The water absorption test is a fundamental laboratory procedure used to quantify how much moisture a material takes up when immersed in water, providing direct insight into open porosity, density, and long-term durability. This guide explains the complete water absorption test steps for ceramics, plastics, composites, and stone in accordance with recognized standards such as ISO 62, ISO 10545-3, ASTM D570, and EN 13755. Whether you are performing routine quality control or material qualification in an accredited third-party laboratory, following these procedures will help you achieve accurate, repeatable, and auditable results.
Technical Overview
Water absorption is defined as the ratio of the mass of water absorbed by a specimen to the mass of the same specimen in the dry state, expressed as a percentage. It is one of the most widely used indicators of open porosity: the more interconnected pores a material contains, the more water it draws in during immersion, and the more susceptible it may be to frost damage, staining, dimensional swelling, and mechanical degradation. The test principle is straightforward—dry the specimen to constant mass, immerse it under controlled conditions, weigh it again, and calculate the mass gain—but strict control of drying time, water temperature, surface wiping technique, and weighing timing is essential for valid results.
The procedure is governed by different standards depending on the material under test:
- ISO 62 and ASTM D570 — determination of water absorption of plastics
- ISO 10545-3 and GB/T 3810.3 — water absorption, apparent porosity, and relative bulk density of ceramic tiles
- ASTM C373 — water absorption and related properties of fired ceramic products
- EN 13755 and EN 1936 — water absorption at atmospheric pressure and by vacuum saturation for natural stone
- ASTM D5229 — moisture absorption properties of polymer matrix composite materials
Testing Samples
The test is applicable to a broad range of engineering and construction materials. Specimen geometry, quantity, and preconditioning requirements vary by standard, so the applicable document must be consulted before cutting or machining samples.
Ceramic Tiles and Porcelain Products
ISO 10545-3 typically requires ten complete tiles. When tiles are too small, a sufficient number of pieces with a combined facial area of at least 0.01 m² may be grouped and tested as one specimen. Broken edges are acceptable provided fracture surfaces are clean and free of loose debris.
Plastic Specimens
ASTM D570 uses discs approximately 76 mm in diameter and 3.2 mm thick, while ISO 62 prefers square plates of 60 mm × 60 mm with a thickness of 1 mm for sheet materials. Specimens must be free of stresses, burrs, and surface contamination that could bias water uptake.
Fiber-Reinforced Composites
Per ASTM D5229, specimens are usually cut to dimensions agreed between parties, with edges sealed or left unsealed depending on whether edge effects are to be included. Composite testing is often continued over an extended immersion schedule to construct a full absorption-versus-time curve.
Natural Stone and Masonry Units
EN 13755 specifies test pieces of approximately 50 mm cubes or equivalent prisms, with a minimum of five specimens per sample lot. All faces should be sawn, and specimens must be representative of the block or consignment being evaluated.
Testing Items
Depending on the selected method and standard, a water absorption test campaign may report the following parameters:
- Water absorption after 24-hour immersion in water at ambient or standard temperature
- Water absorption after immersion in boiling water
- Water absorption by vacuum saturation
- Apparent (open) porosity and exterior volume by hydrostatic weighing
- Relative bulk density and apparent density of the dry material
- Long-term immersion absorption kinetics, including saturation level and diffusion characteristics
Testing Methods
The general workflow consists of drying, immersion under a defined regime, surface drying, weighing, and calculation. The steps below consolidate the common requirements of the principal standards.
Step 1: Specimen Preparation and Oven Drying
Remove loose particles and contaminants from the specimens. Place them in a ventilated oven at 110 ± 5 °C (for ceramics and stone) or at the drying temperature specified by the plastics standard, and dry until constant mass is reached—that is, until successive weighings at least 24 hours apart differ by less than approximately 0.1 percent. Cool the specimens to room temperature in a desiccator, then weigh each one to 0.01 g accuracy and record the dry mass m1.
Step 2: Immersion — Cold Water Method
Immerse the dried specimens completely in deionized or distilled water maintained at the specified temperature, typically 23 ± 1 °C for plastics (ISO 62, ASTM D570) or 20 ± 5 °C for stone at atmospheric pressure (EN 13755, 48-hour immersion). Keep specimens at least 15 mm apart and at least 50 mm below the water surface so that all faces are freely exposed. For the standard 24-hour determination, leave the specimens immersed for exactly 24 hours from the moment of complete submersion.
Step 3: Immersion — Boiling Water Method
For ceramic tiles per ISO 10545-3, place the dry, weighed specimens in a vessel filled with water at ambient temperature, heat to boiling, and maintain a gentle boil for 2 hours. Then allow the specimens to cool in the same water for 4 hours until they reach room temperature. For plastics, ASTM D570 and ISO 62 specify a shorter boiling immersion (on the order of 30 minutes) followed by a brief cooling period in the water before weighing.
