Herbal infused oils are used to apply herbs externally, with some herbs they are a more appropriate way to apply the herb externally than using a water infusion (i.e. a tea), as some of the active constituents of the herb may not be water soluble.
Infused oils are made from a quantity of herb (fresh or dried) and an oil, for instance sweet almond oil, soya or even plain old sunflower and olive oil. Each oil will impart their own properties (and smell in the case of olive oil) to the final infusion. The best quality oils to get are cold pressed oils, but do not bother using these if you are planning to heat the oil, as the advantage of using cold pressed is then lost.
To make the infused oil place the herb in a jar, cover with oil and seal (with a lid). If you are going to leave oil for a while in the jar (for instance for the sun infused method) then try not to leave too large a gap at the top of the jar to prevent oxidation. Then place the jar in the sunlight, either in the garden or on a sunny windowsill, and leave for several weeks. Herbs often colour the oil (comfrey, marigold and St John’s Wort colour the oil green, orange and red respectively), so you can determine if the oil is ready by the colour. If the colour is not very deep then replace the herbs with fresh herbs and infuse for another few weeks. If the herb does not colour the oil then you may have to guess that it is ready.
If you are suffering from a lack of sunlight (a common complaint here in Scotland) or a lack of time, then try the quicker method: heat the oil and herbs gently in a water bath (by placing the jar without a lid, in a pan of simmering water) for a couple of hours. As before, you can repeat the process to make a stronger infusion.
After the oils have been infused strain the herbs off and put the oil in a (preferably dark) bottle. If using fresh herbs try to decant off any water left in the infusion, otherwise this will encourage bacterial growth. You could also increase the shelf life of the infused oil by adding a small amount of vitamin E (such as from a vitamin E capsule, or by adding wheatgerm oil)