Fume Cupboards and the Screaming Jelly Baby Experiment

When a Demonstration Overwhelms a Fume Cupboard

The “Screaming Jelly Baby” experiment is one of the most well-known chemistry demonstrations performed in schools and colleges throughout the UK.

The demonstration involves dropping a jelly baby into molten potassium chlorate, resulting in a rapid oxidation reaction that produces flames, smoke, heat, expanding gases and the characteristic “screaming” sound from which the experiment takes its name.

While often presented as an engaging classroom demonstration, the experiment provides an important lesson for anyone involved in the design, testing or use of fume cupboards and Local Exhaust Ventilation (LEV) systems.

A video supplied by AES, Glasgow and hosted by LEVCentral demonstrates how the volume of smoke generated can quickly overwhelm the containment capability of a typical school fume cupboard. The rapidly expanding combustion products can exceed the extraction capacity of the cupboard, allowing smoke and contaminants to escape into the laboratory environment.

This resource is relevant to:

School Science Technicians & Teachers
Laboratory Managers
LEV Designers/Commissioners & Testers
Facilities Managers
Health & Safety Professionals
Duty Holders

Source Document

Source: AES Glasgow

Document Type: Demonstration Video

Subject: Fume Cupboard Performance and Containment

Status: Current Resource

Last reviewed by LEVCentral: June 2026

LEVCentral Expert Commentary

Most users assume that if an activity is carried out inside a fume cupboard, contaminants will automatically remain contained.

The Screaming Jelly Baby experiment demonstrates why this assumption can be dangerous.

A standard school fume cupboard is designed to contain contaminants generated during normal laboratory operations. It is not designed to cope with sudden releases of large volumes of hot gases, smoke and combustion products generated within fractions of a second.

During the experiment, the rate of gas generation can exceed the volume of air being extracted by the fume cupboard. Once this occurs, contaminants can escape through the sash opening and enter the laboratory.

The experiment therefore provides an excellent visual demonstration of a fundamental LEV principle:

A containment device can only control contaminants that remain within its design envelope

The same principle applies across many industrial processes.

Examples include:

Welding operations
Thermal cutting
Laser processing
Furnace charging
Chemical reactions
Dust explosions
Process upsets

In each case, contaminant generation rates may exceed the capacity of the extraction system, resulting in reduced containment effectiveness.

Key Learning Points

Fume Cupboards Have Design Limits

Fume cupboards are designed for specific contaminant generation rates and process conditions. They cannot guarantee containment under all circumstances.

Containment Depends on Process Behaviour

The effectiveness of any LEV system depends not only on airflow but also on the way contaminants are generated.

Sudden Releases Can Overwhelm Extraction Systems

Rapid expansion of gases or smoke can exceed the extraction capacity of a fume cupboard and result in contaminant escape.

Recirculatory Systems Require Additional Consideration

CLEAPSS guidance notes that sparks generated during the experiment have ignited paper pre-filters within recirculating fume cupboards, demonstrating the importance of understanding process hazards before selecting filtration systems.

Visual Demonstrations Can Reveal Containment Failures

The experiment provides a striking example of why smoke visualisation remains a valuable tool when assessing containment performance.

Why This Resource Matters to LEV Professionals

Although the experiment takes place in a school laboratory, the lessons extend far beyond education.

Many LEV systems are designed around expected contaminant generation rates. Problems arise when actual process conditions differ from those assumed during design.

The Screaming Jelly Baby experiment demonstrates:

The importance of understanding source strength
The limitations of airflow measurements alone
The relationship between contaminant generation and containment
The value of visual containment assessments
The need to consider abnormal operating conditions

For P601 engineers and occupational hygienists, it provides an excellent training example of why satisfactory airflow readings do not always guarantee effective contaminant control.

Further Resources

Recommended Learning

LEVCentral Observation

The Screaming Jelly Baby experiment provides a memorable reminder that successful contaminant control depends on understanding both the extraction system and the process it is intended to control.

The demonstration shows how rapidly generated contaminants can exceed the design capability of a containment system and highlights a lesson applicable to every LEV installation: engineering controls must be designed around realistic process conditions, not assumptions.

For many LEV professionals, the short video remains one of the most effective visual demonstrations of containment failure available.