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Things You'll Need
Instructions
Pick up the doorknob and study its weight in your hand. According to "Antiques Roadshow Primer," basic glass is composed of silica (sand, flint or quartz) and soda or potash. Crystal, on the other hand, contains oxide of lead in place of soda (hence the name "lead crystal"). The addition of lead actually increases the weight of the glass. A crystal doorknob should have a substantial, hefty feel. A lightweight knob is probably not crystal. If the vendor has several glass doorknobs for sale, compare ones of similar size and shape. A heavier specimen might be your first clue that you've found crystal.
Examine the pattern on the doorknob (many are incised with pinwheels, diamonds and similar designs) and run your fingers over the surface. The edges on a cut crystal piece are sharp and precise. If the edges are dull or rough, the doorknob is pressed glass. If the doorknob is smooth, the surface should have a polished sheen without raised seams, which are mold marks and indicate pressed glass.
Hold the doorknob up to the light. Lead gives crystal exceptional clarity and brilliance, even in colored examples. An ordinary clear glass doorknob, on the other hand, will look murky (like peering through an ice cube) and may even have a gray or greenish cast. Colored glass doorknobs are dull as well. If the doorknob is patterned, a crystal version will refract light like a prism. If you see distinct rainbow reflections when you hold up a glass doorknob, it's crystal.
Try a lead testing kit if you still have doubts whether the doorknob is crystal. Naturally, you can't take one of these kits into a flea market or antique mall to test a doorknob before you buy. However, once you get your treasure home, you'll find a basic swab or wipe-on lead testing kit is ideal for your needs. You can get results quickly, and most kits are inexpensive (cost depends on the manufacturer and the amount of surface you can test with the kit). Lead testing kits are available from hardware stores and online retailers. (And, no, there is no danger from lead in a crystal doorknob since you're not using it with food or drink. The test kit is helpful, though, for scientifically confirming that a glass knob is crystal.)