Screen printing (sometimes known as silk screening) is a process in which a screen with a design in it is used to apply ink to fabrics, such as T-shirts and cloth bags. The screen is prepared using a photosensitive liquid that is exposed to light. There are many options for the type of light to use (including sunlight), and one of these options is an unfiltered black light.
Filtered and Unfiltered Black Lights
A filtered black light appears to have a dark purple surface on the bulb, while an unfiltered black light looks like a normal fluorescent light, with a white surface on the bulb. A black light is actually an ultraviolet light. The characteristic purple glow of purple black lights is not the ultraviolet light (which is invisible to the naked eye), but the result of the filter.
Will a Filtered Black Light Work?
A filtered black light will harden the emulsion used to make screens. However, it is not particularly reliable. The results may be splotchy, and could result in more work in the long run. While it may be more convenient--black lights are relatively common, especially around Halloween--using unfiltered black light is going to be a more reliable method.
Where to Get Unfiltered Black Lights
Unfiltered black lights are obtainable through most screen printing supply websites and a few speciality stores. It is unlikely to be in a hardware or electronics store, as it's an uncommon product, but it's not altogether impossible to find there. They might be found in some craft stores, as screen printing is becoming more common, and your local craft or hardware store may special-order them for you.
The most efficient alternative is the sun. On a clear, sunny day, an exposure of about 30 seconds may be enough to achieve the desired effects. Before doing a big project, it is best to test on smaller screens before, and to consult the instructions that come with the emulsion. If it is dark or rainy out, you can probably use a work light--something in the 500-watt range. They're relatively inexpensive at £5 and up. With any light alternative, it's best to test before using the light on a big project.