Bubbles are visible gas inclusions in glass, which not only affect the appearance quality of products but also affect the transparency and mechanical strength of glass. The size of the bubbles in the glass ranges from a few tenths of a millimeter to a few millimeters. According to the size, the bubbles can be divided into gray bubbles (diameter <0.8 mm) and bubbles (diameter>0.8 mm).
The formation of bubbles may be due to chemical reactions occurring in various stages of the melting process, as well as physically dissolved gases, including gases in the interstices of the refractory material or gases brought into the glass when the melt is bubbling, and some easily dissolved gases. Bubbles are caused by vapors of volatile components.
The gases in the bubbles may include: O2, H2, H2O, CO2, CO, SO2, H2S, NO2, N2, etc. Air is also a common gaseous inclusion. The deformation of bubbles is mainly caused during the molding process of the product.
The types of bubbles in glass defects can be divided into primary bubbles (residual bubbles in batches), secondary bubbles, outside air bubbles, refractory bubbles, and bubbles caused by metallic iron.
The one time bubble
The main reason for the generation of primary bubbles is poor clarification, and the solution is mainly to appropriately increase the clarification temperature and adjust the amount of clarifier. According to the two ways to eliminate bubbles in the clarification process (large bubbles escape and very small bubbles are dissolved and absorbed), increasing the clarification temperature is conducive to the escape of large bubbles; lowering the temperature is conducive to the dissolution and absorption of small bubbles. In operation, strictly complying with the correct melting system is an important measure to prevent the generation of primary air bubbles.
So what caused it? The common ones are the uneven thickness of sand particles in the batch, the insufficient amount of clarifying agent, the single gas phase of the batch, or the temperature of the batch and broken glass is too low, the melting and clarification temperature is low, etc., will produce a bubble. If the melting atmosphere conditions and pressure conditions are unreasonable, primary bubbles will also be formed. In addition, an unsuitable redox index of the glass may also cause batch bubbles.
The secondary bubble
After the clarification, the glass liquid is in a certain equilibrium state with the gas dissolved in it. At this time, the glass does not contain bubbles, but there is still the possibility of bubbles. For example, during the cooling process of the glass liquid in some systems, due to the change of the partial pressure of some components in the external gas phase, the dissolved gas is no longer in a state of equilibrium, and bubbles or gray bubbles appear in the clarified glass liquid. There is a possibility that bubbles will occur again. For example, during the cooling process of the glass liquid in some systems, due to the change of the partial pressure of some components in the external gas phase, the dissolved gas is no longer in equilibrium, and bubbles or gray bubbles appear in the already clarified glass liquid, because this The bubbles generated during the cooling process are very small, and the viscosity of the glass increases during cooling. Once the secondary bubbles are formed, it is very difficult to remove them.
To avoid the formation of secondary bubbles, it is necessary not only to adopt the correct temperature and pressure system but also to control the special conditions for generating bubbles in the glass.
There are two reasons for secondary bubbles, physical and chemical. If the temperature of the glass liquid after cooling exceeds a certain limit again, the gas dissolved in the glass liquid will decrease in solubility due to the increase in temperature, and very fine and evenly distributed secondary bubbles will be precipitated. This is the case for physical reasons.
The chemical reasons are mainly related to the chemical composition of the glass and the use of raw materials. For example, the glass contains peroxides or high-valence oxides, and the decomposition of these oxides is easy to generate secondary bubbles. Sometimes, there may be incompletely decomposed thenardite in the glass liquid of the glass using Glauber's salt, which gradually decomposes when it encounters the cooling zone, and the formed bubbles cannot be discharged.
The formation of secondary bubbles is closely related to the glass melting process. If the melting process system is not properly controlled, secondary bubbles will be inevitable. The stability of the melting temperature system is directly related to the quality of the molten glass. If the molten glass, which has been cooled, is heated again due to the increase in the temperature of the melting furnace, it is easy for the residual gas dissolved in the glass to form bubbles.
Similarly, when the amount of broken glass increases, the output of the kiln decreases, the machine stops, or the temperature increases due to the decrease in feeding, the melting zone is shortened, and small bubbles, ash, transparent or with a milky white film may be formed. Bubble. If the iron content in the raw material is reduced, or the oxidant is introduced into the batch, the transparency of the glass liquid increases, and the temperature of the glass liquid increases, which may also cause secondary bubbles.
Figure 1 The bubbles in glass defects
The refractory bubbles
When the molten glass and the refractory are in contact, due to the possibility of metasomatism between the two, such as the dissolution of SiO2 and Al2O3 into the molten glass, the balance will be changed, and the original dissolved gas will be released to form bubbles. In addition, there is still a lot of air and adsorbed gas in the pores of the refractory material, especially when firing in a reducing flame, some carbon may be deposited on the surface or in the pores of the refractory material, and these substances will also grow due to oxidation. for bubbles. The gas composition of refractory bubbles is mainly SO2, CO2, O2, and air.
To prevent the generation of these bubbles, the quality of the refractory must be improved. The furnace building material that is not easy to react with the molten glass to form bubbles should be selected for the part close to the molding, to improve the quality of the molten glass. In operation, the working system should also be stabilized as much as possible, such as the temperature system should be stable, and the temperature should not be too high to avoid aggravating the erosion of materials. The stabilization of the glass level is of great significance to reduce the erosion of refractory materials.
The outside air bubbles
There are also many reasons for the introduction of air bubbles. When feeding, air will inevitably be brought into the gaps of powder particles, but even if these air bubbles are formed during melting, most of them will be discharged with the gas decomposed by the batch material or under the action of the clarifying agent, and only a very small amount remains. in glass.
When stirring the optical glass, air bubbles can also be brought in when the stirring paddle is put into the glass liquid. As for the inappropriate material picking operation during molding, or the poor quality of the tools, it is easy to bring in air bubbles. The characteristics of the outside air bubbles are that the bubbles are relatively large, generally more than 2mm, and often appear in the fixed parts of the product, but the distribution is very irregular.
The bubble caused by metallic iron
In the operation of a pool kiln, iron parts, such as kiln components, tools, etc., are inevitably used. Sometimes due to careless operation, it accidentally falls into the glass liquid, gradually dissolves, and colorizes the glass, and the carbon contained in the iron piece interacts with the residual gas in the glass to form brown streaks, or there are traces of brown streaks attached, or even possible. Filled with dark iron compounds, they range in color from brown to dark green. There is also a special case where the bubbles contain a small piece of metal or its oxide, which can be seen under the microscope as brown to vivid tomato-colored iron silicate crystals.
To prevent the generation of such bubbles, the powder must be carefully prepared, the inspection of the kiln must be strengthened, and the quality of the forming tools, especially the parts immersed in the glass liquid, should be good, and the use method should be appropriate.