The Glass Menagerie, Part Two

Last month, I discussed general glassmaking procedures, what glass types are relevant to neon signage and how to distinguish them. In this column, I’m addressing colored glass and its workability.

Classic colors

Since the early days of glass manufacturing, colored glasses’ vibrancy has made them quite valuable.

Glass can be colored by different means, which lead to different color properties. One main group is metal oxides, in which metal ions create the color. One example is iron oxide, which will create either green or brown tints (think beer bottles). Iron is commonplace; thus, a site where you could mine sand with low iron content was treasured for centuries. Such sand yields very clear glass. The greenish tint in sodalime and lead-free glass stems from iron, as shown in last month’s column (see ST, January 2004, page 18).

The primary ionic coloring agents are blue (cobalt oxide) and green (iron and/or chromium oxide). Oxide-colored glasses are normally quite stable and don’t change color during prolonged flame exposure. Their color impressions vary with thickness; therefore, bends blown thin on the outer periphery will show a lighter shade than an unworked tube.

The next group, the true ruby glasses, achieves color through distribution of fine, metallic particles. The particle size, amidst the light’s electromagnetic wavelength, determines the color. Ruby glasses may change their color during flaming, and, due to their high price, aren’t used for neon tubes.

The third important group of colored glasses, used mainly for orange, yellow and red neon tubing is called striking glasses. With this type, the color separates via precipitation of sub-microscopic crystals (cadmium and selenium salts). Depending on the melting characteristics, the glass can be almost clear during formation, and the color is formed in the temperature range of 932 to 1,292