Bunsen Burner: Flame Types, Aseptic Use, and Why Never in a Safety Cabinet

A student sets up a blood culture subculture on the open bench. The yellow flame is up, loop passing through it, plates opened right beside the barrel. The work looks careful. Two days later the plate grows a contaminant that was never in the sample.

The mistake is a common one, and it is not carelessness. It is a misunderstanding of what the Bunsen burner actually does. The flame is not a sterilizing shield. It does not clean the air around your plate. What it does is far narrower, and knowing exactly what it does (and the one place it must never go) is the difference between clean work and a ruined culture.

Bunsen burner is a gas burner that produces smokeless, nonluminous flame used for heating, sterilizing, and combustion purposes in laboratory experiments. It is named after Robert Bunsen, the German chemist who popularized the design in 1857 (the burner was actually refined by his laboratory assistant, Peter Desaga).

Bunsen burner ignites by the fusion of fuel and air (oxygen). There are two primary fuel sources for bunsen burner: natural gas (methane) and liquified petroleum gas (propane, butane, or a mixture of both).

Parts of a Bunsen Burner

The Bunsen burner is an essential part of laboratory equipment used for heating materials in the laboratory. It consists of seven major components: the base, barrel (chimney),air regulator (collar), air holes, gas valve, gas nozzle, and gas intake tube.

Figure: Parts of Bunsen Burner Image source: DOI:10.13140/RG.2.2.18145.66401

Base: It is a wide and weighty part, present in the bottom of the Bunsen burner, available in variable shapes. It helps to provide support to the burner. It also helps to provide direct contact of the burner with the work surface.

Barrel (chimney): A metal tube, roughly 5 inches long, that rests on the base. Gas and air mix as they travel up the barrel, and the mixture ignites at the open top. The air holes that feed this mixing sit at the bottom of the barrel and are described separately below.

The air regulator (collar): It is a short metallic cylindrical structure around the air holes at the bottom end of the barrel. The primary function of the collar is to control the amount of air entering the barrel.

It works on a screw mechanism: rotating the collar one way opens the air holes and increases air intake, and rotating it the other way closes them. The direction that opens the collar varies by model, so check your own burner rather than assuming.
Air holes: Two air holes are present near the bottom end of the barrel. It allows air to enter the burner to make a mixture of air or any other liquid fluid and gas.

Gas valve:  It is connected internally with the gas nozzle, the gas receiving part. The gas supply to the burner can be controlled by rotating the gas valve right or left.

Gas nozzle (jet): A fine metal jet at the base of the barrel, inside the burner. Gas leaves the nozzle at high velocity as a thin stream, and it is this fast stream that drags air in through the air holes (the venturi effect described below). The nozzle is what meters how much gas actually enters the barrel.

Gas intake tube: The rubber tube on the outside of the burner that carries gas from the bench gas tap to the burner's inlet. This is the part you connect and disconnect; it is not the same as the nozzle.

Principle

Bunsen burner is generally fitted with spiky fittings at the base of the barrel that helps to connect the rubber tube, which is responsible for feeding gas to the burner. The gas passing through the rubber tube goes up the barrel from the bottom of the burner.

The principle of the bunsen burner is based on its ability to mix gases with oxygen by venturi effect before the mixture is ignited. The venturi effect states that the pressure decreases due to an increase in the velocity of the fluid that flows through the constricted pipe or hole. Likewise, in the case of a bunsen burner, gas flowing through the chimney has low pressure than the steady air surrounding it.

This variation in pressure causes air to be drawn into the air hole as the gas flowing passes through it with the help of the venturi effect. As a result, the flame burning at the top of the barrel occurs on the mixing of gas and oxygen.

Figure: Working principle of Bunsen Burner Image source: Russell Thomas

Thus, the amount of air supplied, which can be regulated by the collar (adjustable valve), is directly proportional to the strength and color of the flame. With a closed valve, a minimal amount of air (oxygen) passes, and a smoky yellow (low temperature) flame is produced. Whereas, with opened valve, a sufficient amount of air enters, and the roaring flame is produced hot, nearly colorless.

