Water absorption testing is one of the most important quality control procedures for sanitary ceramics such as toilets, washbasins, urinals, and bidets, because it directly measures the degree of vitrification of the fired ceramic body. This article explains the testing principle, applicable standards such as GB/T 6952, EN 997, ISO 10545-3, and ASTM C373, specimen preparation, boiling and vacuum test methods, and how the results are evaluated against the strict limits required for vitreous china sanitaryware.
Technical Overview
Water absorption (E) is defined as the mass of water that a fired ceramic body can absorb under specified saturation conditions, expressed as a percentage of the dry mass of the specimen. For sanitary ceramics, it is the primary indicator of how completely the body has vitrified during firing: a densely sintered, glassy body absorbs very little water, while an under-fired or porous body absorbs significantly more.
The test compares the oven-dry mass of a specimen cut from the product with its saturated mass after immersion in water under controlled boiling or vacuum conditions. The result is calculated as:
E (%) = (m2 − m1) / m1 × 100, where m1 is the dry mass and m2 is the saturated surface-dry mass.
Low water absorption is essential for sanitaryware performance for several reasons:
- Hygiene: a vitrified body resists penetration by dirty water, urine, cleaning agents, and odor-bearing liquids, and provides fewer pores in which bacteria can lodge.
- Frost resistance: water absorbed into an open-pore body can freeze and expand, causing spalling or cracking when fixtures are installed in unheated locations.
- Dimensional stability: porous bodies undergo moisture expansion in service, which can generate tensile stress in the glaze and lead to delayed crazing.
- Mechanical strength: flexural and impact strength decrease as body porosity increases, affecting the service life of wall-hung and load-bearing fixtures.
The principal standards governing this test and its acceptance limits include:
- GB/T 6952-2015 — the Chinese product standard for sanitary ceramics, which limits the average water absorption of vitreous china products to no more than 0.5%.
- EN 997 — the European product standard for WC pans and WC suites, specifying a maximum water absorption of 0.5% for vitreous china, determined in accordance with EN ISO 10545-3.
- ASME A112.19.2 / CSA B45.1 — the North American standards for ceramic plumbing fixtures.
- ISO 10545-3 and GB/T 3810.3 — the internationally harmonized test method standards describing the boiling and vacuum saturation procedures.
- ASTM C373 — the boiling-method standard widely used for fired whiteware products, including ceramic plumbing fixtures.
Testing Samples
WC Pans and Toilet Suites
Specimens are cut from representative areas of close-coupled, one-piece, wall-hung, and squatting WC pans. Sampling locations are normally chosen on concealed or non-functional surfaces, such as the underside of the bowl, the inside of the trapway, or the rear of the fixture, so that the product remains structurally representative while visible surfaces are not relevant to the body measurement.
Washbasins and Basin Components
Washbasins, pedestal basins, countertop and undercounter basins, and semi-recessed basins are sampled in a similar manner. Fragments are typically taken from the underside of the basin or from the flat back areas where the body thickness is consistent and the manufacturing conditions are representative of the whole article.
Urinals, Bidets, and Cisterns
Wall-hung and floor-standing urinals, bidets, ceramic cisterns, and tank lids are also subject to the same water absorption limits, since all vitreous china components are expected to share the same high degree of vitrification. Samples are cut from hidden edges or unglazed regions where available.
Specimen Preparation
Representative fragments, typically several pieces each measuring approximately 50 mm across, are cut from the product using a diamond saw or by carefully breaking the ware. The following preparation rules apply:
- All cut edges are smoothed and all loose dust, slurry, and debris are removed by brushing and rinsing.
- Grease, oil, and surface contamination are cleaned off so that water can wet the open pores freely.
- Specimens are identified individually so that each result can be traced back to its sampling position.
- Cracked or visibly flawed fragments are discarded and replaced, as pre-existing cracks can distort the saturation behavior.
Testing Items
The core measured parameter is the water absorption of the fired body, expressed as a mass percentage and reported for each specimen together with the average value. Depending on the client's requirements and the referenced standard, the following related items may be reported from the same test series:
- Individual water absorption values for each specimen, normally reported to two decimal places.
- Average water absorption for the sample lot, evaluated against the applicable limit (≤ 0.5% for vitreous china).
- Apparent porosity and bulk density, which can be derived when the hydrostatic (suspended) mass of the saturated specimen is also measured, as provided in ISO 10545-3 and ASTM C373.
- Conformity assessment against the specified product standard, reported as pass or fail.
Testing Methods
Boiling Method (Reference Procedure)
The boiling method is the reference procedure for sanitary ceramics and is specified by GB/T 6952, EN 997 via EN ISO 10545-3, and ASTM C373. The general sequence is as follows:
- Drying: specimens are oven-dried, for example at 110 ± 5 °C per ISO 10545-3 (ASTM C373 uses a higher drying temperature), until constant mass is reached, i.e., until successive weighings agree within 0.1%.
- conditioning: dried specimens are allowed to cool to room temperature in a desiccator to prevent moisture pick-up from the air.
- Dry weighing: each specimen is weighed on a balance accurate to approximately 0.01% of the specimen mass; this value is recorded as m1.
