Coating char length testing is a small-scale flammability evaluation that measures how far carbonization propagates across a coated surface after controlled flame exposure, providing a direct quantitative indicator of the flame-retardant performance of paints, intumescent systems, and back-coated textiles. This article explains the testing principle, applicable standards such as ASTM D1360, FAR Part 25 Appendix F, GB/T 5455, and IEC 60332, specimen preparation, measurement procedures, acceptance criteria, and typical industrial applications.
Technical Overview
Char length is defined as the maximum linear distance between the point of flame application and the farthest extent of carbonized material, measured after a coated specimen has been exposed to a standardized ignition source, allowed to extinguish, and cooled to ambient temperature. Because flame-retardant and intumescent coatings function by interrupting combustion and forming an insulating char layer, the distance over which this carbonized zone propagates is a direct and reproducible measure of how effectively the coating suppresses flame spread. In general, shorter char lengths indicate stronger flame-retardant performance in small-scale screening tests.
Char length testing of coatings is governed by several internationally recognized standards, including ASTM D1360 (cabinet method for fire retardancy of paints, which reports both char length and char depth), ASTM D3806 (two-foot tunnel method), the vertical test of FAR Part 25 Appendix F Part I paragraph (a)(1)(ii) for aircraft interior materials, which sets a maximum average char length of 152 mm (6 inches), GB/T 5455 (vertical method for textiles, reporting damaged length), and the chamber method of the GB/T 15442 series used for decorative fire-retardant coatings in China. Combined with after-flame time, after-glow time, flaming drip behavior, and flame spread index, char length forms part of the core data set used to qualify and classify fire-retardant coating systems.
For intumescent coatings specifically, the test captures the behavior of the expanding char: a well-formulated coating swells rapidly into a thick, closed-cell carbonaceous foam that confines charring to the region directly heated by the burner, while a poorly performing coating allows the carbonized zone to creep outward along the surface.
Testing Samples
Fire-Retardant Paints on Wood Substrates
Decorative fire-retardant paints applied to plywood, hardboard, or lumber panels are the classic specimen type for cabinet-method testing. The coating is applied to the standard substrate at the manufacturer-specified spreading rate, dried to constant mass, and conditioned before exposure. These samples represent interior linings, decorative finishes, and timber treatments in buildings.
Intumescent Coatings on Metal Substrates
Thin-film intumescent coatings applied to steel or aluminum coupons are tested in small-scale flame exposure to screen formulations for char formation quality, char cohesion, and the length of the affected zone before proceeding to full-scale fire resistance testing of structural elements.
Aerospace Interior Coatings and Finishes
Coated decorative laminates, finished panels, films, and surface treatments used in aircraft cabins are prepared as vertical burn specimens in accordance with the applicable aviation flammability test procedures, with char length serving as one of the primary acceptance parameters.
Back-Coated Textiles
Upholstery fabrics, curtains, drapery, and wall coverings bearing flame-retardant back coatings are cut into strips and tested vertically, with the damaged length (the textile equivalent of char length) reported together with after-flame and after-glow times.
Coated Cables and Specialty Materials
Fire-protective coatings on cable jackets and other specialty coated articles can be evaluated by measuring the extent of the charred or affected portion along the sample after a vertical flame exposure, following cable-focused flammability standards such as IEC 60332-1, which limits the charred extent relative to the upper support.
Testing Items
A complete char length evaluation typically reports the following parameters, depending on the governing standard:
- Char length (average and single maximum value, in millimeters or inches)
- Char depth or residual unburned thickness at the deepest point
- After-flame time and after-glow time following removal of the ignition source
- Flaming drip behavior and ignition of an indicator medium placed below the specimen
- Time to ignition and total burning duration
- Mass loss and carbonized volume in chamber-method tests
- Char expansion ratio (intumescence factor) for swelling coatings
- Flame spread index in tunnel-method tests
- Supplementary heat release rate and smoke production data where required for classification
Testing Methods
Specimen Preparation and Conditioning
Coatings are applied to the specified substrate at the declared loading or dry film thickness, which is verified before testing. Specimens are cut to the dimensions required by the standard, edges are prepared or sealed as specified, and all samples are conditioned, typically at 23 ± 2 °C and 50 ± 5 % relative humidity, until moisture equilibrium is reached. Most methods require multiple replicate specimens, commonly three to ten, to support statistical reporting.
Cabinet Method (ASTM D1360)
The coated panel is mounted inside a ventilated metal cabinet, and a gas burner applies a controlled flame to the coated face for a fixed exposure period. After flaming and glowing have ceased and the specimen has cooled, loosely adhering char is carefully removed with a soft brush or similar implement so that the boundary of sound material is revealed. Char length is measured from the reference line to the farthest point of carbonization, and char depth is determined by comparing the residual thickness at the deepest point with the original thickness of the panel.
