Black lights are more formally called ultraviolet lights, or UV for short. Anything from 10nm to 380nm is considered to in the ultraviolet range. Shorter wavelengths are visible light, and longer ones are X-rays. Bees and many other insects can actually see ultraviolet light, but we can't. You may think you have seen some, but that is not strictly true. Ultraviolet light is very energetic, and when it strikes different materials it can cause them to fluoresce. What you see is visible light being generated by the material, not the ultraviolet light that triggers it. Some elements will fluoresce strongly in ultraviolet light, some a little, and some not at all. To make it even more complicated, some elements will only fluoresce in certain wavelengths, and not at all in others. The good news is, that if an element fluoresces, it will always fluoresce the same color. How does this help us make determinations about crystal? Read on.
|Visible light image of 2 Lalique vases and a Sabino vase|
In our first image we have (from left to right) A Lalique crystal "St. Marc" vase from around 1990, a Sabino " Les Abeilles" vase from the 1920's, and a Rene Lalique "St. Marc" vase from 1945. We need to note that the 1945 Lalique vase has some opalescence applied whereas the 1990 vase does not, but that is immaterial to the test. It just makes the 1990 vase look much clearer in the first image. Now lets see what happens when we hit them with some 254nm UV light.
|Crystal vases under 254nm UV light|
As you can see in the image above, they all fluoresce about the same. In real life, the color is quite a bit more purple due to the refraction of the light, but the actual light emitted is really closer to a white. This is because of the lead in the glass. Lead shines white easily in the 254nm UV light. The Lalique vase on the left has 24% lead, and the Lalique vase on the right has only 12%, but no real difference can be seen. Plain unleaded glass will not fluoresce at all, so we know for certain each of the three pieces is indeed leaded crystal and not just a cheap glass reproduction. Good to know, but not enough information. Lets step it up a bit.
|Crystal vases under 365nm UV light|
Switching to a longer wavelength UV light (365nm) tells a whole different story. The newer Lalique vase on the left still fluoresces a bit, because the lead in it is the only thing that actually CAN fluoresce. But the Sabino vase does an odd trick... it starts to shine with a bit of an orange hue. You will find this to be true of every authentic Sabino crystal piece from the 1920's and 1930's. Sabino had his own formula for crystal, and it included Arsenic. It is the Arsenic that gives off the orange tinge to authentic Sabino crystal under 365nm UV light, and it is very distinctive. But take a look at the pre-1946 Rene Lalique vase on the right. It glows with a brilliant green! This is attributed to Manganese in the crystal, used before 1946. So, in this case 365nm UV light can be very helpful.
But we must keep in mind that there are other things to consider. Older pressed glass often used Uranium Oxide, which will also fluoresce green in 365nm UV light. Some unscrupulous individuals will mark them Lalique and try to fool people, so you need to know if the shape is a Lalique shape before declaring it as a Rene Lalique piece not just because it fluoresces green.
Additionally, whatever UV light you choose will have a large effect on your results. Most inexpensive UV lights are more of a broader band, and the results are more mixed. The 254nm we used is considered to be medium wave UV, or UVB. The 365nm is considered as long wave, or UVA. Both are dangerous and caution should be taken to ensure no one looks at an active bulb. This can result in permanent damage to vision. Also, both can cause burns to exposed skin at higher wattages. Never point them at exposed skin.
Once you have found the UV light that suits your needs, have fun finding the previously unseen information about your crystal. You never know what you might learn!