Watches provide some of the toughest challenges for phosphorescent glow in the dark pigments. This document, intended for hobbyists and manufacturers, discusses the challenges and how to properly add glow to timepieces. I will start with some basic definitions.
Fluorescent vs. Phosphorescent
Fluorescent materials instantly convert ultraviolet light to visible light. This is often referred to as day glow or black light paint. Phosphorescent materials store light and release it slowly, often referred to as glow in the dark.
Dyes vs. Pigments
A dye dissolves and alters the color of the medium surrounding it. A pigment does not alter the color of other substances. To be effective, pigments must not dissolve and must maintain their original shape. All phosphorescent materials are pigments. The particles are very small and when mixed into paint, it looks like the paint itself glows in the dark.
Varieties and Brightness
Phosphorescent Pigments and Glow in the Dark Powders are the same thing. However, in addition to brand names, glow in the dark powders come in hundreds of varieties and can be classified into categories for chemical makeup, grade, and particle size. These attributes determine the brightness, durability, and color of the glow in the dark pigment.
Chemical Makeup
Chemical makeup and process varies between manufacturers, but they generally fall into Earth Metal Aluminates and Zinc Sulfide. Zinc Sulfide Green was popular in the 1970s. In the 1990s they modified Zinc Sulfide to include red and orange. Zinc Sulfide is rather inexpensive. However, it does not offer the brightness of newer technologies, degrades easily by sunlight, and has a strong odor due to the sulfur.
Earth Metal Aluminates were discovered and patented in 1966 by the Chinese, but did not become widely available till 1990s. They are sometimes referred to as long afterglow. The popular Strontium Aluminate Europium Doped (green) falls under this category. However similar products can produce a purple, blue, turquoise, or orange glow.
While the chemicals are similar, manufacturing techniques vary widely by manufacturer.
Grade
Phosphorescent Pigments are manufactured in grades. The primary differences between the grades are cost vs. brightness. The best grade is currently Grade 0 (aka V10) and the lowest is Grade 7. It is a misconception that particle size determines grade. Each grade will typically be manufactured in about 10 different particle sizes.
Particle Size
Within a grade, larger particles are brighter. However, larger particles pose a few problems. On large surfaces they appear grainy due to shadows created when charging. On small projects where viewers look closely they give a cheap appearance due to unevenness. Larger particles also create a rough surface.
For each of the techniques below, I will offer what I think is the ideal particle size.
There are detailed technical articles on the Glow Inc. website discussing The Grainy Issue and a comparison chart call Which Green? that compares brightness for each grade and particle size.
Coated Pigment
Most phosphorescent pigments are soluble in water. This means that if you put them in any medium that contains unabsorbed water, than the pigments will absorb the water and break apart. The result is a non-glowing pigment.
Some pigments are coated with a special sealer that makes the non-soluble and therefore usable in water based mediums.
Brightness Improvements
To achieve the highest possible brightness, you want to apply the paint as thick as possible. The thickest applications are created by filling recessed or drilled areas that can be up to 1/8 deep.
For thinner applications, you can improve the glow and minimize the grainy appearance by painting over a white surface, preferably fluorescent white. The white acts like a reflector similar to those on flashlights during both the charging and glow stages.
Backgrounds
For watch backgrounds you can purchase vinyl that is impregnated with glow in the dark pigment. While this is the easiest course, these vinyl products are often made with low-grade pigments.
For the brightest glow background, print the numbering on the front side of clear plastic sheet. Paint the rear side with high grade, 55-85 micron glow in the dark paint.
Screen Printing
For manufacturers, screen printing of backgrounds is an appealing option. Add 15-35 micron glow in the dark powder to Plastisol at 10% by volume. Then follow typical screen printing techniques to add the glow, lettering, and sealing layers.
Numbering
This is the method to make printed areas, like numbering, glow in the dark. If your inkjet printer allows you to adjust the ink load, than increase it as much as possible. Print your numbers onto your background stock using yellow ink. Just as the paper exits the printer, dust on very fine glow in the dark powder and allow to dry. Use a sheet of paper to fan off the excess glow in the dark powder. If done properly, only the printed area will now glow. Next, seal it with a clear spray paint like Krylon Crystal Clear. For this to work, you need to use a very fine glow in the dark powder, like 2-8 or 5-15 micron. Warning, due to the small particles, wear a dust mask for this entire procedure.
If you are having issues with static, then spray the paper with static guard and allow to dry before printing. In humid environments, you may need to bake the moisture out of the paper before printing.
It has been tried by many people to add glow in the dark powder to inkjet and laserjet cartridges. There is even a faked online video on the subject. However, it simply does not work.
Hands
Painting the tiny hands of a watch is hard regardless if you are using white paint or glow in the dark paint. I suggest that you mix together clear medium and 15-35 micron glow in the dark powder. Use a very fine paint brush to apply.
Article Information
This document is modified regularly. The most recent version is kept on
http://www.glowforum.com in the "Watches" section. If you have ideas to improve the information, we encourage you to post your suggestions to that forum.
This article was written by Daniel Clark (Skylighter), Senior Technician at Glow Inc., which sells high-end glow in the dark paints and powders. Copywrite Glow Inc. 2009. Feel free to use or reprint this article in its full unedited version.