There are only three distillation methods for oud oil: steam, hydro and CO2 extraction. The latter is seldom used save in large-scale productions. The results are far from impressive, with a pasty, sticky, solid at room temperature wax as the end product. The scent is impaired by the extraction of non-resin particles along with the agarwood essence.
Steam distillation is widely used in Indonesia. Normally the oil is subjected to higher temperatures above 250 degrees fahrenheit. Some of the distillers dislike for steam distillation when it comes to oud. Yet some oud , were steam distilled.
Then we have classic water distillation. Simple chemistry: you boil the wood and the resin rises to the top; from there you funnel it into a glass vessel where it gathers over the course of several days, floating atop the water. This is the oldest, most widely used method in Southeast Asia and Assam.
Distillation can get real high tech, with different material tubes for different steps of the process. You can have, for example, a stainless steel boiler with copper tubes that the oil travels through; or a fully stainless unit; or a fully copper one; or a copper still with stainless tubes; or different material tubes for different parts of the process. The possibilities are endless.
With steam distillation, you get agarwood oil that was heated up to a certain temperature and then separated from the condensed steam, with the resultant oil potentially impaired by the high temperature. In hydro distillation, the raw materials are in close contact with water for a period of several days or even months. The water has an almost magical effect on the oil, changing its character dramatically depending on how long it stays immersed, the type of water it is boiled in (spring, rain, ground water), the chemical breakdown of the water itself, salt and mineral content, etc.
Whether you get a fecal, a fruity or a woody, a dark or a light, a leathery or a green smelling oud oil depends a great deal on the water you use to cook the raw materials!
–Article from Ensar–