Types of Flame on a Bunsen Burner

Generally, the flame produced by the burner can be controlled by two factors; fuel to air ratio and amount of energy.

Fuel-to-air ratio: It determines the intensity and type of flame produced. The amount of air is controlled by an air valve.

Amount of fuel: The amount of fuel is controlled by a gas valve.

Based on the above two factors, the burner produces different flames. These include; safety flame, medium flame, and roaring flame.

Safety flame

• It is one of the coolest flames that are orange or yellow.
• It can be easily seen in a well-lit room and helps to remind the burner is on.
• Reaches temperature of approximately 300℃; not used for heating any materials in the laboratory.
• It is produced when the air valve is closed, but the air needed for combustion comes from the area near the top of the burner.

Medium blue flame

• They are also known as standard, blue, or invisible.
• Difficult to see in a bright room.
• It is mainly used to heat materials in laboratories.
• Reaches roughly 700 to 1000°C depending on air supply, hot enough for routine heating.
• It is produced when the air gap is partially opened.

Roaring flame

• It is the only type that makes noise.
• It reaches roughly 1500°C, the hottest the burner produces
• It is the hottest flame, characterized by the light blue triangle in the middle.
• It is produced when the air gap is fully opened. Gas and air ignite at the top of the barrel on increasing airflow, resulting in a noisy, bluish-colored three-cone flame.

In summary,

Types of Bunsen Burner

There are different types of Bunsen burners to choose from, depending on the gas source and experimental conditions, which include; the Tirrill burner, Teclu burner, and Méker burner.

Tirrill burner

• It has a needle valve at the base that adjusts the gas supply, in addition to the collar that adjusts air, so both fuel and air are controlled at the bottom.
• With both fully open it produces a hot blue flame in the same range as a well-adjusted standard burner (up to roughly 1500°C).

Teclu burner

• It consists of a screw nut at the bottom of the chimney that helps to regulate the gas input.
• The barrel tube is longer than other types of bunsen burner. As a result, gas and oxygen mix well.
• The combustion power of the flame is stronger.

Méker burner

1. The barrel is wider than other burners, so more gas and air mix before ignition.
2. A grid across the flared top splits the flame into many small cones, giving a larger, hotter, and quieter flame than a standard Bunsen burner (its hot zone reaches roughly 1100 to 1200°C over a broad area, and the primary flame is hotter and more even than a single Bunsen cone). This is why the Méker is chosen when a hot, steady, wide flame is needed.
3. The gas supply is controlled by a gas valve below the barrel.

Application/Uses of Bunsen Burner

In the chemical laboratories

• Drying salts
• Analysis of moisture content.
• Dehydration of complexes
• Determination of solvent flash point.
• Determination of compound’s flammability.

Within microbiology laboratories

What the flame actually does for aseptic work

It is worth being precise here, because this is where students are most often misled. A Bunsen burner does three real things for asepsis, and one thing it does not do.

It does: (1) sterilize a metal inoculating loop or wire by heating it red-hot, killing everything on it in seconds; (2) flame the mouths of tubes and flasks, burning off organisms at the rim before and after you open them; (3) create a small rising column of hot air directly above the flame that discourages airborne particles from settling in that narrow zone.

It does not sterilize the surrounding air, the open bench, or an uncovered plate sitting beside it. The "aseptic zone" of a bench Bunsen is only a few centimeters wide, directly over the barrel. Work performed a hand-span away is not protected. This is the single most common misconception about the burner, and it is why open-flame technique demands that you work close to the flame, fast, and with plates open for the shortest time possible.

• The burner helps in achieving a contamination-free area for research laboratory purposes.
• It also helps with sterilizing inoculating loop and spreader used in microbiological experiments.
• The Bunsen burner is used in the fixation of the smear while staining.
• It is also used in sterilizing neck tubes and flask to maintain the aseptic condition.

Never use a Bunsen burner inside a biosafety cabinet.

