Simone Lawson
Bending glass tubing is a handy skill for managing laboratory glassware. Glass is a brittle, amorphous crystalline material that requires heat to be bent successfully without breaking. The type of glass will determine the heat source and temperature needed to bend it. For instance, borosilicate glass requires a hotter flame than flint glass to be manipulated. When bending glass tubing, it is important to heat the section of glass you want to bend, plus a small area on either side, and to rotate the tubing to ensure it is heated evenly. Once the glass is soft and pliable, gentle and continuous pressure can be applied to bend it.
Characteristics and Values of an acceptable bend in glass tubing
| Characteristics | Values |
|---|---|
| Type of Glass | Borosilicate glass, flint glass, soda-lime glass, soda glass |
| Heat Source | Alcohol burner, gas flame, hydrocarbon-air torch, hydrocarbon-oxygen flame, Bunsen burner, kiln |
| Heat Level | High heat, blue flame, yellow flame |
| Heating Technique | Rotate tubing, heat evenly, heat section to be bent plus 1 cm on either side |
| Bending Technique | Apply gentle and continuous pressure, bend slightly cooled glass, blow while bending |
| Cooling Technique | Set on a heat-resistant surface, not a cold, uninsulated surface |
| Safety | Wear safety glasses, keep fingers away from flame, avoid forcing bends |
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What You'll Learn
- Glass type: Borosilicate glass requires higher heat than flint glass
- Heating: Heat glass tubing evenly, rotating it over a flame
- Bending: Apply gentle pressure to bend the glass tubing
- Cooling: Allow the glass to cool before setting it down
- Safety: Wear safety glasses and keep fingers away from the flame
Glass type: Borosilicate glass requires higher heat than flint glass
Glass tubing is usually made of soda-lime glass, which is easy to manipulate with a hydrocarbon-air flame. However, borosilicate glass requires a higher temperature to bend and shape. Borosilicate glass, often sold under the trade name Pyrex, is a type of glass with silica and boron trioxide as its main constituents. It has a much higher proportion of silicon dioxide (80%) compared to soda-lime glass (69%), making it less susceptible to fractures.
Borosilicate glass has a higher resistance to thermal shock due to its low coefficient of thermal expansion, allowing it to withstand temperature differentials of about 330°F (166-170°C) without fracturing. This means it can go from the freezer to an oven rack or be exposed to cold water straight out of the oven without cracking. Its chemical resistance also makes it ideal for storing hot liquids, laboratory applications, and even nuclear waste.
To bend borosilicate glass, a gas-oxygen torch or a strong hydrocarbon-air torch is required. The glass should be heated evenly, and a gentle introduction to a fluffy flame is necessary before shifting to a blue oxidizing flame. This process is more complex than bending soda-lime glass, which can be done with a simple methane-air flame.
The choice between borosilicate and soda-lime glass depends on the specific application. While borosilicate glass is superior in terms of durability and resistance to thermal and chemical changes, soda-lime glass is more common due to its lower cost and wider availability. It accounts for 90% of manufactured glass worldwide and is used for various items, including furniture, vases, beverage glasses, and windows.
In summary, borosilicate glass requires higher heat than flint glass due to its unique composition, resulting in enhanced thermal shock resistance. This makes it ideal for applications where durability and resistance to extreme temperature changes are crucial. However, the increased complexity and cost of working with borosilicate glass mean that soda-lime glass remains the more common choice for everyday glassware.
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Heating: Heat glass tubing evenly, rotating it over a flame
Heating glass tubing evenly requires a steady hand and a good deal of practice. The first step is to identify the type of glass you are working with. The two most common types of glass tubing used in a laboratory setting are borosilicate glass and flint glass.
Borosilicate glass, which may be labelled as Pyrex, requires a higher heat source to soften than flint glass. It requires a gas-oxygen torch, which is not commonly found in laboratories. A strong hydrocarbon-air torch can be used, but it must be cranked up to a high temperature and the tube must be small. It is much easier to use a hydrocarbon-oxygen flame, starting with a gentler, fluffy flame, and then shifting to a blue oxidizing flame.
Flint glass can be manipulated with just about any flame. An alcohol burner is recommended, as too high a heat may cause the glass to melt too quickly to work with.
Once you have selected your heat source, you can begin to heat your glass tubing. Hold the tubing horizontally in the hottest part of the flame. This is the blue part of a gas flame or just above the top of the inner cone of an alcohol flame. You want to heat the section of glass you want to bend, plus about a centimeter on either side of this point. A flame spreader is helpful for a gas flame but not necessary.
Rotate the tubing constantly to ensure even heating. You can also move the flame over the area you want to bend. This is a tricky skill to master, as the difficulty is in achieving an even bend.
If you are working with a small tube or a tube of moderate size made of soft glass, use a fish-tail burner with a flame breadth of 1½ to 2½ inches. Hold the tube with both hands, one on either side of the flame, and rotate it constantly.
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Bending: Apply gentle pressure to bend the glass tubing
Bending glass tubing can be challenging, and it is recommended to plan for as many straight runs as possible, making use of angled fittings. It is also worth noting that pre-bent glass tubing can be purchased from some companies.
