Each herb, seed, bark, root or flower has a certain temperature where the plant begins to release its essential oil. Knowing what that temperature is and the best method of distillation for the plant material you are trying to distill is essential. This takes a bit of research on your part.
When bringing a still up to temperature, it is best to go slowly. It is hard to cool a still so by carefully watching the temperature and beginning to turn the heat setting back as you go, you will have more control over the end result.
It is best to use fresh cut herbs that have been harvested early in the morning as soon as the dew dries after a several day dry spell has occurred. This insures the maximum amount of oil. If their has been a drought that year you may find some plants such as lavender have more oil than on a year where there has been an abundance of water.
This chart will give you an idea at what point some herbs begin to release their oils or volatilize. Never heat your herbs past this point and try to maintain this temperature throughout the distillation process.
Marjoram and Oregano – 163 F (72 C)
Mints – 200 F (93 C)
Sage – 150 F (65 C)
Savory – 176 F (80 C)
Cinnamon Bark – 170 F to 212 F (76 C to 100 C)
Wintergreen – 218 F (103 C)