A naked flame disrupts the cabinet's laminar airflow, the very thing that protects you and your sample. It also builds up heat that can damage the HEPA filter or the adhesives holding the cabinet together, and it introduces a fire and explosion risk with the flammable materials often present. If you need to sterilize a loop while working in a cabinet, use disposable sterile loops or a shielded micro-incinerator, never an open flame. Using a Bunsen burner for biohazard work in a cabinet is not a shortcut; it defeats the protection the cabinet exists to provide.

In zoology and botany laboratories

• Preparation of permanent slides.
• Heating purpose

Advantages of Bunsen Burner

• Easy to set up and operate.
• Cost-effective
• It can be used not only for heating purposes but can also be used for simple glass-blowing work.
• Available in different sizes and types for the operator’s convenience and requirements.

Limitations of Bunsen Burner

1. Using Bunsen burners inside a biological safety cabinet (BSC) is not recommended because it disrupts airflow, compromising the protection of the workers and the product. Bunsen burner causes excessive heat build-up within the cabinet and may damage the HEPA filter, or melt the adhesive holding, thus compromising the cabinet’s integrity. It also presents a potential fire or explosion within the cabinet. Disposable sterile loops or micro-incinerators are suitable alternatives for using bunsen burners in safety cabinets.
2. Temperature control at the required amount is impossible.
3. Risk of fire accidents.

How to Remember

The flame is a tool, not a shield. If you take one idea from this article, make it this: the burner sterilizes what touches the flame (the loop, the tube rim), not the air around it. A shield protects a whole area; this flame protects only what passes through it.

Air hole open = hot and honest, air hole closed = cool and cowardly. The roaring blue flame is open, loud, and does the work. The yellow safety flame hides (it is nearly invisible in a bright room only because it is doing nothing useful) and marks a burner left idling. Open it up when you mean business, close it down when you are between tasks.

"Never a naked flame in a cabinet." The rhyme is worth keeping because the mistake is dangerous. A cabinet's protection is moving air; a flame wrecks moving air.

Key exam facts in one table

Where Students Get Confused

"The flame keeps my whole work area sterile." No. The protected zone is only a few centimeters directly above the barrel. Anything a hand-span away is unprotected. This single misconception causes more preventable contamination than any equipment fault.

"Yellow flame is fine, I can see it better." The yellow flame is cool and sooty. It will not sterilize a loop properly and it deposits carbon on glassware. Its only legitimate use is as a visible "burner is on" indicator between tasks. Heat and sterilize with the blue flame.

"A flame in the safety cabinet is extra protection." It is the opposite. The flame destroys the laminar airflow that is the cabinet's entire protective mechanism, and risks the HEPA filter and a fire. Extra flame is not extra safety.

Nozzle vs intake tube. The nozzle is the internal jet that meters gas and drives the venturi effect. The intake tube is the external rubber tube you connect to the gas tap. Exam questions sometimes test whether you know the nozzle (not the tube) is what creates the air-drawing effect.

"Loop should glow, then straight onto the plate." A red-hot loop will kill the organisms you are trying to subculture on contact. Flame it, then let it cool for a few seconds (touch it to a sterile part of the agar) before picking a colony.

References

1. Jensen WB. The origin of the Bunsen burner. Journal of Chemical Education. 2005;82(4):518. https://doi.org/10.1021/ed082p518
2. Bykowski T, Verma A, Brissette CA, Stevenson B. Aseptic technique. In: Current Protocols Essential Laboratory Techniques. 2012. https://www.researchgate.net/publication/280947477_Aseptic_techniques
3. Grainger Know How. Bunsen burners, Meker burners, and Tirrill burners: what's the difference? https://www.grainger.com/know-how/equipment-information/kh-bunsen-burners-meker-burners-tirrill-burners
4. Bailey & Scott's Diagnostic Microbiology. 15th ed. St. Louis: Elsevier; 2022.
5. U.S. Department of Health and Human Services. Biosafety in Microbiological and Biomedical Laboratories (BMBL). 6th ed. 2020.