How Glass Bottles Are Made

The formula for making glass is as simple as silica sand, soda ash, limestone, and a whole lot of heat. When combined according to centuries-old proportions and melted, the glass that forms from this recipe is known as “water glass.”

Now, if you used molten “water glass” to make a water glass, let it cool, and then filled it with some nice cold H2O, you would be in for a most interesting surprise. “Water glass” is soluble in water!

Through centuries of innovative chemistry and constant creative thinking today’s glass is a true marvel capable of performance characteristics that far exceed water impermeability. This blog will shed some light (and color!) on the basics of making one of the purest and most useful packaging materials around.

The raw materials for making modern glass are silica sand and soda ash (familiar so far?), limestone (added to lower the temperature at which the mix melts), and frequently magnesium oxide and aluminium oxide (to provide for better chemical durability).

Another basic and highly-essential ingredient is cullet. Cullet is recycled glass that helps reduce the amount of raw materials and energy needed to make new glass. A glass batch may consist of 25 to 60 percent cullet by volume. Cullet is crushed glass, generally of the same composition as the mineral mixture, and comes from regrind from previous production runs and recycling sources. It is included because its characteristic of melting in the furnace before the other minerals helps accelerate the batch’s reaction into molten glass. It is an important energy-savings ingredient that both lowers the amount of energy needed for the melting process and in turn helps reduce greenhouse gas emissions. The resulting glass that is produced typically contains about 70 to 74 percent silica by weight.

Every ingredient is carefully measured -- by weight -- and combined into a batch mixture. It is at this stage that additional ingredients may be added to create various colors of glass.

QUICK FACT! The purer the silica sand is, the lower its iron content will be, which in turn means more control over the ultimate color.

Glass that has virtually no color is called colorless glass. Colorless is the preferred term instead of the word clear. Clear refers to a different value: the transparency of the glass and not its color. The proper use of the word clear would be in the phrase “clear green bottle.”

Aquamarine colored glass is a natural result of the both the naturally occurring iron found in most sands, or through the addition of iron to the mix. By reducing or increasing the amount of oxygen in the flame used to melt the sand, manufacturers can produce a more bluish-green color or a greener color.

Opaque white glass is commonly called milk glass and sometimes called Opal or white glass. It can be produced by the addition of tin, zinc oxide, fluorides, phosphates or calcium.

Green glass can be made through the addition of iron, chromium, and copper. Chromium oxide will produce yellowish green to emerald green. Combinations of cobalt, (blue) mixed with chromium (green) will produce a blue green glass.

Amber glass is produced from the natural impurities in sand, such as iron and manganese. Additives that make Amber include nickel, sulfur, and carbon.

Blue glass is colored with ingredients like cobalt oxide and copper.

Purple, amethyst and red are glass colors that are usually from the use of nickel or manganese oxides.

Black glass is usually made from high iron concentrations, but can include other substances such as carbon, copper with iron and magnesia.

Whether the batch is destined to be clear or colored glass, the combined ingredients are known as the batch mixture and is transported to a furnace and heated to a temperature of about 1565°C or 2850°F. Once melted and combined, the molten glass passes through a refiner, where trapped air bubbles are allowed to escape and then it is cooled to a uniform yet still formable temperature. A feeder then pushes the liquid glass at a constant rate through precisely-sized openings in a heat-resistant die. Shear blades cut the emerging molten glass at the precise moment to create elongated cylinders called gobs. These gobs are individual pieces, ready for forming. They enter a forming machine where, using compressed air to expand them to fill a die of the desired final shape, are made into containers.

Newly formed containers are H-O-T and naturally start cooling rapidly. Left unchecked, this rapid cooling will occur unevenly -- the outside cools more rapidly than the inside -- and result in stress within the glass that can lead to both immediate failure or failure long after they have been formed. That’s why these newly-made containers pass through a special type of kiln called a lehr. The lehr precisely reheats the containers to a point just below the solidification point of glass, then very gradually and evenly cools them to room temperature, relieving stresses. This process is called annealing. Anneal cooling rates are generally tens of degrees Celsius per hour for a typical glass container. Annealing is a key reason why glass is such an incredibly versatile and strong packaging material perfectly suited for so many applications.

Once cool, the glass containers undergo rigorous final inspections to ensure the highest quality. Scanning equipment and technicians review the containers and those that do not meet stringent design specifications are remelted (cullet!) and the process begins again.

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