchiata). Also popular as a terrarium substrate.