Much like the viscosity of honey, glass is liquid when heated at temperatures approximately from 1400-1600˚C. Unlike water, glass has no melting or freezing point, just a difference from stiff solid to liquid mass as and when the temperature is increased.
This ‘variable viscosity’ makes glass special and understanding that property is how glass is made into items of beauty or utility. Sand alone is enough to make glass. However the temperature required to melt it will be much higher. For this reason, soda is added as a modifier. Limestone is added to make the glass more durable.
Until the second half of the 19th-century glass was made by hand gathering, blowing and finishing the neck. It was only after 1850 that an automated process was put in place replacing the older more time-consuming method.
Glass production involves two main methods – the float glass process that produces sheet glass, and glassblowing which produces bottles and other containers.
However, there are places that still make glass using the old process making them richer and more valuable in character. The Manufacturing process begins with the raw materials being automatically mixed and fed into a furnace. Where they are super-heated and fused. This molten glass or ‘gobs’ is then poured into different machines designed for two methods.
The Blow & Blow Method
Heated liquid glass, ‘gobs’, are poured into a mould called the parison or the blank. A puff of air blown down into the base of the blank mould pushes the liquid glass to form the neck. A second blast of air is then applied through the already formed neck pushing the liquid into the walls of the parison mould. This mould is then transferred to the final mould. Where the glass is reheated to be able to take the final moulds shape. This final moulding is usually done with a combination of compressed air or vacuum.
The Press & Blow Method
The gobs are delivered into the parison mould and instead of air a plunger pushes the glass to give shape. The finals stages are the same as the Blow & Blow method. Where the blank shape is transferred into a blow mould for the final shape.
The Westlake Process
Starting by gathering the ‘gobs’ from the furnace the machine mimics the movements and method of a hand blower. The machine forms the parison mould and blows to form the item in a cast iron mould. This is just one of the actions now multiply that by as many as you can as long as it is an even number because the spindles worked in pairs. All these moulds, blowers and spindles along with the gobs and cooled glass move in circular motion around a central column.
The glass is gathered by vacuum into a pair of blank moulds. And each exchange the blank glass to each other’s spindles. The spindles are up when this happens and then they rotate and swing down as air is blown into the parison giving the glass a profile and a distribution. Thereafter another blow of air is required in the wetted mould to take the final shape of the bottom half of the glass container.
The moulded glass is taken by the spindle and transferred to a stemming machine. This machine reheats and stretches the neck to the required specifications. Then the almost finished glass moves to the burn-off stage where oxygen powered gas flames remove the waste glass also referred to as ‘moil’. The finished product is left to cool down and harden.
All this is done at breakneck speeds at times producing more than 75,000 units in a single working day.
The two most commonly methods used are free blowing and mould blowing.
This involves spooling hot liquid glass onto a blowpipe and blowing short puffs of air from one end stretching the glass glob. The method dates back to the 1st Century B.C. and a skilled glassmith can make any shape he or she can imagine. This is, therefore, one of the most popular methods for artistic purposes. In addition to blowing, the glassmith can swing the pipe to cool the glass and manipulate the shape.
During blowing the thin layers of glass cool faster and become more viscous than the thicker layers, allowing the final product to have a uniformed thickness. The ancient method of glass blowing was done with clay blowpipes 30-60cms thick as this was the ideal length to be simple to handle, easy to manipulate and it was reusable several times.
This method involves spooling the blowpipe with liquid glass, placing the glob in an open shut mould and blowing so that the glass takes the shape of the interior of the mould. This made it possible to mass produce glass and encourage the widespread distribution of glass products.
In earlier days the mould was made using wood, however metal is now considered the best material for the mould. Single piece moulds are used for making functional vessels for storage and transportation whereas the multi piece mould is used for more sophisticated surface modelling texture and design.
The basics to modern glass blowing is more or less the same since it first started all those years ago. The process has been, however, streamlined to be more effective. Modern Glassblowing has evolved to use three furnaces.
The first furnace has a crucible of molten glass. It is attached to the area where the raw materials and recycled glass can constantly feed it.
The second furnace is called the glory hole where the glassmith can reheat the article of glass he is working with in between steps.
