The 45-degree burn length determination is a standardized flammability test that measures flame spread time and the resulting char or damaged length on a textile specimen mounted at a 45-degree incline. Anchored by 16 CFR Part 1610, ASTM D1230, and related international methods, it serves as the primary regulatory screen for clothing textiles and many other fabric categories. This article explains the test principle, sample requirements, procedure, classification criteria, and typical industrial applications.
Technical Overview
The 45-degree burn test evaluates the flammability of flexible materials by igniting the lower portion of a specimen held at a 45-degree angle to the horizontal and measuring how rapidly a flame front travels up the fabric. Depending on the governing standard, the reported result may be the flame spread time across a fixed gauge length, the char length (also called damaged length or burn length) remaining after the specimen is extinguished, the damaged area, or the number of flame applications required to sustain ignition. The 45-degree orientation was deliberately chosen because it represents an intermediate fire hazard scenario: it is less severe than vertical burning but more severe than horizontal burning, closely approximating the behavior of a fabric draped or worn on a sloping surface such as a garment front, a shirt collar, or a curtain fold.
The most widely applied version of this method is the clothing textile flammability test specified in 16 CFR Part 1610 of the U.S. Consumer Product Safety Commission (CPSC), which is technically equivalent to ASTM D1230. Closely related procedures include the Canadian 45-degree burner test (CAN/CGSB 4.2 No. 27.4), the Chinese 45-degree determination of damaged area and ignition count (GB/T 14645), and the 45-degree, 30-second Bunsen burner test used for aircraft interior materials under FAR/CS 25.853 Appendix F Part I, where burn length is a principal acceptance criterion.
- Principle: controlled ignition of a 45-degree inclined specimen with a small butane flame
- Primary metrics: flame spread time, char or damaged length, damaged area, ignition behavior
- Core standards: 16 CFR Part 1610, ASTM D1230, CAN/CGSB 4.2 No. 27.4, GB/T 14645, FAR 25.853 Appendix F
- Typical output: flammability classification (Class 1, 2, or 3) or pass/fail burn length limit
Testing Samples
Apparel and Clothing Textiles
Plain-surface woven and knitted fabrics intended for garments are the core sample category. Specimens are cut at 50 mm × 165 mm (2 in × 6.5 in), with the long dimension aligned with the lengthwise direction for one set and the crosswise direction for another. Because the CPSC evaluates garments in the state in which they are sold, fabrics must undergo one laundering or dry-cleaning cycle according to their care label before specimens are cut and conditioned.
Raised-Surface Fabrics
Fleece, terry cloth, velvet, velour, plush, brushed knits, and other pile or nap constructions are tested with special preparation. Prior to ignition, each specimen is brushed with a standardized wire brush to raise loose fibers to their worst-case configuration, since uncontrolled fiber nap can dramatically accelerate surface burning. Raised-surface fabrics are also scrutinized for surface flash, a rapid propagation of flame across the fiber tips without ignition of the base fabric.
Home Furnishing Textiles
Curtains, draperies, bedding fabrics, and decorative textiles are frequently screened with the 45-degree method as part of a broader flammability assessment. Although some interior furnishing categories are governed by separate mandatory standards, the 45-degree test provides a rapid comparative index of burning behavior for product development and risk screening.
Technical and Transportation Materials
Aircraft interior fabrics, films, and coated materials are qualified using the 45-degree, 30-second burner test of FAR 25.853 Appendix F Part I, in which burn length, flame time, and glow time are compared against defined acceptance limits. Automotive and rail applications may also reference 45-degree screening data, although horizontal burning rate methods such as FMVSS 302 dominate the automotive sector.
Testing Items
Depending on the specified method and end-use regulation, a 45-degree burn length determination program can include the following measurable items:
- Flame spread time over a 127 mm (5 in) gauge length, recorded to the nearest 0.1 second
- Char length or damaged length measured after the specimen has cooled
- Ignition outcome classification: did not ignite (DNI), ignited but extinguished (IBE), or full spread
- Surface flash occurrence and duration on raised-surface fabrics
- Afterflame time and afterglow time
- Damaged area traced and calculated on grid-marked specimens
- Number of flame applications required to achieve sustained ignition in repeated-contact methods
- Flammability classification per 16 CFR Part 1610: Class 1, Class 2, or Class 3
- Burn length compliance evaluation for aviation materials per FAR 25.853 Appendix F Part I
Testing Methods
Apparatus and Test Conditions
The test is performed inside an enclosed, draft-free chamber fitted with a specimen holder inclined at exactly 45 degrees. A micro-burner supplied with butane gas delivers a 16 mm (5/8 in) flame that impinges on the lower surface of the specimen at a defined position. A thin stop thread stretched across the specimen 127 mm above the ignition point is linked to an electromagnetic or mechanical timer, which stops the moment the advancing flame consumes the thread. Testing is conducted with the chamber door closed to eliminate air movement, since even slight drafts alter flame spread results.
