Glass coloring and color marking
The reason glass shows different colors comes down to how light interacts with its contents. In an extremely pure piece of glass, without bubbles, coloring ions, crystals, or tiny phases, all visible light would pass through, making it completely transparent. But when impurities are present, they absorb certain wavelengths of light. That makes the glass appear colored.
Glass coloring and color marking can happen in several ways. It occurs through adding coloring ions, using tiny colloids (like in striking glasses such as gold ruby or selenium ruby), embedding colored inclusions (as seen in milk glass and smoked glass), scattering light in phase-separated glass, applying dichroic coatings for dichroic glass, or using colored surface coatings.
Standard soda-lime glass looks clear when thin. But iron oxide impurities give it a greenish tint. That tint becomes visible in thicker pieces or under study with instruments. You can add other metals and metal oxides during this glass-making process to change its color and boost its visual appeal.
Color additives and their effects
Iron(II) oxide added to glass results in a bluish-green hue. That tint is common in beer bottles. When combined with chromium, the green becomes richer, common in wine bottles.
Combining sulfur with carbon and iron salts creates iron polysulfides. This leads to amber glass that ranges from yellowish to almost black. In borosilicate glass (rich in boron), sulfur gives a blue tint. Paired with calcium, it creates a deep yellow tone.
Manganese added in small amounts removes the green tint from iron, while in higher amounts, it brings out an amethyst or purple shade. Manganese is one of the oldest coloring agents. Purple manganese glass was already used in early Egyptian history.
Manganese dioxide, which is naturally black, can also remove green coloration. Over time it slowly converts into sodium permanganate, a dark purple compound. In some New England houses built over 300 years ago, window glass has a faint violet tint from this change. Those antique panes are prized for their subtle color. This transformation is sometimes confused with desert amethyst glass, where glass exposed to intense ultraviolet sunshine in arid regions develops a delicate violet tint. The outcome varies because the process depends on the glass composition, sunlight strength, and exposure conditions.
Adding small concentrations (about 0.025 to 0.1 percent) of cobalt yields blue glass. This works best with glass containing potash. Very small amounts of cobalt can also serve to decolorize glass, canceling out unwanted tints.
Using about 2 to 3 percent of copper oxide produces a turquoise color.
Nickel’s effect varies by concentration: thin levels give blue glass, higher amounts bring violet, and heavy concentrations can yield black glass. Lead crystal with added nickel takes on a purplish tone. And in lead glass, nickel combined with a bit of cobalt can act as a decolorizer, neutralizing undesired tones.
Chromium
Chromium is a strong coloring agent. In small amounts, it gives glass a dark green shade. In larger doses, it can even make glass appear black. If you mix chromium with tin oxide and arsenic, you get emerald green glass. There’s also a special type called chromium aventurine. That’s made when chromium oxide exceeds its solubility in glass. As the mixture cools, parallel plates of chromium(III) oxide form, giving a shimmering, aventurescent effect.
Cadmium
Cadmium paired with sulfur forms cadmium sulfide. This mix creates a deep yellow tone. The color is strong and often used in glazes. But cadmium is toxic. When cadmium, selenium, and sulfur combine, they produce vivid reds and oranges.
Titanium
Titanium on its own tints glass a yellowish-brown. It’s not common by itself. But it’s used to make other colors more vibrant and stronger.
Uranium
Adding uranium (usually 0.1 to 2 percent) gives glass a fluorescent yellow or green glow. Uranium glass isn’t usually radioactive enough to be harmful. But if you sand it down into dust and inhale it, that dust could be carcinogenic. If you use uranium in high-lead glass, you can get a deep red color.
Didymium
Didymium can yield green or lilac-red colors. It’s often used in UV filters.
Selenium
In small amounts, selenium acts like manganese and decolorizes glass. In larger amounts, it gives a red tint. This effect comes from nano-sized selenium particles in the glass. Selenium is key to pink and red glass shades. Mix it with cadmium sulfide and you get “Selenium Ruby,” a brilliant red.
en.wikipedia.org
Some glassmakers confuse selenium ruby with other red glasses, but it’s really about selenium nanoparticles dispersed in the glass.
Copper
Pure metallic copper makes glass dark red and opaque. It’s sometimes used instead of gold to create ruby-like glass.
Gold
Tiny amounts of metallic gold, around 0.001 percent, can turn glass a rich ruby-red. That’s called Ruby Gold or Rubino Oro. If you use slightly less, you get a milder cranberry shade. The color depends on the size and spread of gold particles. These glasses are usually lead-based and use tin to help form the color.
Silver
Silver compounds, like silver nitrate and silver halides, produce colors from orange-red to yellow. The heating and cooling process changes the final shade. Photochromic lenses and photosensitive glass often rely on silver.
Purple of Cassius
This is a purple pigment created when gold salts react with tin(II) chloride. It gives a deep purple tone.
Decorative Coatings
There are two main coating methods for pattern and color. Enamel painting is used on glass bottles and stained glass. Glass paint, usually black, and silver stain create shades from yellow to orange on stained glass. These methods are fired in a kiln or furnace. They last for centuries. “Cold-painted” glass, using oil paint or other mixtures, does not. It typically lasts only a few hundred years.
Tin Oxide Mixes
Mixing tin oxide with antimony and arsenic oxides yields opaque white glass, also known as milk glass. The Venetians first used this to mimic porcelain. It was often decorated with enamel. Other smoked glasses use colored inclusions, but ionic coloring can also give dark tones.
Scattering Effects
When glass has two or more phases with different refractive indexes, the Tyndall effect kicks in. If these phases are the size of visible light wavelengths or bigger, light scatters in blue and violet tones. The remaining transmitted light looks yellow or red.
en.wikipedia.org
Dichroic Glass
Dichroic glass gets its color from nano-scale coatings. These coatings, often metals, metal oxides, or nitrides, give the glass optical effects that change with viewing angle. That’s why some fancy windshields appear blue; they’re dichroic too.
