Glass Colouration — Transition Metal Oxides A deep blue colour is imparted to glass primarily by the presence of which oxide?

Introduction / Context:
Coloured glass is produced by adding small amounts of metal oxides to molten silica-based mixtures. Each oxide imparts characteristic hues due to electronic transitions within the d-orbitals of transition-metal ions. Recognizing which oxide yields which colour is a staple in basic materials science and art glasscraft quizzes.

Given Data / Assumptions:

• We match oxide additives to observed glass colour.
• Typical soda–lime glass composition; low concentrations of colourants.
• Oxidation state and coordination influence hue but the canonical associations hold.

Concept / Approach:
Cobalt(II) oxide (CoO) is renowned for producing a rich, intense blue in glass and glazes (“cobalt blue”). The color arises from d–d electronic transitions of Co2+ ions in the glass matrix. Other oxides give different or variable shades: cupric oxide can give turquoise/green, nickel oxide often results in brown/gray, while iron oxides yield greenish or amber tones depending on Fe2+/Fe3+ ratios.

Step-by-Step Solution:

Verification / Alternative check:
Pottery and glass references consistently list cobalt compounds as the most reliable blue colorants across firing atmospheres.

Why Other Options Are Wrong:

• cupric oxide: Often green to turquoise depending on conditions.
• nickel oxide: Brown/gray tints.
• iron oxide: Green/amber; not deep blue.

Common Pitfalls:
Confusing copper blues (seen in some glazes) with glass; cobalt is the archetypal and most stable deep blue in glass.

Final Answer:
cobalt oxide