I recently had the chance to visit a factory making glass bottles and I did learn one thing or two. So let me share what I learned visiting Verallia in Oiry, Champagne.
In many regards, glass making is very similar to baking. First, you need to follow a recipe mixing raw materials and adapting quantities depending on the quality and colour you wish to achieve.
Glass is one of the most easily recycled materials as all of it can be used to make a new piece. Used glass is broken into small pieces to serve as raw material it is called calcin (calcin in french). When melted again to be shaped into new glassware it is mixed with other ingredients to adjust quality and colour. But if cullet usually represent 80 to 94% of the raw materials it can only produce coloured glass. The factory I visited only does 3 colours. In their process, the colour is achieved by mixing raw materials in the oven. The oven is working and kept continuously at 1550°C. As the oven is never emptied, the colour change is gradual and slow. It takes a bit more than 24 hours to go from one colour to the next without residue from the previous shade.
To achieve and maintain a temperature of 1550°C, at which glass melts, it is fueled with natural gas and fuel. The temperature can also be boosted with 1500kW electric shocks by putting electrodes in the molten glass. Surprisingly liquid glass, unlike solid glass, is a conductor of electricity.
Such ovens have a lifespan of 12 years and are only stopped and emptied twice, once midlife for about 90 days to be cleaned and inspected, and one last time after many years of intense use. In Oiry's factory, it is 560 tons that are melted into the oven each day and 50 000 bottled are produced from it. (Meaning 60 million bottles a year!)
The molten glass, once leaving the oven is blown into a first mould to get pre-shaped, then into a second to get its final shape. Each mould needs to be oiled manually every 15 to 30 minutes to preserve it from the intense heat. Once out of the second mould, bottles are placed on a ventilated pad to cool them down and help them keep their shape before being put onto a treadmill. They will pass through an annealing lehr, inside which the temperature of 550°, to solidify them and cool them down progressively.
Last but not least, the bottles are undergoing strict quality control. Many aspects are controlled such as the colour, the quality, the size and shape and more. As Oiry's factory mainly produces bottles destined for sparkling wines and Champagne (it is located inside the Champagne wine region) bottles are also tested for pressure.
A standard Champagne bottle weight 835g and must resist a pressure of 23 bar to ensure the safety of those handling the preciously filled bottles.
Although the changes they induced are sometimes complicated and costly (Champagne) bottles can come in many different shades, shapes, sizes and patterns.
More (in french) on Verallia's website.