I've explored numerous methods of extracting THC into tinctures. The alcohol method has a tendency to sting the mucus membranes of the mouth. I just finished a batch of the following for those looking for a recipe with less alcohol and a sweet taste to it
Step1) Take a previously made marijuana tincture
Step 2) Pour the tincture into the top of a double boiler . Add an equal amount of glycerin
(4 cups of tincture to 4 cups of glycerin, for example.)
Step 3) Over very low heat, a slow cooker on low works the best to cook the mixture until it has reduced in volume by half, or 50%. The alcohol will "cook off" while you are doing this.
Cooks Note *****Depending on the type of alcohol (for example Everclear 100 proof) and the percentage of the water volume in the alcohol it could take several hours to cook down. I left mine on low for about 8 hours and used Everclear to make my basic tincture recipe.
Step 4) Once the mixture has cooled, store in glass bottles refrigerated.
The key here is to reduce the alcohol and not loose the THC by cooking at to high of a temperature. Never boil at extreme tempatures or you will destroy the THC in the tincture.