Causes of Streaks in Glass Colored by Chrome Ore Powder

Streaks are alternating light‑and‑dark color bands along the glass flow direction, caused by uneven distribution of chromium ions in molten glass. The root causes fall into raw‑material‑related and melting‑process‑related factors.

1. Problems originating from chrome ore powder (most frequent)

First of all, excess coarse particle size. The melting point of chrome ore is around 2150 °C, much higher than the melting temperature 1450‑1550 °C of common soda‑lime glass. Coarse particles melt and dissolve slowly, leading to local chromium enrichment. Partially‑dissolved ore powder flows along the channel and forms dark streaks, together with black spots and color mottling. For glass‑coloring grade chrome ore powder: 325‑mesh is required. Oversized particles over 100 μm must be strictly controlled.

Secondly, poor dispersion and agglomeration. Moisture‑induced agglomerates cannot be broken up during batching. Clumps with locally high chromium content enter the furnace, fail to diffuse fully, and generate flow‑oriented color streaks.

Finally, batch‑to‑batch composition fluctuation. Variation in Cr₂O₃ and Fe₂O₃ content, or unstable Ca / Mg impurities, changes melt viscosity and hinders chromium‑ion diffusion, triggering vertical color streaks.

2. Batching & mixing issues

1. Insufficient mixing time, uneven blending Chrome ore powder is added in low dosage (generally 0.1‑2 %). Inadequate mixing results in spot‑to‑spot chromium concentration differences in the batch, which turn into visible streaks after melting.
2. Improper feeding method Concentrated feeding of chrome ore powder without thorough dry‑blending with silica sand, soda ash and other raw materials creates local high‑chromium zones.

3. Furnace melting process factors

To begin with, low melting temperature & insufficient holding time. Under‑temperature melting prevents full decomposition and dissolution of chrome ore. Chromium ions cannot homogenize, producing color streaks. Recommended melting temperature: 1480‑1550 °C.

Next, inadequate homogenization & stirring. Weak stirring in the homogenization zone fails to eliminate concentration gradients inside molten glass. Concentration differences show up as flow streaks during glass flow.

Most importantly, reducing atmosphere inside furnace (critical factor). Local reducing agents such as carbon or sulfides convert Cr³⁺ (normal green tone) into Cr²⁺ (bluish tone). Inconsistent local color produces blue‑green alternating streaks or hazy defects.

4. Unstable channel temperature & fluctuating glass flow Temperature swings in the forehearth change melt viscosity. Layers of glass liquid with different concentrations form streaks after forming.

Troubleshooting sequence

1. Check sieve residue of chrome ore powder for coarse particles; inspect powder for moisture‑caused agglomeration.
2. Verify batching procedure: pre‑mixing of minor‑additive chrome ore powder and sufficient mixing time.
3. Confirm peak melting temperature and holding time.
4. Check furnace atmosphere for local reduction; add sodium nitrate if needed.
5. Strengthen stirring for glass homogenization and stabilize forehearth temperature.