Black lighting should be used as an ‘indicator,’ not as a litmus test as to whether or not the glass is from the American Brilliant Period. Only longwave blacklight is effective and it must be used in a dark room or in a special ‘box’ that would eliminate outside light. We have made a box in our photography room that is nothing more than a space between two shelves that we lined completely with black fabric and created a ‘flap’ at the front so that we could put the pieces in. We let the flap down and then ‘peek in the side’ to see how the piece fluoresces. The fixture that holds the blacklight bulb was screwed to the bottom of the top shelf; the fixture has a switch on it and a cord that goes to a nearby electrical outlet. The fixture is not important but the bulb is. The bulb we use is a two-pin fluorescent bulb 18-inches long and an inch in diameter. It was made by GE and has the following identification: F (or P, I can’t read it) 15 T 8 – BLB black light 15 watts. Our tube is marked USA; we have found that some GE tubes made overseas do not fluoresce correctly.
To get a feel for the color you are looking for, it is best to take a signed piece or pieces that we are positive are of American origin and fluoresce them. Pieces by Libbey and Pairpoint would be excellent as they made their own glass and we have no documentation that they purchased foreign blanks as some companies did. The color we are looking for is somewhat like a light, apple green. The green indicates that there are traces of manganese in the glass which characteristic of American-made glass. Very few, if any, foreign blanks from Baccarat or Val Saint Lambert and other foreign sources had manganese in them. Once you have the proper bulb and set-up, and have seen a few pieces that have the various shades of apple green, you will get a feel for what to look for.
Another problem is that some pieces are so heavily cut and so highly polished that all you are reading is the reflections of the blacklight itself on the glass. On such a piece, you might be able to find a ‘thick spot’ at the bottom or very top where you can get a true reading. There are other problems, also. One of the biggest is that some of our earliest cutting houses (such as Egginton and Straus) used Baccarat blanks and other foreign blanks. We have pieces in our collection that do NOT fluoresce green but are definitely from these two companies and are shown in their catalogs. Another problem is that some ‘modern’ glass is now turning up with manganese and hence fluoresces the apple green color. As far as I know, there is no one source that would tell you exactly what the different colors indicate. A difficult subject and a decent amount of controversy; it all leads back to blacklighting as an indicator, NOT a litmus test.
We have recently found that a ‘made in Japan’ USH10 Black Light Blue F15T8BLB Hg (in a circle), 18-inch, 2-pin fluorescent, 15 watt bulb produces the required results.