Water absorption testing measures the percentage of water a material takes up under controlled saturation conditions, providing a direct indication of its open porosity, density, and long-term durability. This technical guide explains the gravimetric principle behind water absorption determination and the major international standards, including ISO 10545-3, ASTM C373, ISO 62, ASTM D570, and ISO 535. Whether you are performing in-house quality control or outsourcing to accredited third-party laboratories, this article will help you select the correct method, prepare specimens properly, and interpret results accurately.
Technical Overview
Water absorption is the mass of water that a dry material takes up under specified saturation conditions, expressed as a percentage of the dry mass of the specimen. It is one of the most widely used indicators of open porosity and microstructural compactness, and it correlates directly with performance characteristics such as frost resistance, mechanical strength, stain resistance, chemical durability, and dimensional stability in humid environments.
All recognized water absorption test methods share the same gravimetric principle. The specimen is first dried to constant mass and weighed, then saturated with water by boiling, atmospheric immersion, or vacuum saturation, and finally weighed again in the saturated surface-dry state. The result is calculated as:
W = (m2 - m1) / m1 × 100%, where m1 is the dry mass and m2 is the saturated surface-dry mass of the specimen.
Because the measured value is highly sensitive to saturation technique, water temperature, drying temperature, immersion time, and the way surface water is removed, results are meaningful only when the test is performed in strict accordance with published standards from organizations such as ISO, ASTM, EN, GB, and JIS. Commonly applied standards include ISO 10545-3 and ASTM C373 for ceramic tiles, ISO 62 and ASTM D570 for plastics, ISO 535 for paper and board, and EN 13755 for natural stone.
Testing Samples
Ceramic Tiles and Fired Whiteware
For testing to ISO 10545-3 or the equivalent GB/T 3810.3, a minimum of 10 whole tiles is normally required, although 5 tiles may suffice for products with a face area greater than 0.04 square meters. ASTM C373 requires at least 5 representative specimens. Where cutting is permitted, test pieces of at least 100 mm × 100 mm are taken from the tile body. All ceramic specimens are dried at 110 ± 5 °C until the difference between two successive weighings is negligible, then cooled to room temperature in a desiccator before the initial weighing.
Plastic Materials
For ASTM D570, the standard specimen for molded plastics measures 76.2 mm × 25.4 mm × 3.2 mm, while sheet materials are tested at full product thickness. ISO 62 specifies square specimens of approximately 60 mm × 60 mm cut from 1 mm sheet, or representative molded discs and bars. Specimens are dried at 50 °C for 24 hours, or to constant mass for strongly hygroscopic polymers, and cooled in a desiccator so that only immersion uptake, not residual moisture, is measured.
Paper and Paperboard
For the Cobb method (ISO 535), specimens are cut to define a test area of 100 cm², and approximately 10 specimens are tested, covering both sides of the sheet where required. Because paper moisture content strongly influences the result, specimens are conditioned at 23 °C and 50% relative humidity before testing.
Rigid Cellular Plastics (Foams)
For ASTM D2842 and ISO 2896, blocks with smooth cut faces, commonly 76 mm × 76 mm × 50 mm or the full product thickness, are used. Specimens are dried to constant mass at 40 to 70 °C, and the dry apparent volume is determined so that water uptake can be expressed on a volumetric basis with a correction for open cells exposed by cutting.
Natural Stone, Concrete, and Other Construction Materials
EN 13755 for natural stone uses typically six representative specimens, such as 50 mm cubes or prisms, dried at 70 ± 5 °C to constant mass. ASTM C642 for hardened concrete uses representative fragments or cores of adequate volume, dried before sequential immersion, boiling, and vacuum saturation stages.
Testing Items
A water absorption determination typically reports the following parameters, depending on the selected standard:
- Water absorption by mass, expressed as a percentage of the dry mass
- Water absorption by volume, mainly for cellular plastics and foam materials
- Apparent porosity, derived from saturated and immersed (suspended) masses
- Bulk density and apparent relative density of the fired or cured material
- Individual mass values: dry mass (m1), saturated surface-dry mass (m2), and immersed suspended mass (m3)
- Cobb value in g/m² for paper and board at defined contact times (Cobb60, Cobb120)
- 24-hour immersion, boiling, and vacuum saturation absorption values for plastics and concrete
- Thickness swelling after immersion for wood-based panels, as a related durability index
Testing Methods
Common Conditioning Steps
All methods begin by drying specimens to constant mass at the temperature specified by the standard, followed by cooling in a desiccator and weighing to obtain the dry mass m1. Analytical balances with readability of 0.01 g or better, controlled water temperature, and a consistent surface blotting technique are essential for reproducible results.
Boiling Water Method (ISO 10545-3, ASTM C373)
The boiling method is the reference procedure for ceramic tiles. Specimens are immersed in water, which is brought to a boil and maintained for 2 hours per ISO 10545-3 or 5 hours per ASTM C373, ensuring the specimens remain covered by at least 50 mm of water throughout. The specimens are then allowed to cool in the same water for 4 hours per ISO 10545-3 or 24 hours per ASTM C373. Each specimen is removed, quickly wiped with a damp absorbent cloth to remove surface water, and weighed within a defined short time to obtain the saturated surface-dry mass m2. Boiling drives trapped air out of open pores and guarantees near-complete saturation of the porous body.