Vertical Flame Method
Specimens are clamped in a vertical holder and a standard burner flame, typically about 38 mm (1.5 inches) in height, is applied to the lower edge for a defined duration, commonly 12 or 60 seconds depending on the standard and application. After-flame time, after-glow time, and flaming drips are recorded during the test. After cooling and removal of friable char, the char length is measured from the point of flame application to the farthest carbonized point. In textile testing per GB/T 5455, the damaged length is determined by folding the specimen and tearing it under a specified hanging weight, with the distance from the upper clamp to the tear endpoint reported. For aircraft interior materials tested under FAR Part 25 Appendix F, the 60-second vertical test requires an average char length not exceeding 152 mm (6 inches), an average after-flame time not exceeding 15 seconds, and no flaming drips averaging more than 3 seconds.
Tunnel and Flame Spread Methods
ASTM D3806 evaluates fire-retardant coatings in a small two-foot tunnel by comparing flame spread over the coated surface against reference materials, while ASTM E84 and UL 723 employ the 25-foot Steiner tunnel to assign flame spread classifications to coated building products. These methods do not report char length directly, but the extent of the flame front and the visual char pattern provide complementary evidence of surface flame propagation behavior.
Supplementary Quantitative Methods
Where a fuller fire performance profile is required, char length data can be complemented by cone calorimeter testing (ISO 5660-1 or ASTM E1354) for heat release rate, radiant panel flame spread index (ASTM E162), smoke density (ASTM E662), and limiting oxygen index (ISO 4589 or GB/T 2406). Accredited third-party laboratories often combine these techniques to support classification under frameworks such as GB 8624 or EN 13501-1.
Data Processing and Reporting
Results from all replicate specimens are averaged, and the maximum single value is reported alongside the mean. The test report documents the substrate, coating system, dry film thickness, conditioning history, exposure conditions, measured char length and char depth, after-flame and after-glow times, and any deviations from the standard procedure. Acceptance is judged against the limit values of the applicable specification, for example the 152 mm average char length criterion in aviation vertical burn tests.
Applications
Building and Construction
Char length testing supports the qualification of decorative fire-retardant coatings for wood linings and interior finishes, helping manufacturers demonstrate compliance with building fire safety regulations and contributing input to fire hazard classification of coated construction products.
Aerospace and Rail Interiors
Cabin liners, seat dress covers, decorative finishes, and coated panels in aircraft and rolling stock must meet strict flammability limits in which char length is a decisive criterion, making this test central to material selection, certification, and change control.
Textiles and Furnishings
Back-coated upholstery, drapery, and contract furnishings are screened using vertical flame tests with damaged length measurement, enabling suppliers of public-venue and hospitality textiles to verify flame-retardant durability after cleaning and aging cycles.
Research, Development, and Quality Control
Formulators of intumescent and fire-retardant coatings use char length as a rapid screening metric when optimizing char-forming resin systems, blowing agents, and acid or carbon sources, and production quality control laboratories rely on the same measurement to monitor batch-to-batch consistency of finished coatings.
FAQ
What does a short char length indicate?
A short char length indicates that carbonization was confined to the region directly exposed to the flame, meaning the coating effectively resisted lateral flame spread and self-extinguished quickly. It is one of the strongest positive indicators of flame-retardant performance in small-scale tests.
What is the difference between char length, damaged length, and burn length?
The three terms are closely related measurement concepts used by different standards communities. Char length emphasizes the extent of carbonized material, damaged length is the corresponding parameter in textile flammability testing, and burn length typically refers to the consumed or affected distance measured in horizontal burn or cable tests. All describe the linear extent of thermal damage after flame exposure.
Which limits are typically applied to char length?
The most widely cited limit is the aviation vertical burn criterion of 152 mm (6 inches) average char length after a 60-second flame exposure. Textile specifications frequently set damaged length limits on the order of 150 mm, and customer or industry specifications for fire-retardant coatings may define their own comparative or absolute thresholds.
How many specimens are required?
Most standards require three to ten replicate specimens per condition, with the average and maximum values reported. Additional specimens are recommended when results fall near a limit value or when evaluating durability after washing, aging, or environmental conditioning.
How is char length measured when the coating intumesces?
Intumescent coatings produce a fragile, swollen char that must first be gently removed after the specimen cools so the boundary of undamaged material is exposed. The measurement is then taken from the reference point to the farthest point of carbonized substrate or coating, ensuring the result reflects true damage extent rather than foam height.
How long does a typical test program take?
The flame exposure and measurement itself takes only minutes per specimen, but the overall program, including coating application, full curing, conditioning, replicate testing, and reporting, commonly requires one to two weeks. Programs that include aging, washing, or environmental preconditioning prior to the burn test take correspondingly longer.