Glass color index

Why every color also needs to physically match

Chemistry sets a glass's color; physics decides whether it can actually be combined with other colors. Every glass has a Coefficient of Expansion (COE): how much it expands when heated and contracts when it cools. The number is in units of 10⁻⁷ per °C: COE 104 means 104 × 10⁻⁷ per °C, which is 10.4 parts per million per °C. If two colors in the same piece have different COEs, they pull against each other as the whole thing cools after being worked, building up internal stress. That stress shows up as cracking, sometimes immediately and sometimes weeks later, and it happens regardless of how well the colors chemically get along. This is separate from a color's chemistry: a compatible COE match doesn't guarantee a nice color combination, and a great color combination is worthless if the COEs don't match. Common named systems include COE 104 (soft lampworking glass, with long working time and the most color variety), COE 96 and COE 90 (fusing systems), and COE 33 (borosilicate, high-temperature). The specific number for any given colored rod depends on its manufacturer's formulation, not the colorant family alone, so this app doesn't assign COE numbers per color for that reason, and neither should you without checking the actual supplier data for the glass in hand.

Does murrini need encasing in clear?

Often yes, for two reasons, not just convention. First, encasing a cane in clear glass protects a striking color's pattern through the repeated reheating and handling a pull requires; striking colors (like the gold ruby and copper ruby entries below) specifically need that controlled reheat to develop, so shielding the pattern during it matters. Second, some colors (notably opal whites and certain commercial purples) are documented as prone to devitrification with repeated reheating; a clear casing cuts down how much direct heat that color sees. Not every color needs it, and encasement always changes a cane's size and proportions, so it's a real design tradeoff, not a free win.

Cobalt Blue

Made with Cobalt oxide (CoO). A transparent color.

One of the strongest colorants in glass: tiny amounts produce deep blue. Cobalt glasses need more energy to melt and cool noticeably faster than most colors; at high saturation they work stiff, which is why some suppliers blend in extra flux specifically to soften cobalt's working range.

Copper Blue / Turquoise

Made with Copper oxide (oxidizing atmosphere). A transparent color.

Copper gives a blue-green/turquoise color under normal oxidizing furnace conditions, the everyday result of working with copper.

Copper Ruby

Made with Copper (reducing atmosphere). A transparent color. It strikes: it develops its full color when reheated.

The same copper that gives turquoise under normal conditions instead gives a deep red ("copper ruby," historically used as a cheaper substitute for gold ruby) under a reducing furnace atmosphere with careful heat treatment. Same colorant, opposite result, purely from atmosphere control, which makes it a genuine strike/redox-sensitive color rather than a fixed tint.

Gold Ruby / Cranberry

Made with Colloidal gold (Purple of Cassius). A transparent color. It strikes: it develops its full color when reheated.

Color comes from gold metal nanoparticles suspended in the glass, not a simple dissolved tint. Particle size sets the hue, with smaller particles (~60nm) reading red and larger ones shifting toward purple, then blue. Historically one of the most expensive colorants used in glass, which is part of why cranberry glass reads as a luxury color. It strikes: gold ruby often comes out of the first heat nearly colorless and only turns red when it's reheated, as the gold particles form.

Amethyst

Made with Manganese dioxide (MnO2). A transparent color. Some versions are prone to devitrification.

Same element does double duty: enough manganese gives a purple/violet glass, but small amounts are also used as a decolorizer to cancel out the green tint that iron impurities cause in "clear" glass. Some specific commercial purples are documented as notably prone to devitrification with repeated reheating. This varies by manufacturer, not by manganese content alone.

Emerald Green

Made with Chromium oxide (Cr2O3). A transparent color.

A very strong colorant: small amounts give a clear, dark green; push the concentration higher and it goes nearly black.

Iron Blue

Made with Iron oxide, Fe²⁺ (ferrous). A transparent color.

Iron in the ferrous (Fe²⁺) state reads blue-green. Which oxidation state you get depends on the furnace atmosphere during working, not just the batch recipe. The same base glass can swing toward this or its ferric sibling.

Amber (Iron/Sulfur/Carbon)

Made with Iron + sulfur, reduced with carbon. A transparent color.

Classic bottle-glass amber isn't iron alone: it's a specific chromophore formed when iron and sulfide sulfur combine under a reducing (carbon-added) melt. Different chemistry from the plain iron entries above even though the visual family (brown/amber) can look similar.

Opal White

Made with Tin oxide / phosphate-crystal opacifiers. An opaque color. Prone to devitrification with repeated reheating.

Not a tint but an opacifier. It works by growing microscopic crystals in the glass that scatter light instead of transmitting it, which is what makes it opaque rather than colored. Opal and opaque whites are commonly documented as the colors most prone to devitrification (a whitish, rough, crazed surface) after too much time in the heat, so plan shorter reheats around it.

Black

Made with Manganese + chromium + cobalt (or iron + manganese + cobalt). A transparent color.

True black isn't one colorant: it's a multi-oxide recipe, most commonly manganese dioxide, chromium oxide, and cobalt oxide together (an iron/manganese/cobalt combination is also used). Each of those three shows up elsewhere in this index on its own; stacked together in the right ratio, they push the glass past any single hue into black.