What is glass?

Glass is made from liquid sand. You can make glass by heating ordinary sand (which is mostly made of silicon dioxide) until it melts and turns into a liquid. You won't find that happening on your local beach: sand melts at the incredibly high temperature of 1700°C (3090°F).

When molten sand cools, it doesn't turn back into the gritty yellow stuff you started with: it undergoes a complete transformation and gains an entirely different inner structure. But it doesn't matter how much you cool the sand, and it never quite sets into a solid. Instead, it becomes a kind of frozen liquid or what materials scientists refer to as an amorphous solid. It's like a cross between a solid and a liquid with some of the crystalline order of a solid and some of the molecular randomness of a liquid.

Glass is such a popular material in our homes because it has all kinds of beneficial properties. Apart from being transparent, it's inexpensive to make, easy to shape when it's molten, reasonably resistant to heat when it's set, chemically inert (so a glass jar doesn't react with the things you put inside it). It can be recycled any number of times.

Major Types of Glass:

• Type I - Borosilicate Glass
• Type II - Treated Soda Lime Glass
• Type III - Soda Lime Glass

The materials used to make glass include approximately 70% sand along with a specific mixture of soda ash, limestone and other natural substances – depending on what properties are desired in the batch.

When manufacturing soda-lime glass, crushed, recycled glass, or cullet, is an additional essential ingredient. The amount of cullet used in the batch of glass varies. Cullet melts at a lower temperature which reduces energy consumption and requires fewer raw materials.

Borosilicate glass should not be recycled because it is heat-resistant glass. Because of its heat resistant properties, borosilicate glass will not melt at the same temperature as Soda Lime glass and will alter the viscosity of the fluid in the furnace during the re-melt stage.

All of the raw materials for making glass, including cullet, are stored in a batch house. They are then gravity fed into the weighing and mixing area and finally elevated into batch hoppers that supply the glass furnaces.

Ingredients for making glass

Ingredients for making glass
There are a few main ingredients used in glassmaking plus many additives for particular properties.

1. Sand or Silica
The main ingredient of glass making is Silica, which as a very high melting point of over 2,000 deg C. It is possible to make highly specialised products from just Silica such as halogen lamps or one of the materials in fibre optic cables. This is because it has such a high melting point and is more durable than regular glass.

2. Sodium Carbonate
Sodium Carbonate lowers the melting point of Silica to about 1,000 deg C and is therefore added to make the process more efficient. The Sodium Carbonate will, however, cause the polished glass to be water-soluble, which is not desirable in glass making. Sodium Carbonate was initially found in the ash of individual plants – soda ash – but is now commonly produced from table salt.

3. Lime or Calcium Oxide
Calcium Oxide is extracted from limestone and counters the effects of the Sodium Carbonate, making the glass non-soluble in water. Magnesium Oxide and Aluminium Oxide can also be used to enhance the properties of the glass.

These ingredients, along with other parts, are carefully weighed and mixed to form ‘batch’. The batch is placed in a furnace with cullet (recycled glass) where it melts and can be used for the production of glass products.

4. Other Additives
Several different ingredients can be added to the glass to change the properties of the finished product, for example:

Lead – to make leaded or crystal glasses. The lead has better reflective properties, and therefore the lens seems to ‘sparkle’. This kind of glass also lends itself to be cut to form decorative patterns on the glass.

Boron – this changes the thermal and electrical properties of the glass and is used to make Pyrex glassware which can withstand extremes of heat and cold.

Lanthanum Oxide – this has excellent light-reflective properties and is used to make high-quality lenses in glasses. Thorium Oxide was formerly used, but due to its radioactivity, this is no longer the case.

Iron – used to absorb infrared energy in, for example, heat-absorbing filters in movie projectors.

5. Colour Additives A range of additives can be used to make glass into different colours. Although glass appears to be clear, it is green. It is tough to make visible glass. Metals and oxides can be used to colour glass.