- Boiling: specimens are fully immersed in water, which is then brought to and maintained at a rolling boil. ISO 10545-3 specifies 2 hours of boiling, while ASTM C373 specifies approximately 5 hours followed by immersion during cooling; the required period is defined by the referenced method.
- Cooling and soaking: the heat source is removed and the specimens remain immersed while the water cools to room temperature, typically for at least 4 hours (or up to 24 hours under ASTM C373), ensuring complete saturation of the open pores.
- Saturated weighing: each specimen is removed, and surface water is removed by wiping with a damp, well-wrung cloth or chamois until the surface is visibly damp-dry, and the saturated surface-dry mass m2 is determined immediately to avoid evaporation losses.
Vacuum Saturation Method (Alternative Procedure)
The vacuum method achieves saturation by evacuating the air from the open pores rather than displacing it by boiling. Dry specimens are placed in a vacuum vessel, the pressure is reduced to a low residual pressure on the order of 10 kPa and held for a defined period, and water is then admitted while the vacuum is maintained or immediately released so that atmospheric pressure forces water into the evacuated pores. After a soaking period, the saturated surface-dry mass is determined in the same way as for the boiling method. This procedure is faster and is commonly applied to ceramic tiles under ISO 10545-3; where it is used for sanitaryware, the acceptance criteria of the product standard still apply.
Calculation and Evaluation
Water absorption is calculated for each specimen as E = (m2 − m1) / m1 × 100, and the results are evaluated against the limit in the governing product standard. For vitreous china sanitary ceramics, both GB/T 6952-2015 and EN 997 restrict water absorption to a maximum of 0.5%. Results are normally assessed on the average of the specimens, with some specifications also capping individual values. A result significantly above the limit indicates an under-vitrified body, commonly caused by insufficient firing temperature or soak time, an unsuitable body formulation, or inadequate green density from casting or pressing.
Equipment and Quality Assurance
The test requires a forced-convection drying oven capable of controlled temperature, a clean boiling vessel with a heat source, a desiccator, a calibrated precision balance, a damp cloth or chamois for surface drying, and, for the vacuum method, a vacuum chamber with a pressure gauge. Accredited third-party laboratories performing this test maintain traceable calibration of the balance and oven, verify constant-mass criteria before recording dry masses, and condition specimens under controlled ambient conditions to minimize weighing errors.
Applications
Water absorption testing of sanitary ceramics is applied across the entire product life cycle:
- Factory quality control: routine monitoring of kiln output to verify that firing curves and body formulations consistently deliver vitrified bodies within specification.
- Type testing and certification: demonstration of conformity with product standards such as GB/T 6952 and EN 997 as part of market access, CE marking support, and listing programs for plumbing fixtures.
- Procurement and incoming inspection: verification by buyers, contractors, and building owners that supplied fixtures meet the specified absorption limit before installation.
- Export inspection: batch testing performed by accredited third-party laboratories to support trade documents and customer declarations of conformity.
- Failure analysis: investigation of in-service defects such as crazing, cracking, frost damage, staining, and odor retention, where excessive body porosity is frequently a contributing factor.
- Research and development: optimization of body recipes, flux content, firing temperature, and kiln dwell times to balance vitrification against deformation risk and energy consumption.
FAQ
What is the maximum water absorption allowed for vitreous china sanitaryware?
The dominant requirement for vitreous china products is an average water absorption of no more than 0.5%, as specified by GB/T 6952-2015 in China and EN 997 in Europe. North American ceramic plumbing fixture standards impose comparable limits for vitreous china bodies. Fireclay products, where permitted, are governed by their own product standards with different limits.
What is the difference between the boiling method and the vacuum method?
The boiling method displaces air from the open pores through prolonged boiling and subsequent soaking, and is the reference method for sanitary ceramics under ISO 10545-3 and ASTM C373. The vacuum method removes air by applying a strong vacuum before water admission, achieving saturation more quickly. Both methods determine saturated surface-dry mass, but the boiling method is generally preferred for conformity assessment of sanitaryware because it is the procedure cited by the product standards.
Where should test specimens be taken from on the product?
Specimens are cut from concealed or non-functional areas of the fixture, such as the underside of a basin or the hidden regions of a WC pan, so that the sampled body is representative of normal production. Edges are smoothed and cleaned before drying so that water can freely enter the open pores of the freshly exposed body.
What causes excessively high water absorption in sanitary ceramics?
Common causes include firing at too low a temperature or with insufficient soak time, a body formulation lacking sufficient fluxing materials, excessive pore-forming impurities in the raw materials, and low green density from poor casting or pressing practice. Because absorption reflects the finished body, corrective action typically involves adjusting the firing schedule or the body recipe rather than the glaze.
Does the glaze layer affect the test result?
Water absorption is fundamentally a property of the fired body. The glaze is a thin, essentially non-porous glass layer, so its contribution to the specimen mass change is negligible. When specimens are cut, the exposed body edges provide the dominant path for water entry, which is why freshly cut edges must be cleaned and smoothed before testing.
How long does a complete water absorption test take?
A complete test typically takes one to two working days: drying to constant mass may require 24 hours or more depending on specimen thickness, followed by boiling (2 to 5 hours depending on the method), cooling and soaking (4 to 24 hours), and final weighing. Accredited third-party laboratories factor this timeline into their reporting schedules for conformity testing.