Vacuum Saturation Method (ISO 10545-3, EN 1936, ASTM C642)
In the vacuum method, dry specimens are placed in a vacuum chamber, and the pressure is progressively reduced to approximately 10 kPa and held for the dwell time specified by the standard. Water is then admitted while the vacuum is maintained, the chamber is returned to atmospheric pressure, and the specimens soak for a further defined period before being weighed saturated surface-dry. Vacuum saturation is the most aggressive technique and is used when complete saturation must be guaranteed, for example when apparent porosity is derived from the same measurements or when testing dense, low-porosity products such as porcelain tiles.
Immersion Method at Ambient Temperature (ASTM D570, ISO 62)
For plastics, dry specimens are fully immersed in water at 23 °C for a fixed period, most commonly 24 hours. They are then removed, dried quickly with a lint-free cloth, and weighed within one minute of removal to prevent evaporative loss. ISO 62 also defines boiling water immersion, typically 30 minutes of boiling followed by 15 minutes of cooling in water at 23 °C, for accelerated results, as well as long-term immersion to saturation and humidity exposure methods for equilibrium moisture studies. For natural stone, EN 13755 specifies full immersion at atmospheric pressure for 48 hours. Because diffusion time depends strongly on thickness, specimen geometry must always be reported together with the result.
Cobb Method for Paper and Board (ISO 535)
The Cobb test measures water absorptiveness over a defined contact time rather than total saturation. A specimen is clamped under a metal ring defining a 100 cm² area, and 100 mL of water is poured onto the surface. After the specified contact time, 60 seconds for Cobb60 or 120 seconds for Cobb120, the water is poured off, the surface is blotted with standard blotting paper under standardized pressure, and the specimen is reweighed. The Cobb value, expressed in g/m², evaluates the degree of sizing and predicts behavior during printing, gluing, and coating.
Calculation and Reporting
For all gravimetric methods, water absorption is calculated as W = (m2 - m1) / m1 × 100%. When the suspended immersed mass m3 is also measured, the same dataset yields bulk density and apparent porosity using the formulas of ASTM C373 or ISO 10545-3. Test reports must always state the standard used, specimen geometry and thickness, conditioning procedure, saturation method, and both individual and mean values, because results obtained by different methods are not directly comparable.
Applications
Water absorption testing is applied across many industries for quality control, product classification, certification, and materials research:
- Ceramic tile manufacturing: classification of tiles into groups ranging from BIa porcelain with absorption not greater than 0.5% up to BIII products above 10% according to ISO 13006, and verification of suitability for exterior and freeze-thaw exposure
- Plastics engineering: assessment of dimensional stability, mechanical property retention, and electrical insulation behavior of hygroscopic polymers such as polyamides in humid service environments
- Paper, packaging, and printing: control of sizing degree, printability, and glueability through Cobb values
- Building and construction: evaluation of natural stone, concrete, mortar, and wood-based panels for durability, moisture resistance, and coating adhesion
- Foams and thermal insulation: determination of water uptake relevant to buoyancy, insulation performance, and long-term aging of rigid boards
- Compliance and trade: support for product certification, customs verification, and supplier qualification using test reports issued by accredited third-party laboratories
FAQ
What is water absorption and why is it tested?
Water absorption is the mass of water a dry material takes up under defined saturation conditions, expressed as a percentage of the dry mass. It quantifies open porosity and predicts how the material will behave in humid or wet environments, including strength loss, swelling, frost damage, and stain susceptibility.
Which method should be chosen: boiling, vacuum, or immersion?
The choice follows the product standard for the material. Ceramic tiles are normally tested by boiling according to ISO 10545-3, with vacuum saturation used for low-porosity porcelain products; plastics use 24-hour immersion at 23 °C according to ASTM D570 or ISO 62; paper uses the Cobb method according to ISO 535. Vacuum saturation generally produces the highest value because it removes trapped air most completely.
What is the difference between ISO 62 and ASTM D570?
Both determine the water absorption of plastics gravimetrically, but they differ in specimen dimensions, drying conditions, immersion times, and optional procedures such as boiling immersion. Results from the two standards follow the same principle yet should not be compared directly without stating the method used.
Why do different methods give different values for the same material?
Temperature, pressure, and saturation time control how completely water penetrates the open pores. Boiling and vacuum methods force air out and achieve near-total saturation, while cold 24-hour immersion measures only the water that diffuses in during that window. For this reason, the test method must always accompany the reported value.
How many specimens are required for a valid test?
Typical requirements include 10 tiles for ISO 10545-3, 5 specimens for ASTM C373, at least 3 specimens for ASTM D570, and approximately 10 specimens for Cobb testing. Accredited third-party laboratories select the exact sampling plan according to the applicable standard and the purpose of the test.
How long does a water absorption test take?
Depending on the method, between one and four days. Boiling methods require drying to constant mass, typically 24 hours, boiling for 2 to 5 hours, and a cooling soak of 4 to 24 hours; immersion methods require 24 to 48 hours or more; and vacuum methods can shorten the saturation stage considerably while still achieving full saturation.