Glass is a solid-like material that can be shaped into a variety of shapes. It’s a versatile material that can be poured, blown, press-formed and moulded into plenty of different products.
Historically, glass was made from natural raw materials like sand and seashells mixed with silica, lime, and soda. Today, different processing techniques and added chemicals are used to create various types of glass for many applications.
The Raw Materials
The raw materials used to make glass include silica sand, soda ash, dolomite and limestone. These are all fused together at a high temperature before they can be further processed into various forms of glass.
The three main components are weighed and then sorted into batches before being delivered to a glassmaking facility. This mix is known as a batch recipe and varies from manufacturer to manufacturer depending on the type of glass, desired quality, raw material purity/availability and furnace design.
The mixture of ingredients can also contain broken pieces of glass called cullet, which can be recycled and reused as part of the manufacturing process. This is a major energy saver in the manufacturing process as it allows broken glass to be used for the manufacture of new glass.
The Furnace
The furnace is a critical part of glassmaking. It uses high energy to heat the raw materials and then fuses them together to form a glass mixture.
Most glass melt furnaces use natural gas as their fuel source. However, they can also run on oil or propane.
Glass melting furnaces are designed to minimize the amount of waste heat lost from the melting process. They also feature a flue-gas heat recovery system to increase energy efficiency.
Regenerative furnaces, which use refractory bricks called checkers, pre-heat the combustion air by passing it over hot regenerator bricks before it enters the melter. This helps to maximize energy efficiency while reducing CO2 emissions during the process.
The Press & Blow Method
The Press & Blow Method is the process of creating bottles and jars from molten glass. This process uses a plunger and compressed air to form the gob into a pre-form shape called a parison.
The parison is then transferred to a blow mould. A further blast of compressed air is used to blow the parison into its final shape.
This is a type of bottle forming method that is used by early automatic and semi-automatic glass machines. These machines are capable of producing very large quantities of bottles.
The Finishing Mould
The finishing mould used in glassmaking produces the body, shoulder and neck of a bottle. It is a relatively simple process in which a parison (smaller than the final size of the container) is formed in the press and blown to form the bottle.
The finished bottle is then either tooled or ground top. The finished bottle varies greatly in appearance, but usually is quite similar.
One mold type that has a distinctive and very pronounced shape is the “key mold.” This mold typically has a very round/oval base flange that archevely fits into the other forming a slight depression in the center of the base. This was a common feature on keyed base bottles produced from the 1850s through the early 1900s.
The Annealing Lehr
In glassmaking, a special annealing process is required to relieve internal stresses that have been introduced into the glass by quenching. This cooling process helps make the glass more durable.
The annealing process is typically carried out in a long oven, called a lehr. It consists of multiple heating zones that are designed to remove a specific amount of stress.
This enables the glass to be cut and manipulated in a consistent manner. Depending on the product, the lehr may be equipped with a conveyor belt or rollers.
The endless belt 20 passes over a top discharge roll 140 mounted in pillow blocks at the front of the lehr and around a drive roll 142, a snub roll 144 and return rolls commonly designated at 146 and 148. The lehr also includes an adjustable slot roll 158 and a return charge roll 160 to properly align the return flight of the belt during its travel through the cooling sections 14 and 16.
