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Stripes – Glass in Glass

Definition of Stripes

Stripes are a common type of glass homogeneity defect. Due to the various forms in which stripes can appear, their meaning has been perceived as not entirely clear over the years. According to the American Material Testing Standards, stripes in glass products are defined as “elongated glass-like inclusions with optical properties and other properties different from the main glass.”

Stripes can be understood as a transitional phase before the complete homogenization of the melt or solution through chemical, physical, and structural permeation. Therefore, the concept of “stripes” refers to glass-like inclusions that can be completely mixed in glass. If conditions permit, they can be thoroughly mixed and frozen along with the surrounding material at the melting temperature. Glass-like inclusions can have different chemical compositions, thermal states, and/or structural states from the surrounding material.

The presence of stripes can lead to various issues related to changes in physical nodules, ranging from minor aesthetic defects to severe reductions in glass strength.

Outer surface stripes and inner surface stripes
Outer surface stripes and inner surface stripes

Formation of Various Types of Stripes
  1. Unreasonable particle size of glass raw materials.
  2. Incorrect variations in glass composition.
  3. Suboptimal design of glass composition.
  4. Improper melting in the furnace.
  5. Unstable output volume.
  6. Erosion of refractory materials.
  7. Volatilization of alkaline materials.
Solutions for Stripes

1. Stripes Formed due to Uneven Mixing or Weighing of Batch Materials

This category can be divided into stripes rich in silica and those deficient in silica. If the glass contains aluminum oxide, stripes rich and deficient in aluminum oxide may also occur. Stripes with excess or deficiency of calcium oxide or magnesium oxide are relatively rare.

2. Stripes caused by variations in the raw material mix can often lead to serious safety issues with bottles. This type of striping exhibits characteristics of different intensities and multiple layers, typically appearing in a spiraled pattern and extensively distributed within cross-sectional slices. The stripes can manifest as either tensile stress or compressive stress patterns, depending on the severity of the misalignment in the composition of the raw materials.

Streaks caused by incorrect ingredients

The uniformity of batch materials should be considered from the following aspects:

  1. Particle size distribution determines the contact area between participating substances (raw materials), and a larger contact area leads to a larger reaction area. Therefore, the particle size distribution should be as large as possible.
  2. The distribution of each component should be as statistically uniform in space and time as possible to ensure thorough mixing of raw material components. Regular monitoring is essential.
  3. To accurately measure the amount of added batch materials, the moisture content in the batch materials should be known at all times. Quartz sand, for example, is usually required to contain 4-5% moisture (by weight). When using dry sand, some moisture can be added to the batch materials during or after mixing.

Density stability can serve as an indicator of possible stripe problems.

If the density fluctuates more than ±0.0020g/cm³ in a day, changes in components caused by density may lead to stripes.

1. The homogenizing effect of the clarifying process should be noted.

A large number of bubbles during melting and clarifying can cause many displacement reactions, making the composition of the glass finally close to an average value within statistically acceptable limits. The smaller the deviation of each part’s composition from this average value, the more uniform the melt.

The “hotspot” is the hottest area in the clarifying zone, and it has two major functions: decomposing the clarifying agent and expelling gases from the melt; forming large and small circulations in the glass melt pool. Large circulations assist in mixing bubbles, causing intense diffusion deformation, resulting in uniformity in the melt. The hotspot can be strengthened by two measures: electric-assisted heating and “bubbling.”

After electric-assisted heating, the refractive index of sodium-calcium glass increases by Δn = 0.0003, and the density increases by 4.5%. The degree of non-uniformity (measured using the Christiansen filtration method) decreases by about 2/3. Bubbling with air in the glass melt can reduce density fluctuations from the original ±1.1 kg/m³ to ±0.45 kg/m³.

2.Stripes Caused by Batch Material Changes

It takes a long time to introduce the same batch materials to make the melt in the melting pot reach a truly uniform composition. Changes in the chemical composition of the batch materials need some time to be analyzed in the final glass products.

In the pot furnace, there is always a layer of glass that does not flow with the flowing liquid glass. This layer may be the source of defects. Especially in places where the pot furnace suddenly narrows (flow hole), vortexes are likely to form, and substances dissolved in each independent vortex may differ, leading to temporary glass layers that may form stripes at their boundaries.

