How Is Glass Made (Part One)
Glass making is a very ancient craft. Archaeological data proves that glass making can be traced back to before 2500 BC. Making glass was once a very rare and precious art, but now it has evolved into a common industry. Glass products are used for commercial and household purposes as containers, insulators, reinforcing fibers, lenses and decorative arts. Although the materials used to make glass are different, the process is roughly the same, which will be explained as below.
Use stove or kiln
Prepare silica sand. Silica sand, also called quartz sand, is the main material for making glass. If you want to make a transparent glass sheet, you need glass without iron impurities, because the presence of iron will make the glass green.
If you are dealing with ultra-fine particles of silica sand, please wear a face mask, otherwise it may cause throat and lung discomfort if you accidentally inhale it into your body.
You can buy silica sand online, usually the price is not too expensive. If you want to operate on an industrial scale, specialized retailers can provide extremely favorable prices for large orders.
If you cannot find silica sand that does not contain iron impurities, you can add a little manganese dioxide to offset the coloring effect caused by iron. Alternatively, if you want to make green glass, let the iron stay in it.
Add sodium carbonate and calcium oxide to the silica sand. Sodium carbonate (commonly known as washing soda) reduces the temperature required for commercial glass production. However, sodium carbonate causes water to flow through the glass, so calcium oxide or lime must be added to make the glass insoluble in water. In addition, you can also add magnesium oxide and/or aluminum oxide to make the glass more durable. Generally, these additives account for no more than 26 to 30% of the glass mixture.
Add other chemicals according to the intended use of the glass. The most common additive for decorative glass is lead oxide, which can make crystal glassware sparkle, increase softness, make the glass easier to cut, and lower the melting point. Spectacle lenses may contain lanthanum oxide because of its refractive properties, while iron helps the glass absorb heat.
Lead crystal glass (artificial crystal) contains up to 33% lead oxide. However, the more lead oxide is used, the better technology is required to shape the glass melt. Therefore, many artificial crystal manufacturers choose to use a lower lead content.
Add chemicals to achieve the desired color (if any) of the glass. As mentioned above, the iron impurity of the quartz sand is the green color of the manufactured glass, so you can add iron oxide to increase the green hue, and copper oxide can also play the same role. Sulfides can produce pale yellow, amber, brown or even black hues, depending on how much carbon or iron is also added to the glass mixture.
Put the glass mixture into a heat-resistant crucible or holder. The container should be able to withstand extremely high temperatures in the kiln. Depending on the additives you use, the melting point of the glass mixture is in the range of 15000 to 2500 degrees Celsius. Metal hooks and rods should also easily control the container you use.
Melt the mixture into a liquid state. Commercial silica glass can be put into a gas stove to complete, while special glass needs to be manufactured using electric melters, crucible furnaces or kilns.
Quartz sand without additives will turn into glass at 2300 degrees Celsius (4172 degrees Fahrenheit). Adding sodium carbonate (soda, washing soda) can reduce the temperature required to make glass to 1500 degrees Celsius (2732 degrees Fahrenheit).
Make the glass melt uniform and remove bubbles. This means you need to stir the mixture to a fixed consistency and add chemicals such as sodium nitrate, sodium chloride or antimony oxide.
Shape the molten glass. You can shape glass in one of seven ways:
Pour the molten glass into the mold and let it cool. The ancient Egyptians used this method, and this is how many lenses are shaped today.
You can concentrate a large amount of molten glass on the end of a hollow tube, while rotating, while blowing air from the other end of the tube. The air entering the hollow tube, the action of gravity, and the tools used by the glass blower shapes the glass.
Pour the molten glass into a tank of tin bath, float on the surface of the relatively dense tin liquid, and slowly shape it under the action of pressurized nitrogen. The glass produced by this method is called float glass and has been used to produce flat glass since the 1950s.
Let the glass cool down. Do not place the glass in a place where it will be disturbed, dust, leaves, or water droplets. Exposing a cool substance to hot glass can cause the glass to burst.
The glass is strengthened by heat treatment. This process is called annealing and removes any stress points formed on the glass during the cooling process. Glass that has not been annealed is significantly weaker. Once this process is completed, the glass can then be coated, laminated, or other treatments to improve strength and durability.
The precise temperature of annealing varies, depending on the exact composition of the glass, as low as 399 degrees Celsius (750 degrees Fahrenheit) and as high as 538 degrees Celsius (1000 degrees Fahrenheit).  The cooling rate of glass is also different. Generally, the cooling rate of a larger piece of glass must be slower than that of a smaller glass. Investigate the correct annealing method before starting annealing.
Another related process is tempering. The polished shaped glass is heated to at least 600 degrees Celsius (1112 degrees Fahrenheit) in an oven and then rapidly cooled (quenched) with air under high pressure. Annealed glass will become fragments under a pressure of 6000 psi (421.94 kg/cm2), while tempered glass will become fragments under a pressure of no more than 10,000 psi (703.23 kg/cm2), usually about 24,000 psi (1687.76 kg/cm2).
To be continued in Part One…