Stages of Glass Manufacturing and Production
The production of glass and the art of glassmaking have an ancient history in Iran. According to archaeological research, glass production dates back to more than 2,500 years before Christ. Iran has a long-standing tradition in the glassmaking industry, and this craft is considered one of the country’s oldest industries. Glass products are used both in industry and in households as tableware and decorative items. Although the raw materials used for different types of glass vary to some extent, the overall glassmaking process is almost the same. The process of producing glassware and crystal is nearly identical worldwide. This article reviews the general stages of glass production. In general, three methods are used to produce glass and crystal: traditional, semi-automatic, and fully automatic. The methods used in Iran are traditional and semi-automatic. In the following, join Jahan Bolor as one of the active centers in traditional glass production.
Review of Raw Materials in the Glass Manufacturing Process
The production of glass begins with silica as the most important raw material, which is also known in the industry as quartz. The purer the silica used in glassmaking, the clearer and higher quality the final glass will be. If silica contains iron impurities, the produced glass tends to have a greenish color.
Silica is a hazardous material, and if inhaled through the throat or lungs, it can cause respiratory problems. For this reason, safety precautions must be strictly observed when working with silica during the production process. Silica is an inexpensive material and is widely available in the industry. If silica contains a high amount of iron impurities, manganese oxide is added to counteract the green coloration.
Use of Mineral Materials in Glass Production
Sodium carbonate and calcium oxide are among the raw materials used in glass production. These materials are added to reduce the melting temperature of glass. The higher the furnace temperature required for glass production, the higher the production cost. Magnesium oxide or aluminum oxide is also used to increase the strength of glass. In general, the percentage of additives in glass should not exceed 30%.
Methods of Producing Colored Glass
By adding metal oxides to glass compounds, different colors can be produced. As mentioned earlier, the presence of iron oxide in silica can cause a green color in glass. Adding metal oxides in varying percentages can create yellow, brown, turquoise, and even black colors in glass.
To color glass, the following metal oxides can be added to the glass melt:
- Iron oxide: Produces light to dark green colors.
- Iron oxide and cobalt: Adding cobalt oxide to iron oxide produces a very light blue color.
- Chromium oxide: Produces green, yellow, or blue-green colors.
- Copper oxide: Usually produces green or turquoise; in some cases, it can also produce red.
- Copper oxide and nickel: Can produce blue, purple, and even black colors in glass.
One of the most important metals used to create color in glass is gold. This precious metal is used in very small amounts (about one per thousand) and can produce a beautiful ruby color.
Adding calcium in different percentages results in yellow shades with varying tones. If selenium is added to the primary glass compounds, red and pink colors can be produced. Combining calcium and selenium creates different shades of orange in glass. Adding titanium produces yellow-brown glass. Uranium (0.1% to 2%) can be added to produce fluorescent yellow or green glass.
An important point when adding extra materials to glass to achieve color or desired quality is ensuring uniform grain size of the added materials.
Glass Production Methods
01. In the first stage, the raw materials used to produce glass and crystal are fed into the furnace to melt. Inside the furnace, the materials melt and mix together until a homogeneous molten substance is formed. Any air bubbles inside this molten material must be completely removed to ensure acceptable product quality. Before entering the furnace, the materials must be properly mixed so that they melt almost simultaneously. This stage is considered the most important step in glass production. The melting process follows a critical flow path that defines the minimum temperature and shortest time the molten glass experiences inside the furnace. Monitoring this temperature and time is extremely important, as they directly affect the quality of the final glass. The raw materials begin to melt at temperatures between 600 and 900 degrees Celsius. During melting and chemical reactions, gases such as carbon dioxide and other gases are released, which are the main cause of bubbles forming in glass.
02. After the bubbles are removed from the molten liquid, the temperature is rapidly increased. To eliminate bubbles from the molten mixture, chemical agents such as sodium chloride, sodium sulfite, arsenic oxide, and others are used. Once all bubbles are removed, the mixture becomes homogeneous, meaning the proportions of raw materials are uniform throughout the melt. The temperature is gradually increased until it reaches approximately 1300 to 1400 degrees Celsius. At this stage, the molten glass is refined: remaining bubbles are removed and the melt becomes fully uniform. Any slag formed inside the molten material or on its surface must be carefully removed.
03. After the refining process, controlled cooling begins. In the first cooling stage, the temperature of the glass is reduced by about 200 to 300 degrees, reaching approximately 1100 degrees Celsius. This temperature reduction is necessary to achieve the proper viscosity for shaping. At this stage, the molten material is suitable for glass and crystal production. The molten glass is transferred into a mold into which air is injected, forming the initial shape. This is not the end of the shaping process. While the glass is still hot, the mold is moved and air is injected again to achieve the final shape of the desired vessel.
04. The final stage is shaping the molten material and forming the glassware into the desired design. This stage must be carried out slowly, as rapid cooling can cause cracks or breakage and result in poor-quality products. Even after shaping, the process is not complete. To relieve internal stresses and achieve the required strength, the finished glassware is reheated and gradually cooled in annealing furnaces. At this stage, the produced glass and crystal items gain sufficient strength to prevent premature breakage.