Due to changes in batch materials, it is almost impossible to avoid glass with stripes (at least temporarily). This also affects other properties of the glass and can cause problems in the forming operation.

3.Stripes Caused by Volatility of Materials and Dust

Both phenomena have adverse effects and can be the roots of defects. This situation mainly occurs in low-alkali borosilicate glasses, as boron oxide (which is easy to volatilize) and high temperatures are required for glass melting, making it prone to the loss of alkaline substances from the glass liquid surface in the furnace, resulting in the glass being rich in silicate. If this glass is introduced into the glass flow system, it can create stripes.

  1. Changing the composition of the glass, although local, can lead to obvious stripes over a large surface of the melt.
  2. The formation of slag on refractory materials above the glass liquid level. This not only causes stripes in the glass but also corrodes the refractory material itself.

Generally speaking, this type of stripe can be subdivided by category and only exists in a portion of the circumferential cut. It appears as vertical stripes on the bottle and exhibits a shape resembling “nodules” in the circumferential cut.

Circumcision shows a nodule-like shape
Circumcision shows a nodule-like shape

In the feeder channel, at lower temperatures, the erosion of glass and refractory materials is slower. However, the glass liquid still has an impact on refractory materials, and after erosion, it is more challenging to diffuse due to the lower temperature. However, when the temperature in the feeder channel changes, it flows along the bottom of the feeder channel into the glass product. In such stripes on the same group of products, the basic shapes are essentially the same, forming “cat’s claw”-shaped stripes.

The solution to this type of stripe is to add a stirring device in the feeder channel.

Aluminum-containing stripes (including zirconia) and corundum-containing stripes
Aluminum-containing stripes (including zirconia) and corundum-containing stripes

Additionally, in the feeder channel structure, increasing the intersection angle radius between the feeder channel wall and the bottom, reducing dead angles to avoid the formation of slow-flowing “dead material,” enhancing insulation at the bottom of the feeder channel, increasing the temperature of the bottom glass liquid, thereby reducing its viscosity and increasing its flow and diffusion.

An advanced structure abroad is the discharge device on the feeder channel.

Stripes caused by improper melting
  1. The unstable furnace temperature, sometimes high and sometimes low, can cause movement in the stagnant layer (or slow-moving layer) at the original pool bottom when the temperature rises. Due to differences in composition and viscosity, stripes may form, presenting a plate-like appearance.

Due to temperature changes, different compositions of retained materials in the furnace may mix with the majority of glass in the pot furnace, also resulting in stripes.

Changes in furnace operation, such as significant changes in discharge quantity, unbalanced combustion on one side of the furnace, drastic changes in melting electrodes or bubbling, and uneven glass flow in the pot furnace, can lead to stripes.

Siliceous streaks contain unmelted quartz sand particles with high surrounding compressive stress
Siliceous streaks contain unmelted quartz sand particles with high surrounding compressive stress
  1. Stripes caused by improper furnace atmosphere

The furnace atmosphere refers not only to the composition of gases in the molten space but also the partial pressures of various gases, including the temperature of gases. The impact of the furnace atmosphere on the glass melt is significant. As stated by Noguchi, “The role of the furnace atmosphere is almost equivalent to one component in the batch, and it must be precisely controlled like the batch itself.” This is especially crucial for color glass and sulfate glass, which are highly sensitive to fluctuations in the reducing degree of the flame. Keeping temperature conditions constant is also essential, as each temperature corresponds to an equilibrium state and must not be disturbed. Temperature fluctuations leading to transitional states inevitably result in uneven outcomes.

Some glasses contain multivalent cations or sulfides, selenides, etc., which can change color through redox reactions. The furnace atmosphere’s influence may lead to the formation of colored stripes.

The reasons for brown stripes are diverse. In addition to the reduction of glass and selenium, iron in crushed glass or iron parts in the furnace structure entering the glass can contribute to brown stripes. When changing glass color varieties, failure to clean the old glass liquid thoroughly, improper positioning of small furnaces, combined with the reduction of sulfate-containing clarification, and the introduction of carbon debris in crushed glass, may increase the sulfur content in the glass, causing it to color. The occurrence of amber-colored stripes and bands in the glass is similar to this situation.

