Causes of Striae (Streaks) in Glass

In the glass manufacturing process, one of the common quality issues is the presence of striae (streaks) or cords in the final product. These defects not only affect the visual appearance of glass bottles and containers but may also compromise product performance and brand image. Understanding the root causes of striae formation is essential for producing high-quality glass packaging.

Here are the main causes of striae in glass and the factors that influence their occurrence:

1. Striae Caused by Uneven Melting

During glass melting, the homogenization process allows different parts of the molten batch to diffuse and eliminate irregularities. If homogenization is incomplete, varying levels of non-uniformity remain in the glass body.

Factors that contribute to this problem include:

• Inconsistent batch composition and particle size.

• Excessive use of cullet (glass scrap).

• Fluctuating melting temperatures that disturb the homogenization regime.

• Influence of furnace atmosphere (oxidizing vs. reducing gases), which alters glass surface tension and promotes rolling movements in the melt.

These conditions can lead to visible striae and cords, often accompanied by bubbles or stones if melting quality is poor.

2. Striae from Dripping of Furnace Crown or Sidewalls

When silica bricks in the crown or sidewalls of the furnace are corroded, glass droplets rich in SiO₂ or Al₂O₃ fall into the melt. These droplets have high viscosity and low diffusion rates, making them difficult to dissolve fully. As a result, they remain as striae or cords in the finished glass.

3. Striae Caused by Refractory Corrosion

This is one of the most common sources of striae. As molten glass corrodes furnace refractories, the deteriorated material may enter the melt in either crystalline or glassy form.

• Crystalline inclusions may become stones.

• Glassy inclusions increase viscosity and surface tension locally, forming striae.

The problem is most severe near sidewalls, furnace joints, and throat blocks where alkali vapors can penetrate. Measures such as improving refractory quality, sealing vertical and horizontal joints, and applying a cullet barrier layer at the bottom of the furnace are effective in reducing corrosion-related striae.

4. Striae from Dissolving Stones

Sometimes striae form when stones (unmelted inclusions) dissolve at different rates in the molten glass. Even after partial dissolution, the surrounding liquid may retain different chemical compositions, creating streak-like defects.

For example:

• Clay-based refractories can release alumina-rich streaks.

• Dissolved stones often form envelopes or halos in the glass, leaving behind long “tails” of striae.

Conclusion

Striae in glass are primarily caused by uneven melting, refractory corrosion, furnace atmosphere, or incomplete dissolution of stones. These factors can significantly impact the optical and mechanical quality of glass packaging.

At QLT Glass Products Manufacturing Co., Ltd., we minimize striae through:

• Strict raw material selection and precise batch control.

• Stable melting regimes with advanced furnace temperature control.

• High-grade refractory materials to resist corrosion.

• Continuous quality monitoring to detect and correct irregularities early.

This commitment ensures that our glass bottles and containers deliver exceptional clarity, consistency, and strength, helping clients worldwide enhance product presentation and brand value.