Ranging in complexity from a full distillation to the simpler setup described below, hydrosols are a great way to capture some of the volatile oils (mono- and sesquiterpenes, phenolic compounds, alcohols, ethers, aldehydes, ketones) and essential oils of plants. For obvious reasons, these are preparations best reserved for the more aromatic herbs (it would be relatively fruitless, for example, to try a Dandelion leaf hydrosol), and they provide a wonderfully light, clear impression of some of the most subtle and memorable qualities of the plants.
To prepare a simple, although rather dilute, hydrosol (still very effective, although not as concentrated as a pure essential oil), you will need: A large pot (2 gallon) with a snugly fitting lid, some waterproof tape, two bowls, and small bag of ice. Take the herb you wish to distill, clean and garble it thoroughly, and chop it to a fine consistency. It may be a good idea to dry it for a few days first, to allow the essential oils to concentrate; fresh herbs work well too. Place one of the bowls upside-down in your big pot, then add the herb all around it, and cover with water. Place the second bowl on top of the first, base-to-base, and put the lid on the pot, again upside-down, so that the curvature of the lid points downward into the second bowl. Seal the lid with tape, and place the bag of ice on the top of the whole thing. This is your basic distillation setup: the idea is that, as the steam rises, it meets the cool lid, condenses, and drips into the second bowl for collection. The essential oil, whose constituents boil at a lower temperature than the water, will rise with the steam.
Now, apply a very gentle heat to the bottom of your distillation setup. It will take several minutes before you hear the first ‘ping’ of water starting to simmer in the pot; at this point, turn the heat down even further. If you have a clear glass lid (recommended), you should be able to see little droplets forming on the underside of the lid and running (slowly, at first) into the center. This process should continue for 10 of 15 minutes, perhaps more depending on the amount of herb used, but not for too long, as the volatile oils tend to be drawn off fairly quickly and further heat (and steam) only dilutes the hydrosol.
When you feel the time is right, turn off the heat, remove the distillation setup from the burner, and allow it to cool for another 10 minutes. Then unseal it, take the top bowl which should contain a fair amount of liquid, and drain it into a graduated cylinder or other appropriate container. Place a cork or lid over this, and allow it to sit for a few hours. If you want, you can then take a dropper and draw off the top of the liquid – this is where the majority of the oils (which float on top of water) will be. However, it’s fine to use the whole hydrosol too. Preserve this final product with 20% alcohol (meaning, add ¼ of the volume of hydrosol in alcohol: 1 oz hydrosol, ¼ oz alcohol, etc…), keep it in the fridge to avoid spoiling, or add it to a tincture or formula.