Steam distillation can take three forms. The first uses just water where the plant material is placed in boiling water. The rising steam and oils are then captured and then separated. Clove oil is distilled this way.
There is also a combination water/steam method where boiling water and steam are pushed through and around the plant matter. This is how nutmeg essential oil is distilled.
The final method is straight steam, where steam is pushed through the plant material, picking up the essential oils. Lavender is distilled in this way.
In each of these processes, as the steam rises it ruptures the oil membranes in the plant and releases the essential oil. The steam carrying the essential oil molecules, rises to a condenser, where the oil-steam mixture condenses and reliquifies. As the steam condenses back to water, the lighter essential oil collects and floats on the top. This liquid is then sent to a separator where the oil is separated from the water.
There are many variants in steam distillation. Subtle differences in equipment and processing conditions can translate in to huge differences in the essential oil quality. The size and material of the cooking chamber, the type of condenser and separator, and the degree of temperature and pressure can all have a huge impact on the oil. Distillation is as much a science as it is an art. If the pressure or temperature is too high, or if the cooking chambers are constructed from reactive materials this can affect the quality of the oil.
Temperature also has a distinct effect. At certain temperatures the oil fragrance, as well as the chemical constituents, become altered. High pressures and temperatures seem to cause a harshness in the oil. Even the oil pH and the electrical polarity are greatly affected.
In France, lavender produced commercially is often distilled for only 15 to 20 minutes at 155 pounds of pressure with a steam temperature approaching 350° F. Although this high temperature, high pressure oil costs less to produce and is easily marketed, it is of poor quality. It retains few if any of the therapeutic properties of high-grade lavender distilled at zero pounds of pressure for a minimum of 1 hour and 15 minutes!