When bending glass tubing, it is important to wear the appropriate safety gear, including laboratory gloves and safety goggles, and to work in a well-ventilated area. Ensure that you are using the correct type of glass for your purpose, as some glass is more difficult to bend than others. The two main types of glass used in a lab are flint glass and borosilicate glass. Flint glass is typically unlabeled and can be bent using almost any flame, whereas borosilicate glass (which may be labeled as Pyrex) requires a higher heat to soften and should be bent using a gas flame.
To bend the glass tubing, follow these steps:
- Heat the section of glass you want to bend, plus about a centimeter on either side of this point. This can be done using a gas flame, with the tubing held horizontally in the hottest part of the flame (the blue part of a gas flame).
- Rotate the tubing slowly to ensure even heating.
- As you heat and rotate the tubing, apply gentle and continuous pressure where you want it to bend.
- Once the glass starts to yield, release the pressure and heat the tubing for a few more seconds.
- Remove the tubing from the heat and allow it to cool for a couple of seconds.
- In a single motion, bend the slightly cooled glass to the desired angle.
- Hold the bent tubing in position until it hardens, then set it on a heat-resistant surface to cool completely.
It is important to note that if the tubing starts to bend under its own weight, it has been overheated. Additionally, avoid setting the hot glass on a cold, uninsulated surface, as this may cause it to crack or break.
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Cooling: Allow the glass to cool before setting it down
When shaping glass tubing, it is important to allow the glass to cool before setting it down. This is because glass is susceptible to thermal shock, which can cause it to crack or break. The glass should be set on a heat-resistant surface to cool completely.
The process of gradually cooling glass to room temperature is called annealing. Annealing is necessary to prevent the glass from cracking or breaking due to rapid cooling. There are different types of annealing ovens, such as upright ovens with shelves and top-loading annealers, which open like a cooler. The size of the glass piece determines the length of time required in the annealer, with larger pieces taking longer to cool down properly.
Proper oven annealing is especially important if there are joints involved in the glass tubing. Additionally, a polariscope can be used to check the annealed sections of the glass. Annealing with a flame is possible, but it is best to use an oven for more controlled and even cooling.
To bend glass tubing, the section of glass to be bent is heated in a flame, typically a gas flame or an alcohol flame, depending on the type of glass. The glass is rotated to ensure even heating. Once the glass becomes pliable, it is removed from the heat and gentle pressure is applied to bend it to the desired angle. After shaping, the glass is allowed to cool and harden before being set down on a heat-resistant surface.
In summary, allowing glass tubing to cool before setting it down is crucial to prevent thermal shock and ensure the integrity of the glass. Annealing is a technique used to gradually cool the glass, and proper annealing techniques, such as using an annealing oven, contribute to the overall success of the bending process.
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Safety: Wear safety glasses and keep fingers away from the flame
Bending glass tubing is a potentially dangerous activity that requires safety glasses and extreme caution to avoid injury. When working with glass tubing, it is essential to prioritize safety and take the necessary precautions to protect yourself. Here are some detailed safety instructions to follow when bending glass tubing:
Wear safety glasses: Before beginning the bending process, ensure that you are wearing safety glasses to protect your eyes from any flying glass fragments or hot particles that may be generated during the process. Safety glasses are essential to safeguard your eyes from potential injuries.
Keep fingers away from the flame: When handling glass tubing near a flame, always maintain a safe distance between your fingers and the heat source. Keep your fingers at least 4-5 cm away from the flame and the point of bending. This precaution will help prevent burns and reduce the risk of injury.
Use appropriate tools: When working with short pieces of glass tubing, it is recommended to use a clamp to secure the glass to a ring stand or a light vise. This allows you to manipulate the tubing without bringing your hands close to the flame. Using the proper tools helps you maintain a safe distance from the heat source.
Heat the tubing gradually: Start by heating the tubing slowly and evenly. Apply a gentle, continuous flame to the desired bending point while rotating the tubing to ensure uniform heating. Avoid excessive heat or rapid temperature changes, as these can increase the risk of cracking or shattering the glass.
Avoid direct contact with the flame: Whenever possible, avoid touching the glass tubing directly with your fingers while it is in the flame. Use heat-resistant tools, such as tongs or heat-proof gloves, to handle the tubing and protect your hands from the heat.
Allow for controlled bending: Once the glass is evenly heated and pliable, remove it from the direct flame. At this point, you can carefully apply pressure to bend the tubing to the desired angle. Always bend the tubing in a controlled manner, avoiding forceful or abrupt movements that could lead to breakage.
By following these safety guidelines and maintaining a cautious approach throughout the process, you can significantly reduce the risks associated with bending glass tubing. It is important to prioritize your safety and well-being when working with potentially hazardous materials and equipment.
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Frequently asked questions
Soda-lime glass tubing is the easiest to bend and can be manipulated using a hydrocarbon-air flame.
Heat the section of glass tubing you want to bend, plus about a centimetre on either side, using a gas flame. Rotate the tubing to ensure it is heated evenly.
Glass tubing is brittle and can easily break at room temperature. If your tubing cracks, try using a flame spreader adapter to ensure even heating.
If the inside of the tube seals, it means the glass got too hot. Remove the tubing from the heat sooner next time.
When the glass tubing is ready to bend, it will feel like a cooked noodle. At this point, remove it from the flame and bend it to the desired angle.