Specimen Preparation and Conditioning
Specimens are cut with clean edges, conditioned to standard atmosphere, then dried in a ventilated oven at approximately 105 °C for about 30 minutes and cooled in a desiccator immediately before testing. This drying step standardizes the moisture content, because residual moisture slows flame spread and would otherwise bias results. Raised-surface specimens are brushed with the prescribed number of brush strokes before conditioning. A complete evaluation typically requires at least five specimens in each principal direction so that both lengthwise and crosswise burning behavior are characterized.
Test Procedure
- Mount the conditioned specimen in the holder at 45 degrees and secure the stop thread.
- Adjust the butane flame to 16 mm and verify burner alignment against the gauge.
- Apply the flame to the specimen for the specified 1.0-second ignition period.
- Record the flame spread time between ignition and severing of the stop thread, or record DNI/IBE if the flame self-extinguishes.
- Observe and note surface flash, flaming drips, afterflame, and afterglow behavior.
- Allow the specimen to cool, then measure the char length along the specimen axis from the ignition point to the farthest edge of damaged material.
- Repeat for all specimens in both fabric directions and calculate the arithmetic average.
Measurement of Burn and Char Length
Burn or char length is determined after the test by laying the cooled specimen flat and measuring the maximum extent of destroyed, charred, or embrittled material along the direction of flame travel. For damaged-area methods, the damaged outline is traced against a millimeter grid and the area is computed from the traced boundary. In aviation 45-degree tests, burn length is specifically defined as the distance from the original edge of the specimen to the farthest evidence of combustion damage, which makes it a direct indicator of whether fire could propagate beyond a localized ignition source.
Calculation and Classification
Under 16 CFR Part 1610, results are averaged and compared with the regulatory thresholds. Plain-surface fabrics with an average flame spread time of 4 seconds or greater are Class 1 (normal flammability). Raised-surface fabrics with an average of 7 seconds or greater, including fabrics that do not ignite at all, are also Class 1. Raised-surface fabrics averaging between 4 and 7 seconds are Class 2 (intermediate flammability), while plain-surface fabrics below 4 seconds and raised-surface fabrics below 7 seconds, or those exhibiting unacceptable surface flash, are Class 3 (rapid burning). Class 3 textiles are prohibited from use in wearing apparel in the United States, and testing laboratories report any Class 3 result as a regulatory failure.
Applications
The 45-degree burn length determination supports compliance, safety engineering, and materials development across multiple industries:
- Regulatory compliance testing for apparel and textiles sold in the United States and Canada
- Import and export verification for garment manufacturers, traders, and customs authorities
- Quality control of flame-retardant finishes, fiber selection, and fabric construction
- Product development and benchmarking for home furnishing and decorative textiles
- Qualification of fabrics, films, and coated materials for aircraft cabin interiors
- Forensic and fire investigation support for evaluating fabric contribution to fire spread
- Academic and industrial research on textile combustion behavior and FR chemistry
FAQ
What is the difference between the 45-degree test and the vertical flame test?
The 45-degree test uses a partially inclined specimen and a small, brief ignition flame, making it suitable for screening general apparel flammability. Vertical tests, such as those in 16 CFR Parts 1615 and 1616 for children's sleepwear, expose a vertically hung specimen to a larger flame for a longer duration, impose far more severe conditions, and enforce strict char length limits, typically 17.8 cm (7 in) average.
Why is char length or burn length measured instead of only timing the flame?
Timing captures how fast the flame travels, while char length captures how much material is actually destroyed. Some specimens smolder or char extensively with a slow flame front, so combining time data with measured damaged length gives a fuller picture of hazard. Aviation and damaged-area methods rely on burn length as their primary quantitative criterion.
Do raised-surface fabrics require any special handling before testing?
Yes. Specimens of fleece, velvet, terry, and brushed fabrics must be mechanically brushed before conditioning so that surface fibers stand in their worst-case orientation. This step prevents a fabric from passing simply because its nap had been flattened during storage or finishing.
Must fabrics be washed or dry-cleaned before the test?
Under 16 CFR Part 1610, garments must be tested in the condition in which they will be used, so fabrics undergo one laundering or dry-cleaning cycle per the care label prior to specimen preparation.
How many specimens are tested?
A full evaluation normally requires at least five specimens in the lengthwise direction and five in the crosswise direction, giving ten results that are averaged for classification. Retesting with additional specimens is required when results fall near class boundaries or when surface flash behavior is ambiguous.
What happens if a fabric fails and is classified as Class 3?
Class 3 fabrics exhibit unacceptably rapid burning and may not be used in wearing apparel sold in the United States. Manufacturers typically respond by redesigning the fabric, applying an effective flame-retardant treatment, changing fiber content, or adjusting surface finishing before resubmitting samples for testing.
