Overview
ISO 3378:2002 specifies a laboratory method to determine the resistance of heavy leathers to grain cracking and to calculate a grain crack index. The procedure is applicable to all heavy leathers and is intended to provide repeatable, comparable results for quality control, specification and product development.
Key elements of the standard include: sample selection and preparation, a defined test apparatus (roller, clamp and a set of mandrels with specified diameters), the bending test procedure and the method for expressing results. Normative references used for sampling, conditioning and thickness measurement include ISO 2418, ISO 2419, and ISO 2589.
Key Topics
- Purpose: Measure how the grain of heavy leather resists cracking when bent grain-outwards around mandrels.
- Apparatus: Test machine with a clamp, a cylindrical roller, and a set of mandrels (diameters specified in the standard), plus a press knife and thickness gauge.
- Sampling & preparation: Cut six test pieces from the sample (three parallel and three perpendicular to the backbone), condition them per ISO 2419 and measure thickness per ISO 2589.
- Test procedure: Bend each test piece grain-outwards over mandrels by rotating the roller through 180° in a controlled time and observe the grain for cracking. Tests can be done on one or multiple mandrels.
- Grain crack index: Determined by identifying the largest mandrel that causes cracking and multiplying its mandrel number by the test-piece thickness (n · t). Special notations are used if cracking occurs on the largest or smallest mandrel.
- Annex A: Explains the derivation and interpretation of the grain crack index.
Applications
The standard supports practical applications in:
- Quality control and incoming inspection: Use pass/fail over specified mandrels or report the grain crack index to accept or reject batches.
- Product specification and procurement: Provide objective performance data when specifying leather for heavy-duty footwear, saddlery, belts or technical goods.
- Material comparison and R&D: Compare leather treatments, tanning processes or finishes to optimise resistance to grain cracking.
- Supplier communication: Use standardised test reports to document compliance and discuss improvements with tanneries.
Related Standards
- ISO 2418 - sampling location for leather
- ISO 2419 - sample preparation and conditioning
- ISO 2589 - thickness measurement
For reliable implementation, follow the full ISO 3378:2002 text for apparatus details (mandrel sizes), timing and reporting requirements. A compliant test report should reference ISO 3378:2002 and include mandrel diameters used, pass/fail outcomes or grain crack index, conditioning atmosphere, any deviations and full sample identification.
Keywords: ISO 3378, leather testing, grain crack index, resistance to grain cracking, heavy leathers, quality control.
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