In Malaysia our main potting medium is charcoal.Here our rainfall is around 100ins a year and it rains almost every late afternoon. We grow our orchids outdoors 24/7 in lathe houses for shade-loving orchids, or right out in the open with no shade for semi-terete and terete vandas,arachnis, aranda, mokaras. The high rainfall and temperature cause most conventional potting medium to rot and break down very quickly but charcoal remains intact for quite a long time. Moreover it remains 'sweet' over a much longer period of time. Sometimes we add broken bricks for better retention of water and also, if placed on top, to prevent excessive absorption of the sun's heat which might result in the burning of new emerging root-tips.