Monk Life and Natural Dyeing
Here is another picture about the natural dyeing process. They boil the bark chips for a long time. When a drop of dye is dropped in a glass of water, it will stay together and sink. At that point, they will empty the dye into containers and the fill the same chips with water again for another boiling and making more dye.
The dye is added by rolling it on and then drying it. It is better to store the freshly dyed robe in a plastic bag overnight or longer for better setting. Chemical dyes are done much differently by making a dye bath and then boiling the robe in it for about 25 minutes.
The ancient rules (vinaya) do not mention chemical dyes. Pa-Auk does not allow chemical dying by monks even though the cloth is factory colored beforehand. Nauyana, Sri Lanka, allows chemical dying by monks, but recommends a natural dye on top.