Additive(s) Colour Comments
Iron Oxide Blue-green Used for beer bottles
Iron Oxide & Chromium Richer green Used for wine bottles
Sulphur, carbon & iron salts amber, yellow Can be almost black
Manganese Removes green Can give a purple colour
Selenium Red or Pink Copper and gold both produce similar colours
Cobalt Blue Tiny amounts required
Tin oxide, antimony & arsenic White opaque Used to mimic porcelain
Copper Oxide Turquoise Nickel Blue, violet or black Chromium Dark green Titanium Yellow / brown Uranium fluorescent yellow or green Not radioactive in the glass but Can be if ground down.
Silver Orange or red The way the glass is cooled affects the colour.

How Are Glass Bottles Made

Although traditional glass-blowing and blow-moulding glass methods are still used by artists and for custom applications, most glass bottle manufacturing is an automated process. The development of glass bottle machining peaked with the advent of feed and flow machines, which enabled manufacturers to generate larger production runs than was previously possible. Glass production is broken down into two general categories: container production and sheet production. Bottle machining is part of glass container production.

Glass Bottle Manufacturing Process
The glass bottle manufacturing process takes place at a glass container factory in multiple steps.

Hot End Processes
The first stage of the glass bottle production process begins with the hot end processes, which typically employ high amounts of heat to produce and shape a glass container. A furnace is first used to mould molten glass, which fed to the stove as glass feedstock. Soda-lime glass stock accounts for the majority (around 90 per cent) of the types of glass products and is typically comprised mainly of silica, with about 10 per cent each of calcium oxide and lime. Small amounts of aluminium oxide, ferric oxide, barium oxide, sulfur trioxide, and magnesia also account for about 5 per cent of soda-lime glass. Before melting, cullet (recycled glass) is added to the stock, accounting for anywhere between 15 and 50 per cent of the final glass composition.

Once the stock has been fed into the furnace, temperatures inside can be as high as 1675 degrees Fahrenheit. Next, one of the two method forming methods is applied: press-and-blow or blow-and-blow.

Press-and-Blow Molding Glass
Press-and-blow formation takes place in an individual section (IS) machine and is the more commonly used method in glass-container production. IS machines have between five and 20 articles, all identical, which can each carry out the glass-container forming process wholly and simultaneously. The result is that five to 20 containers can be produced with one machine at the same time.

When the molten glass reaches between 1050 and 1200 degrees Celsius, it is said to be in its plastic stage, and it is during this phase that press-and-blow formation begins. A shearing blade is used to cut and shape the glass into a cylindrical shape, called a gob. The cut gob falls and, using gravitational force, rolls through the appropriate passage to reach the moulds. A metal plunger presses the gob into the blank mould, where it assumes the mould’s shape and is then termed a parison. Next, the parison is moved into a final mould, where it is blown into the mould to assume its final dimensions. This process is typically used for wide-mouthed glass containers, but can also be used to manufacture thin-necked bottles.

Blow-and-Blow
Like press-and-blow formation, blow-and-blow takes place in an IS machine, where a gob is released during the plastic stage and moved along to the moulds. However, in blow-and-blow formation, the gob is forced into the blank mold using compressed air to push the gob into place. The gob, now a parison, is then flipped into a corresponding final mold where it is blown again, to form the interior side of the glass container. Glass bottles of varying neck thickness can be made using blow-and-blow formation.

After formation, bottles often undergo internal treatment. This process makes the inside of the bottle more chemically-resistant, an essential factor if the bottles are intended to hold an alcohol or other degrading substances. Internal treatment can take place during formation or directly after, and typically involves treating the jars with a gas mixture of fluorocarbon. Glass containers can also be treated externally, to strengthen the surface or reduce surface friction.

Annealing Glass Bottles
Once the formation is complete, some bottles may suffer from stress as a result of unequal cooling rates. An annealing oven can be used to reheat and cool glass containers to rectify tension and make the bottle stronger.

Cold End Processes
At this stage in glass production, the bottles or glass containers are inspected and packaged. Inspection is often done by a combination of automated and mechanical control to ensure the integrity of the final product. Common faults include checks (cracks in the glass) and stones (pieces of the furnace that melt off and are subsequently worked into the final container), which are essential to catch because they can compromise the component. Packaging methods will vary from factory to factory depending on the specific type of bottle and the size of the production run.