The interior of the tubes may, or may not, be coated with a thin phosphorescent powder coating, affixed to the interior wall of the tube by a binding material. When processed, the tube is filled with a purified gas mixture, and the gas ionized by a high voltage applied between the ends of the sealed tube through cold cathodes welded onto the ends. What you see when you observe an illuminated neon tube is only the glowing interior gas, which may or may not depending on the tube's intended color, be used to illuminate the phosphor on the interior of the tube. Different phosphor coated tubing sections may be butt welded together using glass working torches to form a single tube of varying colors in the span of the same filled tube, for effects such as a sign where each letter displays a different color letter within a single word, such as shown in the OPEN sign in the photo above right.
Even though the term "neon" is used to denote the general field, this is a misnomer; inert neon gas, which glows reddish orange by itself, is only one of the two types of tube gases principally used in commercial application and in fact pure elemental neon gas is used to produce only about a third of the colors. The greatest number of colors is produced by filling with another inert gas, argon, which itself produces very little light.
What makes these tubes work is a tiny droplet of mercury (Hg) which is added to the tube immediately after purification. When the tube is ionized by electrification, the pure mercury droplets disperse into tiny drops, migrate evenly throughout the tube, coat the phosphors, and evaporate into mercury vapor, which fills the tube and produces strong ultraviolet light. The ultraviolet light thus produced excites the various phosphor coatings that come coated inside the tubing which contain exotic rare earth phosphors designed to produce different shades of color. Even though this class of neon tubes have absolutely no neon in them whatever, they are still denoted as "neon." In this sense there is little difference between the physical ionization and light production process of a standard fluorescent tube and a neon tube, except for the cathodes, the diameter of the tubes, the bending, and the gas interior chosen.
Each type of neon tubing produces two completely different possible colors, one with neon gas and the other with argon/mercury. Some "neon" tubes are made without phosphor coatings for some of the colors. Clear tubing filled with neon gas produces the ubiquitous yellowish orange color with the interior plasma column clearly visible, and is the cheapest and simplest tube to make. Traditional neon glasses in America over 20 years old are lead glass that are easy to soften in gas fires, but recent environmental and health concerns of the workers has prompted manufacturers to seek more environmentally safe special soft glass formulas.
One of the vexing problems avoided this way is lead glass' tendency to burn into a black spot emitting lead fumes in a bending flame too rich in the fuel/oxygen mixture. Another traditional line of glasses was colored soda lime glasses coming in a myriad of glass color choices, which produce the highest quality, most hypnotically vibrant and saturated hues. Still more color choices are afforded in either coating, or not coating, these colored glasses with the various available exotic phosphors.
