Kopi joss is sweet black coffee with a glowing ember of charcoal dropped into the glass. It comes from Yogyakarta in central Java, where street stalls have served it since the 1960s.
The name is onomatopoeic — the joss is the hiss when the charcoal hits the coffee.
What it does
The charcoal is glowing hardwood, briefly quenched. It fizzes violently, then sits at the bottom.
The claimed effect is that it reduces acidity — which is chemically plausible, since ash is alkaline and would neutralise some acid. Vendors also claim it settles the stomach, which is folklore.
What it definitely does is add a smoky note and a distinct texture from the fine ash.
Is it safe
Charcoal from hardwood is essentially activated carbon, which is consumed medicinally. The practical concerns are what the charcoal was made from and whether anything was used to light it.
Street vendors use plain hardwood embers from a brazier. Do not attempt this with barbecue briquettes, which contain binders and accelerants.
Making one
Kopi tubruk — fine coffee and sugar in a glass, boiling water poured on — then a small ember of plain hardwood charcoal from a fire, tonged in.
The honest note
It is a spectacle as much as a drink, and it is a genuine local tradition rather than a tourist invention.