Chemistry of Glass "Part 5": Glass melting.

In a combustion chamber, the composition is melted at 1500–1550 °C after all of the raw materials are combined according to a predetermined weight ratio. Some processes make up glass melting, and they all take place in a furnace where endothermic reactions are most common:

1- Silicate formation:

The composition within the oven dehydrates as a result of the high temperature; carbonates, sulphates, nitrates, etc. breakdown and release gases (CO2, SO2, SO3, etc.) as a result:

Na2CO3 ======= Na2O + CO2

CaCO3 ======= CaO + CO2

Compounds based on silicates are created as a result of the reactions between the composition components. When a particular temperature is achieved, the liquid phase emerges, and from this point on, the chemical reaction speed accelerates dramatically. Chemical reactions first occur in the solid state.

A glass made of CaCO3, Na2CO3, and SiO2. Around 600°C, CaNa2(CO3)2 is initially produced. The materials are then still solid, and around 870°C, an eutectic is formed. around 813°C, the eutectic melts, and silica SiO2 interacts vigorously with the molten substance.

Following the dissociation of calcium carbonate (CaCO3), lime (CaO) combining with silica to generate lime silicate (CaSiO3), sodium carbonate mixes with silica to form (Na2SiO3). All of the silicates melt around T=1100- 1200 °C.

Ca+ SiO2 ===== CaSiO3

2- Glass formation:

This stages progress quite slowly and require a lot of heat.

3- Glass refining and homogenization:

The vitreous mass, which exists after the glass has fully formed, is made up of many gaseous bubbles and is neither homogenous nor consistent in terms of its chemical makeup or these physical characteristics.
The most crucial phase in glass melting is refining, which entails getting rid of these gas bubbles. The gases in the balls include CO2, SO2, SO3, O2, and N2. Since refining occurs at a high temperature—the highest in the melting process—the clearance of gases is a result of the temperature increase.

Furnace from HORN Glass Industries, Leonardo glass factory, Bad Driburg

References:

• [General and inorganic chemistry book- M. Shkhashirou- H. Birqdad- Y. Qodsi- University publications. Algeria]
• Glass and Ceramic Technology Course (2021). Professor Khelifa- Department of Materials Process Engineering- University of Mostaganem. Algeria.
• Les matériaux au cœur du processus d'innovation- Clefs CEA No 59.
• Neumann, Florin. "Glass: Liquid or Solid – Science vs. an Urban Legend". Archived from the original on 9 April 2007. Retrieved 8 April 2007.
• Helène Tregouët. Structure et cristallisation de verres d’oxydes simples riches en bore et en terres rares. Chimie-Physique [physics.chem-ph]. Université Pierre et Marie Curie- Paris VI, 2016. Français. NNT: 2016PA066032. tel-01358710.