Each scale, if separated from the parent bulb and placed in a soil-less mix, can produce as many as three to five (or even more) bulblets that will eventually grow into full grown plants.
Lily bulbs contain overlapping scales or plates similar in appearance to an artichoke. Scaling is the process of severing these scales from the main bulb and placing them in a sealed bag of moist peat until they root. This is a great way to increase your stock quickly and inexpensively, because a fair number of scales can be picked off the parent bulb without injuring the plant or lessening its ability to produce flowers. A few scales from a lily bulb can easily produce several dozen new plants in as little as a few years.
The best time to collect scales from lily bulbs is when the plants are flowering, or immediately thereafter. You need to collect them by midsummer if you want to be sure of having usable new bulbs the following spring.
To remove the scales, you can either lift the entire plant out of the ground to remove of the scales (and replant the parent plant), or you can dig down around the bulb, and carefully remove some of the scales while the plant remains intact.
You can safely remove up to 1/2 of the bulb's scales without injuring the plant. Look for firm plump scales, and try to break them off as close to the base (basal plate) of the bulb as possible. This tissue is where the bulblets will eventually form. Discard any shriveled scales or scales that show signs of damage or decay. Wrap the collected scales in a damp paper towel to prevent drying and set them aside while you replant the parent lily or refill your hole.
To convert scales into bulblets, "plant" them in a gallon size plastic bag in mix of damp peat/sphagnum moss or vermiculite. As an extra precaution, you may want to dust the scales with a fungicide before placing them in the bag. The mix should be moist but not wet-just damp to the touch.
Position the scales in the mix so they remain visible through the bag. Seal the bag and punch a small ventilation hole near the top to let in some air. Store the bag in a location where the temperature remains around 70 F to 80 F. Check on it every few weeks to make sure the mix hasn't dried out.
After six to eight weeks, you should see bulblets beginning to form at the base of each scale. When they grow to the size of a pea (some may also have roots), take them out of the bag and carefully separate them from the scales. Separated scales can be returned to the bag to produce another round of bulblets. As this point, the newly separated bulblets (from Asiatic and Trumpet lilies) can be planted into pots or transplanted into a nursery bed in your garden.
Some lily species, like Oriental lilies for example, require vernalization (a cold treatment) to break dormancy and develop leaves. This can be done by relocating the pea-sized bulblets to cooler temperatures (a refrigerator at 35 to 45 F), for a period of 4 to 6 weeks (or longer). After the cooling period, these bulblets can then be removed from their bag, separated from their scales, and planted in pots or garden beds.
Cultivating lilies (no matter what method you use) is not a speedy process, but in return for your patience, you'll be rewarded with a steady supply of free plants. If you have the room, designate a small "nursery" bed for developing new plants. It takes a few years for plants to reach maturity and produce flowers, so labeling the color and species of developing plants will make your life easier in the long run.
During their first few years of growth you may wonder if your new bulbs are ever going to reach adulthood. Expect nothing more than a single leaf the first year, and depending on the species you're cultivating, you may have to wait from 2 to 5 years before seeing your first flowers.