Cork is the outer bark harvested from the cork oak tree, Quercus suber, making it an organic substance. Since it is a natural, carbon-based product, cork will combust when exposed to sufficient heat, meaning it is not strictly fireproof. However, the unique structure and chemical composition of the bark provide it with a natural resistance, making it significantly less flammable than many other common wood products. This ability to resist ignition and slow the spread of flame makes cork a valued material in building and consumer applications.
Cork’s Flammability Profile
The reason for cork’s fire-resistant properties lies in its chemical makeup, which includes a high percentage of a waxy substance called suberin. Suberin constitutes up to 40% of the cork’s weight and acts as a natural flame inhibitor by delaying thermal decomposition. When subjected to heat, the material requires a high ignition temperature, typically taking over a minute to begin burning.
Once combustion occurs, cork tends to smolder slowly instead of rapidly propagating a flame across its surface. This slow burn rate is aided by the material’s honeycomb cellular structure, which contains millions of air-filled cells. As it burns, the surface forms a protective layer of char, which acts as an insulator that further slows the transfer of heat to the interior. Unlike many synthetic materials, cork releases no toxic gases like cyanide or chloride compounds when it combusts, primarily producing carbon gases and aromatics.
Fire Resistance Ratings and Standards
To classify cork’s fire performance, building materials are subjected to standardized testing, most commonly the ASTM E-84 test, also known as the Steiner Tunnel Test. This test measures surface burning characteristics using two metrics: the Flame Spread Index (FSI) and the Smoke Developed Index (SDI). The FSI quantifies how quickly flames travel across the surface, and the SDI measures the density of smoke produced during combustion.
The ASTM E-84 test results place materials into one of three classifications: Class A (FSI 0-25), Class B (FSI 26-75), or Class C (FSI 76-200). Many manufactured cork products, such as flooring or acoustic wall tiles, achieve ratings in the Class C or better range. The final rating depends on the density of the cork and any treatments or binders used; some specialized cork agglomerates qualify for a Class B rating. The maximum allowable SDI for all three classes is 450, a limit cork products typically meet due to their low smoke emission properties.
Practical Safety Considerations
The fire performance of cork products is often determined by the manufacturing process, particularly the binders used to create agglomerated cork. Binders like polyurethanes, natural rubbers, or thermoplastic powders hold the cork granules together in products like flooring and insulation boards. These binding agents can sometimes be more flammable than the natural cork itself, necessitating careful formulation to maintain the material’s resistance.
When using cork, understand that its natural resistance does not equate to being fireproof. Even resistant materials require proper installation and adherence to local building codes. Consumers should confirm the specific fire rating of the final product, especially when installing cork near heat sources, such as fireplaces or kitchen appliances. Ensuring adequate clearance from intense heat remains a precaution for any organic material used in a home or commercial setting.