Decoctions are used for hard bits of the plant, such as roots, twigs, branches and barks. In order to extract the medicine in these dried herbs, boiling is required.
A decoction is a fancy word for making a long boiled "tea." After adding the dried herbs and water to a glass or ceramic pot (herbs react with metal), the water is brought to a boil, and then left to simmer, covered, for 20-30 minutes. The usual way to gauge the time is, once the water has boiled down to half (through the process of slow evaporation), it is ready to be drunk.
The standard ratio of herb to water is 2-4 TBSP dried herb for every 4 cups water. The amount left in the pot after boiling should be reduced by half, and you should therefore be left with 2 cups of decoction or "boiled tea."
To strain out the herbs, simply use a fine mesh sieve. If you find there are still bits in the tea, then line the sieve with a coffee filter and strain through. If you like, you can make another decoction by adding 4 additional cups of water and letting simmer once again for 20-30 minutes. The herbs can be composted, and your beautiful "tea" can be drunk right away. Any sweetener and non-dairy milk or creamer can be added to the brew. Remainders will keep refrigerated for 2-3 days. Standard dosage is 2-4 cups daily or 2-3x/week.
A double decoction means that the amount of water left in the pot has been reduced to 1/4th its amount. This means that the 4 cups of water added to the pot with the herbs has been cooked down to make 1 cup of boiled "tea."
Another way to make a double decoction is to do this: once the 4 cups of water has cooked down to 2 cups, add 2 cups more water (the amount that was lost) to the pot. Let it come to a boil, and simmer it down again so that only 2 cups are left to drink.
Whichever method is used, making a double decoction means a much more potent tea medicinally. In terms of medicinal potency equivalency: 1 cup of infusion (flower/ leaf) = 1/4th cup decoction = 1 TBSP double decoction. (To find out how to make flower and herbal infusions, see HERE).
Once decocted, strain out the herbs using a fine mesh sieve (lined with a coffee filter, if you prefer, to ensure all small bits are removed). Let cool and drink. As with decoctions, you can add milk and creamer, and sweeten to your liking. Keep any remainders in the fridge up to 2-3 days. Standard amount to be drunk: 1/2-2 cups daily or 2-3x/week.
Some Interesting Tidbits:
To your lovely decoction adventures ahead!
As always, Enjoy :)