According to researchers at the University of Mississippi, a mature marijuana plant yields about equal weight of bud and leaf. Although most successful growers find their plants produce 1/3 to 1/2 less weight in leaf than in buds, this is still a lot of leaf material. While the bud, with perhaps 5% THC, is considered a valuable commodity, the leaf, containing around 1% THC, is commonly considered trash. Some growers throw it away, others give it away to bakers or sell it for $100-200 a pound. Trim contains 1-2% THC and is valued only slightly more than leaf. So growers may be tossing or undervaluing 10 to 20 % of their THC yield.
The reason that the leaf and trim are so undesirable is not their potency, which is comparable or higher than some grade-B Mexican. It is that they are unpleasant to smoke. Nobody likes to gag on smoke from burning leaves just to get high. The solution is to remove and concentrate the THC bearing glands, often called trichomes, which dot the upper surface of hte leaves. Using a photographic loupe or magnifying glass, these glands are easily observed on leaf. Screening the grass is the way to concentrate these THC-rich trichomes.
In 1986, John Gallardin introduced the Master Sifter andthe Motorized Master Sifter, which were featured as prizes at the 1998 and 1999 Cannabis Cup awards. Both of these used standard 100-strand stainless steel mesh. Just as John's company was starting up, the DEA and the Drug War harassed him out of business. Unfortunately, John died in 1994.
The Master Sifter consists of a framed screen with a glass plate below it. Both of these are held by a crafted wooden frame and a larger wooden frame holds the whole assembly in place. The grass is rubbed over the screen, and the glands drop down onto the glass where they are easily collected. The Motorized Master Sifter does the same thing but has a larger screen and is hooked up to a motor and vibrator for automated sifting.
Mila's "Pollinator" has rekindled interest in using leaf. Her device looks like a front-loading washing machine. There are two main differences: no water, and the sides are made of screen mesh so that the tumbling action dislodges the THC-bearing glands from the leaf. Some of the Pollinator models are small enough to fit in a refrigerator, or drop-in freezer. The screening works best at those temperatures because the low humidity and brittleness from the freeze increase the percentage of glands which break off the leaf.
If the glands are to be used for cooking, they can be rubbed or tossed vigorously. Even though there is more extraneous material in the mix, the goal is the greatest number of glands. Sifting for smoke is a more subtle matter. Just like pressing olives for olive oil, the lightest rub for the least tim produces the purest product. Each additional rub produces more glands, but also a higher percentage of extraneous vegetation. In any case, even a vigourously rubbed mixture is much cleaner than leaf or trim. In Morocco, hashish is defined by its rubbing order. "Zero-Zero" is the finest grade, obtained by passing the grass over the screen. Next come Zero, and then lesser grades.
Once the glands have been separated from the grass, they can be used as is. They are a much more potent concentrate than the grass to which they were originally attached, so only a tiny pinch of them need be used at a time. If they are stored in a cool, dry place, they dry out a bit over several weeks and acquire a slightly more hashy flavor and high, which is probably caused by a change in the THC molecule. A bake in a microwave oven under "low" for a few minutes can approximate this aging process.
This separation and aging of glands can also be the first step towards making hash, which is an agglomeration of glands made by pressing them under heat. I've seen some being made by a grower using the leaf produced from his last crop. First he spooned the glands from a bowl into a cellophane pouch. (He used real cellophane, which is processed from wood and does not melt. Plastic would melt and stick to the glands.) He evened the glands in the pouch to a pile 3/8" high. He wrapped this in moistened newspaper, placed it in a pan and heated it on the stove, keeping the paper moist to prevent burning or overheating. Once the package was heated, which took about five minutes, he unwrapped the paper and placed it in the press, which had a die with metal block into which the pouch fit. Once in place, the press was put to work. Ten minutes later it was released, and the pressed hash was ready to smoke.