The third furnace is called the lehr or the annealer and it is used to cool the glass slowly over a period of days to weeks depending on the size of the article.
The Tools of Modern Glass blowing;
- Blowpipe – The now steel pipe that is used to gather glass on one end and blow from the other.
- Marver – Traditionally a flat slab of marble it is now mostly steel and is used to roll the glass.
- The Bench – This the glassblower work station and consists of a place to sit, a place for all the tools and two rails to roll the pipe on while working.
- Blocks – They look like ladles and are used to shape and cool the glass during early stages of working.
- Jacks – They look like large tweezers and are used and are more or less the fingers of the glassmith. How well a glassmith handles this tool is a way to judge the skill level.
- Paddles – These are usually flat pieces of wood or graphite and are used to make the flat ends of the glass articles.
- Straight Sheers – They are used for linear cutting the glass and look like big scissors.
- Diamond Sheers – This is a blade when partially open form a diamond shape that is used for cutting off masses of glass.
The Ribbon Machine
For the high speed manufacture of glass the Ribbon machine was manufactured to increase mass production. The main difference from the Westlake process is that the glass is fed from a furnace onto a straight conveyor belt or ribbon. On one side of the belt you have a continuous chain of blow-heads blowing air.
On the other side you have a continuous rotation of mould that form the shape of the light bulb. The rotation of the spindle ensures the glass is annealed and it is later dropped onto a conveyor belt. The remaining glass on the ribbon gets transferred back to the furnace as cullet. The glass is cooled by water and air during the process. Depending on the bulb size each machine can produce up to 60,000 bulbs per hour.
Flat Glass Manufacture
Flat glass is used in homes, vehicles, public buildings, offices, hotels, shops etc. And the most popular method of production used is the float glass process.
The Float Glass Process
This method of making flat glass is the most popular method used by manufacturers. And requires a great deal of initial capital to start up. These float glass manufacturing plants will work for 24 hours a day the whole year round non-stop for 15 years!
The machine required to produce flat glass was first designed by Sir Alistair Pilkington in 1952. It is now the standardised method of making flat glass. The process line from furnace to flat glass of variable thickness and length can be half a kilometre long and the machine can churn out 6,000 tonnes a week amounting to 6,000 thousand kilometres of glass a year. All this production boils down to a six step process.
Step 1 – Melting and Refining
By far one of the most important steps because this can influence the quality of glass highly. In this furnace the glass is melted at approx 1400-1600˚C is refined and homogenised to ensure the glass coming out at 1100 ˚C onto the float bath is removed of any gas bubbles or insertions. However if the finished product requires a certain look and feel the compositions of the methods can take an altering.
Step 2 – Float Batch
The spout pushes out the glass onto a layer of liquid molten tin. The molten glass is of high viscosity and does not mix with the liquid molten tin literally ‘floating’ on top. This with a little help from gravity creates a perfectly levelled contact surface. The thickness is controlled by the speed at which the glass is spread over the line of molten tin.
Step 3 – Coating
Coatings are added to the glass as a more modern day requirement according to customer specifications. A way of applying involves an area in the line that uses advanced high temperature wielding robots at the cooling ribbon of glass. Another way to vary the optical properties of the floating glass is through a chemical vapour depositor (CVD). This can cover the liquid glass moving beneath with a variety of coatings less than a micron in thickness. They can be used to bounce out UV rays or to reflect visible and infrared wavelengths.
Step 4 – Annealing
To further ensure that the glass has no unwanted insertions or bubbles that can occur when cooled unevenly. A long furnace, the ‘lehr’ cools the glass at varying temperatures best suited to keep the glass perfect to customer specs. The later technology developed included a sensor that detects the stress level of the glass and in turn controls the temperatures of the lehr.
Step 5 – Inspection
Occasionally a grain of said, a tremor or a single bubble will create a lower quality output and with improved technology automated highly sensitive methods are used to pick up these glitches. The automated inspection method can control the cutters and make 100 million measurements a second across the ribbon of glass.
Step 6 – Cutting to Order
The final step is also dictated by a computer program where the glass is cut to order and sold to the client. Any waste material that has been cut out due to imperfections are fed right back into the furnace as cullet.