Here’s how I make my own rhododendron charcoal down at Valley Wood.
I make it in a “retort”, modelled on various versions I came across on YouTube. The basic design is an old oil drum, with a smaller steel drum inside it, and a chimney in the lid. The whole thing is clad in ceramic fibre kiln insulation.
The idea is that the rhododendron wood which is to be turned into charcoal is loaded into the inner drum, which is then placed upside down in the outer drum. The outer drum is then packed with dry kindling. The lid is placed on top and a couple of lit firefighters are dropped into the chimney opening. Once the fire is going, the chimney is placed in the lid and off the whole thing goes.
As the fire in the outer drum gets hotter, it drives off moisture and volatile gases (“wood gas”) from the rhododendron in the inner drum, which exits at the bottom of the inner drum and is fed into the fire, adding fuel. This is a relatively clean and efficient way of doing it, as the volatiles contain many climate forcing gases such as methane and carbon monoxide. Burning them converts them into the less potent carbon dioxide.
The whole thing takes about a day, and is left to cool down overnight. A nice big bag of top-quality charcoal is the result.