Question

Which oxide is most favorable for the development of crystals.

• A. Titanium Oxide
• B. Iron Oxide
• C. Tin Oxide
• D. Zinc Oxide

Hint:

Logic Tip: Macro-crystalline glazes = Willemite crystals. Willemite = Zinc + Silica. Therefore, Zinc Oxide is essential.

Answer 1

The Correct Option is D

Concept:
Crystalline glazes are specialty glazes where large, visible macro-crystals grow within the glassy matrix during a very specific, controlled cooling cycle in the kiln.

Step 1:
For crystals to grow in a glaze, the glaze melt must be highly fluid (low in alumina) and supersaturated with a specific mineral that will crystallize out as the temperature drops.

Step 2:
The classic and most reliable macro-crystalline glazes are based on Willemite crystals. Willemite is a zinc silicate mineral.

Step 3:
To create Willemite crystals, the glaze recipe must contain massive amounts of Zinc Oxide ($ZnO$)—often up to 25% or 30% of the recipe—combined with Silica. During the cooling cycle, the zinc and silica bond to form the large, beautiful starburst or floral-shaped crystals.

Step 4:
While Titanium Oxide can act as a "seed" to help crystals start growing, Zinc Oxide is the actual primary building block (the most favorable and necessary oxide) for the large crystals themselves.

Step 5:
Because Zinc is the fundamental element required to form Willemite crystals, Zinc Oxide is the correct answer. \[ \boxed{\text{(4) Zinc Oxide}} \]