Soda–lime glass

Soda-lime glass, also known as soda-lime-silica glass, is the most common type of glass in the world. It accounts for approximately 90% of all manufactured glass. This material is essential because it is transparent, inexpensive, and chemically stable. You can find it in many everyday items, such as windowpanes and containers for food or beverages.

Because it is relatively hard and extremely workable, it is a preferred material for many industries. It is even ideal for recycling since it can be resoftened and remelted numerous times.

The production of this glass begins with a mixture of raw materials called a batch. This batch typically includes silica, soda, and lime, along with materials like dolomite or aluminium oxide. Sometimes, small amounts of fining agents, such as sodium sulfate or sodium chloride, are added. These materials are placed into a glass furnace where temperatures can reach up to 1675 °C.

To keep costs low, manufacturers often use inexpensive minerals like sand, trona, and feldspar instead of pure chemicals. If the raw materials contain iron oxide, the resulting glass will appear green or brown.

To understand how it works, one must look at the chemical roles of the ingredients. Pure silica, or silicon dioxide, is the main component, making up about 70% to 74% of the weight. However, pure silica has a very high melting temperature of 1723 °C. It also has a high viscosity, which makes it difficult to shape. To simplify processing, manufacturers add "soda," or sodium oxide. This acts as a flux, which lowers the melting temperature of the silica from 1580 °C.

Soda also helps the mixture soften as it heats, starting at temperatures as low as 700 °C.

While soda makes the glass easier to melt, it creates a new problem. Sodium oxide makes the glass water-soluble, meaning it could dissolve in liquid. To fix this, manufacturers add "lime," which is calcium oxide. This addition provides better chemical durability. Other substances like magnesium oxide and alumina are also added to increase this durability. These ingredients ensure the glass remains stable even when it holds liquids for a long time. This careful balance of chemistry allows the glass to be both easy to make and tough enough to use.

Technically, soda-lime glass is divided into two distinct categories: flat glass and container glass. Flat glass is used for windows and is produced using the float process. Container glass is used for bottles and jars and is made through blowing or pressing.

These two types have different chemical compositions to suit their specific jobs. Flat glass contains higher amounts of magnesium oxide and sodium oxide. In contrast, container glass has lower levels of silica, calcium oxide, and aluminium oxide. This specific mix gives container glass higher chemical durability against water, which is necessary for storing food and drinks.

The way glass moves and shapes is defined by its viscosity, or its resistance to flowing. As the temperature increases, the viscosity of soda-lime glass increases steadily. This allows workers to perform operations with increasing precision. The glass becomes readily formable when its viscosity reaches 10^4 poises, which usually happens around 900 °C. When the viscosity is less than 10^8 poises, near 700 °C, the glass undergoes steady deformation. Even after it appears to have hardened, it can be annealed. This process removes internal stresses by holding the glass near 500 °C for about 15 minutes.

Soda-lime glass is often used instead of chemically pure silica, known as fused quartz. While fused quartz has excellent resistance to thermal shock, its high melting point makes it difficult to work with. Soda-lime glass provides a much more practical alternative for mass production. Its ability to be recycled and its predictable physical properties make it a cornerstone of modern manufacturing. From the windows in our homes to the bottles in our kitchens, this material is a vital part of our daily lives.