The expression form of essential oil extraction does not involve the use of a heat source. This is the method commonly used to extract oils from the rinds of citrus fruits. In earlier times, rinds were squeezed by hand and a sponge was used to collect the essential oils. The fruit would be removed and then the rinds along with the pith would be soaked in water to make them easier to work with. They'd then be turned upside-down. Turning them upside-down caused the cells containing the oils to break apart. Once broken, the oils would drip out and soak into a nearby sponge. When the sponge became saturated, the oils were squeezed into a container so they could be decanted.
The essential oils of Lemon, Bergamot, Orange and other citrus fruits are obtained by simple pressure. The essential oil of the citrus fruits are found in the outer coloured layer of the rind, and the pulp and white pith must be removed before the oil is extracted. This has long been carried out by hand, by one of two methods. The inside of the fruit is either scooped out, leaving a cup-shaped rind, or the peel is taken off in strips, leaving the pulp intact.
The peel is then squeezed to press out the aromatic oil, with a certain amount of juice. This is left to stand until the oil can be separated off from the top of the juice.
Another traditional method was to roll the fruit around in a barrel lined with spikes to puncture the peel so that the oil and juice could be collected and separated.
Various machine methods are now in use, but the best quality citrus oils are still extracted by hand. By using a new garlic-press (which has never been used for garlic) you can produce very small amounts of Lemon or Orange oil at home, but it is very important that you make sure the fruit has not been sprayed, dyed or coated.
Another expression method involved sticking pins into the skins of fruit for the purpose of damaging the cells that contain the essential oils. The apparatus used had a built-in container for collecting the oils and the other parts of the fruit that ended up in the collection area. The final steps involved separating the essential oils and decanting them. This is called the ecuelle a picuer method.
Those were quite laborious processes, and thankfully, technological advances led to the invention of machines to do this type of tedious work. Nowadays, oils from rinds are extracted using centrifugal force. This rapid process is called Machine Abrasion.