Stripes formed by adding crushed glass

In general, adding crushed glass to the batch material is smoother than melting without crushed glass. However, crushed glass lacks some alkali, which results from the evaporation of alkali in the original batch material. This difference should not exceed 0.2% Na2O to avoid uneven phenomena in the melt. Therefore, the surface of crushed glass tends to absorb components from the air rapidly, especially moisture. In terms of composition, the surface of crushed glass is theoretically different from the interior, forming layers (stripes) separated from the surrounding melt after melting. Attention should be paid to preventing differences in the glass composition between crushed glass and the furnace’s glass. Regularly analyze the composition of externally purchased crushed glass and adjust the glass formula based on the analysis results.

In long-term production practice, people have gradually mastered the technology of “recycling” crushed glass in glass melting. In the past, adding some sodium carbonate to the batch material could use 30-35% crushed glass without harm. Today, successful melting has been achieved in large-scale production trials using 66% to 100% crushed glass, without affecting many important properties of glass, such as flexural strength, impact strength, uniformity, E-modulus, thermal expansion, resistance, etc.

Stripes caused by unstable discharge quantity

In production, the most common occurrence of stripes is after changing products. When the product changes and the flow rate of the glass liquid changes, it is essential to strictly control the rate of change. Especially when the flow rate of the glass liquid needs to increase, it is better to gradually increase the machine speed. Because when the discharge quantity increases, especially when the glass liquid level fluctuates significantly, the high-viscosity melt corroded from both sides of the pool wall flows towards the middle of the melting pool, quickly entering the production flow and forming stripes. When the discharge quantity increases, due to the influence of the glass’s own viscosity, the flow of the original pool bottom is slow or the non-moving layer’s speed increases or fluctuates, resulting in stripes.

The solution is to gradually increase the machine speed and strengthen control measures to stabilize the melting conditions in the furnace.

Another situation is that after changing the product, the flow rate of the glass liquid may decrease. This is generally due to a decrease in the product weight, an increase in the forming temperature requirements, and a need to increase the temperatures of the working section and feeder channel. Due to the temperature increase, the stagnant materials in the working section and feeder channel are driven by the glass liquid flow, mixed into the forming glass flow, and produce stripes. The solution is to gradually increase the temperatures of the working section and feeder channel slowly before changing the product. Strengthening control measures and stabilizing the melting conditions in the furnace.

Compared to stripes formed by differences in batch materials, these stripes tend to be smaller stress levels in the circumferential cut and are not widely distributed.

Stripes formed by the interaction between crystallization and “stones” and the surrounding melt, both related to the shift of solubility equilibrium.
Stripes caused by refractory materials

Refractory materials are non-glass materials that the glass melt contacts, except for the furnace atmosphere. Their resistance is limited, and they will slowly dissolve into the melt, becoming the source of stripe formation.

Stripe formation caused by slagging on the upper structure of the furnace

When the upper components of the furnace are attacked by alkali in the batch material dust, it can also cause slagging, leading to stripes. If the slagging situation is normal, its harm will not be too significant. However, if it develops to exceed the eutectic point and locally softens, forming slag that drips into the glass melt, it may generate a large number of stripes.

Sometimes, the kiln lining of a glass melting furnace may exhibit stalactite-shaped and other forms of slag due to inadequate sealing of joints. The inadequate sealing of joints may be caused by imprecise dimensions of refractory bricks (protrusion, bending, deformation due to uneven firing, inconsistent wedge angles, etc.). It could also result from insufficient attention during construction, improper use or absence of mortar. Additionally, certain types of bricks are more susceptible to erosion by molten slag.

Inappropriate heating of the kiln lining can also cause originally tight joints to open, allowing gas to pass through.

Proper selection of brick sizes and a well-calculated heating curve (expansion due to temperature gradients in the thickness direction of the kiln lining) may, on the contrary, bring about the tight sealing of opened joints. It is recommended to carefully inspect the kiln lining of the melting furnace every 4 to 5 months, repairing severely eroded sections.

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