User safety - Sooting behaviour

The EN 15426 and the ASTM F2326 standards detail the protocol used for testing the soot emission rate of candles (image © Dekra)

If you've read the article titled How do candles work?, you probably remember that by-products of candle combustion are water, carbon monoxide and dioxide and... soot. How much soot a candle produces depends on several factors: type of wax used, amount and type of fragrance oil and, above all, the choice of the right (type and size) wick.

An unsuitable wick or one that is not properly maintained by the consumer (trimmed to about 1 cm before every lighting of the candle) will for sure produce too large an amount of soot that sooner or later will build up on the walls, the ceilings and pretty much everything in the room where candles are burnt. Soot built-ups create damages that are notoriously difficult to repair. Several lawsuits relating to damages caused by candle soot built-up have been filed in the United States and the United Kingdom in recent years.

To keep damages caused by candle soot to homes and to the health of their residents to a minimum, the EN 15426 standard (European Union) and the ASTM F2326 standard (United States) suggest a standard test protocol that allows manufacturers to measure the amount of soot emissions.

The test involves burning the candle inside a wire mesh cylinder on top of which a heat resistant glass plate is mounted to capture the released soot. At the end of the test, the glass plate will be placed between a light source and a photometer to measure what amount of light passes through the soot covered plate. That amount of light gives an indication of the amount of soot released during the test.
Depending on the type of candle (container candle or not) and its size, the test consists of one to three burning cycles, as explained below.

Candle with a mass lower than 25 grams (except container candles)

A single continuous burning cycle ending when the residual height reaches 10 mm.

Container candle with a mass lower than 25 grams (container not included)

A single continuous burning cycle ending when the candle self extinguishes.

Candle with a mass higher than 25 grams and a diameter smaller than 70 mm (except container candles)

Two burning cycles.

• 5 minutes stabilizing period (a candle smokes more when it has just been lit; this stabilizing period allows the candle to reach its "cruise speed". The glass plate is only put in place at the end of the stabilizing period)
• first burning cycle of 240 continuous minutes (4 hours) after which the glass plate is removed
• pause of 60 minutes minimum
• 5 minutes stabilizing period (after which a new glass plate is put in place)
• second burning cycle of 240 continuous minutes OR until the residual height reaches 10 mm

Container candle with a mass higher than 25 grams (container not included)

Two burning cycles.

• 5 minutes stabilizing period (see above)
• first burning cycle of 240 continuous minutes (4 hours) after which the glass plate is removed
• pause of 60 minutes minimum
• 5 minutes stabilizing period (after which a new glass plate is put in place)
• second burning cycle of 240 continuous minutes OR until the candle self extinguishes

Candle with a mass higher than 25 grams and a diameter greater than 70 mm (except container candles)

Three burning cycles.

• 5 minutes stabilizing period (see above)
• first burning cycle of 240 continuous minutes (4 hours) after which the glass plate is removed
• pause of 60 minutes minimum
• 5 minutes stabilizing period (after which a new glass plate is put in place)
• second burning cycle of 240 continuous minutes after which the glass plate is removed
• pause of 60 minutes minimum
• 5 minutes stabilizing period (after which a new glass plate is put in place)
• second burning cycle of 240 continuous minutes OR until the residual height reaches 10 mm

After the glass plates have been analyzed with the photometer, an average is computed if more than one burning cycle were performed. The soot index is then known. The standard requires that the average is not superior to 1.0 per hour and that no burning cycle (if more than one) shows an index higher than 2.0 per hour. A candle whose test results exceed these values does not meet the terms and conditions of the standard.