Written by David Milbradt L.Ac., AHG , Abundance Acupuncture
Percolation is an extraction process that involves the slow descent of a solvent through a powdered substance until it absorbs certain constituents and drips out through the filtered bottom of the container. A common example of percolation is making fresh coffee through a paper filter. The ancient production of lye from wood ash depended upon this technology, but the first modern pharmaceutical use of percolation did not begin until 1833.
The main advantages of percolation are a more complete extraction of constituents, shorter processing time, and increased flexibility in processing. The disadvantages include the need for more equipment, additional complexity in processing, and the incompatibility of percolation with certain herbs.
Both percolation and maceration depend upon the basic principle of diffusion for extraction: diffusion flows from a gradient of greater concentration to a lesser concentration. Percolation, however, has the advantage of a continuous gravity driven flow of fresh, unsaturated solvent that works its way down through the marc. Since the soluble constituents are constantly wicked out of the marc by a less concentrated solvent the process can continue until the constituents are completely exhausted. This process does not require any agitation and can usually be completed in two days. Because the earlier portions of saturated solvent (called the percolate) contains the greatest portion of constituents and the easily dissolved volatile oils this part is often set aside while later portions are reduced by evaporation to a greater concentration. There are many potential variations and elaborations of this basic procedure.
Equipment needed for making a percolation includes the same equipment used for a standardized maceration with the exclusion of the tincture press and the inclusion of the following items. A percolating cone or percolator will be needed. This may be purchased from a supplier of laboratory equipment or improvised by having a glass shop cut the bottom off of a smoothly tapered wine bottle. Any similarly shaped bottle will do, but I would recommend avoiding the use of plastic if possible. The cork or bottle cap should be saved to control the drip rate of the percolate when necessary. Filter paper such as unbleached coffee filters and a tamping device such as a wooden dowel will also be needed. A proper vessel with a stopcock and glass tubing would be useful in this situation, but much can be done with primitive equipment. The well known Eclectic, John Scudder (late 19th century) used to make fine extracts by knocking the bottom off of a clay jug and packing the neck of the vessel with wire grass.
The herb should first be dried and ground to a powder. With most herbs this may be done with a coffee mill or a blender. Some tougher roots are more easily chopped into pieces or grated before they are dry and others require industrial equipment or mechanical creativity to grind into a powder. Sifting the powder through a sieve will help insure the uniformity of the powder. A five-inch diameter kitchen sieve with 16 wires per inch is adequate for most herbs, but one should keep in mind that densely structured herbs will more easily part with their constituents if they are ground into a finer powder.
The next step in the process is to moisten the powder with the solvent or menstruum. Add the powder (4 ounces of herb would be a reasonable amount to start with if you are using a wine bottle percolator) into a one liter measuring cup. Check the volume displaced by the herb, add about two‑thirds as much menstruum to the powder, and stir it in with a stainless steel fork. The herb should be moist but not so soggy that packing it ends up being drippy or wet. If some of the powder is still dry add more menstruum. The purpose of this step is to allow the powder to expand before packing and to soften up the herb to allow it to more readily absorb the menstruum. The moistened powder should be covered and set aside for 1 hour or so before packing.
Once the powder is properly moistened it is time to pack it into the percolator. Start by finding a cork that fits into the end of the percolator. Cut 3/8 to 1/2 inch off of the end of the cork and notch it with a knife in the shape shown in the illustration. Push it just inside the neck of the cone and follow it up with the rest of the cork thereby sealing the bottom of the percolator. Set the percolator on its stand or into the neck of a 5# honey jar or two quart canning jar. Cut and form a small cup shape with the filter paper, add some moistened herb to the cup and gingerly position it down into the tip of the cone. (The bottle is now up side down with the neck sitting inside the jar). Pack this down until it pushes up against the notched cork. Add more herb and pack it down onto the rest in one-inch layers until you have added all four ounces of herb.
The degree of compression used in packing will depend upon the properties of viscosity, adhesion, and capillarity of the herb that is used. Percolation will not work with every herb. Mucilaginous herbs like Comfrey root or resins like myrrh will clog the cone and obstruct the process. If this happens simply dump the marc into a jar and proceed to make a standardized maceration. Next time try a lighter packing process and see if this solves the problem. With experience one can begin to predict the viscosity of a new herb from the consistency of the moistened herb.
Cut a circle of filter paper slightly larger than the diameter of the top of the percolator, insert it into the percolator so it sits just on top of the herb. Weigh the paper down with a pebble or an official glass percolator weight. This keeps the herb from being washed out when the menstruum is added. Set the apparatus up in an area with good lighting and add 8 ounces or so of the menstruum. Watch the column carefully. The solvent should soak down into the herb gradually and evenly. If the packing is irregular or loose in some areas the menstruum will flow through those areas first and fail to exhaust the rest of the herb properly. Loosen the stop cork until a few drops percolate out, reseal the cork, and cover the top of the percolator. The solvent should completely saturate the herb and be left to macerate for 12 to 48 hours depending on the density of the herb.
After macerating it is time to begin the percolation. The top of the percolator should be filled with a measured amount of menstruum. If there is not enough room to hold all of the menstruum (20 ounces total for 4 ounces of herb) care must be taken to ensure that the level of solvent does not drop down to the top of the herb before all of the menstruum is added. If the powder dries fissures may form that will short cut the percolation when more solvent is added. The cork is loosened just enough that a controlled rate of flow is established. Around sixty drops per minute is ideal. If the rate is too fast tighten the cork, if the rate is too slow loosen the cork, and the cork is completely removed and the rate is still too slow there are two options. You can simply wait out a slow drip rate or dump the works into a jar and macerate as described above for two weeks. Next time lighten up on the packing process.
Ideally in a 1:5 tincture the solvent should exhaust the constituents of the herb completely and you will recover five times the weight of the herb in fluid ounces of tincture. There is nothing left in the marc except compost.