Step 4: Immersion — Vacuum Saturation Method
Place the dry specimens in a vacuum desiccator and evacuate gradually to the low absolute pressure specified by the applicable standard, commonly on the order of 10 kPa or lower. Hold the vacuum for the prescribed dwell time, typically 15 to 30 minutes, to evacuate air from the open pores. While maintaining the vacuum, admit water slowly until the specimens are fully submerged, then release the vacuum, allow the vessel to return to atmospheric pressure, and let the specimens soak for the additional period required by the standard. Vacuum saturation forces water into pores that capillary immersion alone would not fill, and it is mandatory for classifying low-absorption products such as porcelain tiles in many certification schemes.
Step 5: Surface Drying and Saturated Weighing
Remove each specimen from the water and rapidly remove surface water with a damp, well-wrung cloth or chamois, working quickly so that water inside the pores is not extracted. ASTM D570 requires the specimen to be weighed within one minute of removal from the water. Record the saturated surface-dry mass m2. Where density and porosity are also required, suspend the saturated specimen in water on a thin wire and record the hydrostatic mass m3.
Step 6: Calculation and Expression of Results
Calculate the water absorption of each specimen as follows:
W = (m2 − m1) / m1 × 100 %
where m1 is the dry mass and m2 is the saturated surface-dry mass. Where the Archimedes measurements are performed, apparent porosity may be calculated as P = (m2 − m1) / (m2 − m3) × 100 %, and the relative bulk density of the dry material is obtained from m1 / (m2 − m3). Report the individual values, the arithmetic mean, the immersion method used, water temperature, immersion duration, and the standard applied. Absorption is normally reported to two decimal places for low-porosity products and to one decimal place for porous materials.
Step 7: Quality Control of the Procedure
The following factors are the most common sources of error and must be controlled during testing:
- Incomplete drying before the initial weighing, which understates absorption
- Residual surface films of water, which overstates absorption
- Delay between removal and weighing, which allows pore water to evaporate
- Water temperature or immersion time outside the specified tolerance
- Contaminated or hard tap water, which should be replaced by deionized water
- Insufficient specimen quantity for statistical validity
Applications
Water absorption data are used across many industries and decision contexts:
- Classification of ceramic tiles into porosity groups (for example, porcelain tiles at 0.5 percent absorption or less) for product certification and marking
- Quality control of incoming polymers and molded plastic parts where moisture affects dimensional stability and electrical properties
- Durability assessment of building stone, roof tiles, and clay bricks against freeze–thaw damage and efflorescence
- Qualification of composite laminates for marine, aerospace, and infrastructure service, where moisture uptake degrades matrix-dominated properties
- Research and development of formulations, firing schedules, and surface treatments aimed at reducing open porosity
- Verification testing for regulatory compliance or dispute resolution in accredited third-party laboratories
FAQ
What is the difference between the boiling method and the vacuum method?
The boiling method drives water into open pores through prolonged thermal immersion and is well suited to medium- and high-porosity products. The vacuum method removes trapped air under reduced pressure before admitting water, saturating even very fine pore networks. For low-absorption porcelain tiles, the vacuum method provides better discrimination and is often required for classification purposes.
How long must a specimen be dried before testing?
Until constant mass is achieved. In practice this means repeating oven drying cycles with weighings at least 24 hours apart until the change in mass is below the threshold defined by the standard, commonly 0.1 percent. Drying time varies from a few hours for thin plastics to several days for thick or dense ceramic and stone specimens.
Why must surface water be wiped off before weighing?
The test measures water held inside the pore structure, not water clinging to the exterior. A thin film left on the surface can add a significant mass fraction, particularly on small or low-absorption specimens, so a quick, consistent wiping technique with a damp cloth is essential to avoid overstating the result.
What absorption values are typical for common products?
Porcelain tiles generally absorb 0.5 percent or less, compact stoneware and glazed floor tiles commonly fall between 0.5 and 3 percent, wall tiles range from roughly 3 to 10 percent, and earthenware or terracotta products may exceed 10 percent. Engineering plastics typically absorb well under 1 percent, while glass-fiber-reinforced laminates may absorb several percent at saturation.
Can the test be performed on site?
The quantitative procedure described above is a laboratory method requiring a controlled oven, precision balance, and temperature-controlled water bath. Simple on-site indicators, such as placing water droplets on a surface and observing spreading, can flag gross porosity but do not replace standardized laboratory measurement when certification or contractual values are required.