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Testing candles for sooting behaviour

Investigating BS EN 15426:2007 – ‘Candles – Specification for sooting behaviour’.

The sootiness of candles has long been a concern for regular users and suppliers alike. In recent years in the United States and the United Kingdom, candle users have brought lawsuits against retailers citing allegedly poor candle quality as the cause of soot damage to their homes, with claimants seeking compensation to cover redecoration. It is important for candle suppliers and retailers to ensure the quality of their products prior to supply.

Candle sooting

Soot is a product of the incomplete combustion of a candle’s hydrocarbon rich fuel – the wax. Soot consists of microscopic, often oily, solid particles which are released into the atmosphere as the candle burns. It is the soot particles in the candle flame which give it the characteristic yellow glow.

Candle manufacturers aim to select the correct combination of wax types, wick sizes and candle shapes and sizes to produce the best burning performance possible. A candle showing a high degree of sooting indicates poor quality of one or more of its components, or a possible mismatch between these components. For example, a wick with poor posture may result in higher levels of sooting occurring during burning. The majority of candle wicks are manufactured with a slight curvature to ensure gradual incineration of the wick. If incineration does not occur and the wick becomes elongated, incomplete combustion of the wax is likely, resulting in the candle burning with a more sooty flame. Once released into the atmosphere, these soot particles may be deposited around the home – especially on walls, ceilings and soft furnishings.

Users of poor-quality candles may notice a cumulative build up of soot as a result of regular candle use. However, in worst-case scenarios, an extremely poor candle is capable of producing enough soot to produce noticeable deposits in a single burn.

Preparing for burning

The European BS EN 15426:2007 standard specifies requirements and the test method for evaluating the sooting behavior of burning candles. The method is applicable to single-wick indoor candles up to a diameter of 100mm. For each candle design, three samples are selected, which are representative of the finished product intended to be sold at retail. The standard does not include information on how to treat different colours and/or scents, although SATRA recommends a separate test sample for each scent and colour variation within a candle design.

The test samples are removed from any packaging and prepared for assessment in accordance with the manufacturer’s instruction, if any are given. For example, the user may be instructed to trim the wick to a certain height, such as 10mm, prior to each burn period.

The burning test

The test equipment comprises wire mesh test cylinders in two sizes for testing different sizes of candle. For tea lights and candles 70mm or less diameter, type 1 (230mm) cylinders are used, and for candles greater than but less than 100mm, diameter type 2 (300mm) cylinders are used.

​ Figure 1: Wire mesh test cylinders

The wire mesh cylinders (figure 1) have a known permeability of 60 per cent. These are mounted with an adjustable height fixture on a vertical stand, on top of which heat-resistant glass test plates can be located. The test candle is placed inside the cylinder, which has its lower surface a minimum of 180mm from the test plate, and is located a minimum of 50mm from the base plate. During testing, any soot emitted by the candle rises vertically upwards from the flame and collects on the underside surface of the glass plate.

The candle is lit and allowed to burn for five minutes to stabilise burning behaviour before soot collection commences. Soot is not collected during this stabilising period, as higher levels of soot (which may not be representative of the rest of the candle’s performance) may be emitted. The glass plate is removed from the test cage during the stabilising period, and the candle is then subjected to a series of burn cycles, depending on its mass and size. Candles weighing less than 40g are tested in a single continuous burn cycle until a residual height of 10mm is reached.

Tea lights are tested in a single continuous cycle until they self-extinguish, which is usually on consumption of the wax. Candles above 40g in weight and with a diameter of less than 70mm are tested in two burn cycles each lasting four hours, with a minimum of an hour’s pause between burning to allow the wax to fully solidify. Candles weighing more than 40g which are equal to or greater than 70mm in diameter are tested in three burn cycles of four hours each, with a minimum of an hour’s pause between burning.

At the end of testing, the sooted plates are removed from the test cylinders and transferred to a measuring unit to ascertain the level of sooting which has collected on the plates (figure 2). The measurement unit incorporates a light source and a photometer. The test piece is placed between the light source and photometer and the level of light reaching the photometer is measured. The plate is removed, cleaned with detergent and water, dried and then returned to the measuring unit, after which the measuring process is repeated. The soot index of each candle is calculated from the ratio of the illuminance resulting from the measurement of the light levels passing through the sooted and the clean glass plates. The soot deposit impedes light transmission through the glass plate onto which it has been collected, so the more soot deposited, the lower the light transmission level. Using the length of burning for the test, the degree of sooting can be expressed as ‘Soot Index per hour’, which allows the sooting levels of candles of different burning life to be compared.

Figure 2: Glass plates are used to measure the level of sooting

The standard requires that the mean soot index result of the three tests of any candle design shall be less than 1.0 per hour, and no individual result shall exceed 2.0 per hour. Any candle which produces results exceeding these levels fails to meet the standards requirements.

SATRA testing services

EN 15426:2007 provides a quantified assessment of the degree of sooting of a candle, which is useful for assisting candle manufacturers, suppliers and retailers in predicting the sooting behaviour of their candles.

When this test is used in conjunction with burning assessment tests, such as BS EN 15493:2007 – ‘Candles – Specification for fire safety’ and SATRA’s own test methods, it can assist with demonstrating due diligence.

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