Identifying lead crystal can be quite difficult. This is because different countries have different standards when it comes to designating glass as lead crystal. For instance, in America, glass can be sold as lead crystal if it contains 1% lead, while in Europe, glass must contain at least 4% lead in order to be sold as crystal. Full-lead crystal, which is the most rare form, can contain between 24 and 32 percent lead.
What methods are used to identify lead crystal? Some collectors insist that weight is the best way to tell the difference between lead crystal and cut glass. While lead will indeed add weight to glass, this is not a very reliable method because there are various types of cut glass that can be just as heavy as lead crystal.
A more accurate test is by examining the optical quality. If you shine a beam of light directly onto lead crystal, you will notice a prism effect. In other words, you should see different colors of the rainbow. If the crystal contains a very high amount of lead, the light will have a slightly grayish cast to it. Even though this test is more accurate than the weight test, it is still not foolproof. Cut glass that has many facets can also be highly-reflective and produce a prism effect.
So what is the best way to tell if you have cut glass or lead crystal? Except for a laboratory analysis, the most accurate way to tell if you have lead crystal is by examining its acoustic properties. Have you ever seen the age-old party trick where someone can make a wine glass "sing" by rubbing the rim? This can only be achieved on glass that contains lead. By lightly tapping the surface of the glass or crystal with a metal object (a fork or spoon works well for this), the tone which is produced will accurately tell you whether you have glass or crystal. Glass will produce a short "clunk" or "clang" sound. Lead crystal, on the other hand, will produce a lovely ringing sound. Generally, the more lead that is in the crystal, the longer the ringing will last.
To be on the safe side, it is always a good idea to have a known specimen of both cut glass and lead crystal which you can use for comparative purposes when examining a piece. If the piece in question is heavy, prismatic, and produces a ring, then you can be sure that you have found lead crystal.