- Primary cause: incomplete melting of coarse chromite particles Chromite powder is a high-melting-point mineral (around 1850°C), while the melting temperature of ordinary glass bottles is only 1450–1500°C. Fine particles can fully dissolve into glass and produce a uniform green color. Coarse particles only melt on the surface, and the core of particles remains undissolved, leaving visible black spots, green spots or stones inside the glass.
Therefore:
- Grade with 90% passing 325 mesh: contains a small amount of coarse particles → prone to black specks
- Grade with 100% passing 325 mesh: no coarse particles → clean glass without black specks
- Excessive impurities in chromite powder High levels of silica and iron impurities in the mineral powder have an extremely high melting point and cannot melt under glass-making temperature, forming permanent black specks and sand defects. Low-silicon and low-iron glaze-grade chromite powder must be used for glass coloring.
- Powder agglomeration and uneven mixing The addition amount of chromite powder is relatively low. Poor mixing or damp caking leads to local agglomeration. The agglomerates cannot melt completely and form dark spot defects.
- Kiln process issues
- Low melting temperature
- Insufficient refining holding time
- Reducing atmosphere inside the furnace
All of the above will cause incomplete dissolution of chromite particles and generate black specks.
Nearly all black specks in glass come from coarse particles in chromite powder, which cannot be fully melted at glass furnace temperature. Switching to ultra-fine chromite powder with 100% passing 325 mesh can fundamentally solve the black speck problem.