Category: Technical Knowledge
Published by: MosaicPeel Technical Team
Last Updated: 2026
Reviewed by: MosaicPeel Product Engineering & Quality Control Department
Quick Answer
Water absorption is measured as the percentage of water a tile body absorbs by weight and is classified into four tiers: Impervious (≤0.5%), Vitreous (0.5%–3%), Semi-vitreous (3%–7%), and Non-vitreous (>7%). Only Impervious-rated tile — porcelain and glass — is safe for pools, outdoor freeze-thaw climates, and constant-submersion applications. This single specification, tested per ASTM C373 or ISO 10545-3, is the most important number on any mosaic tile data sheet for wet or exterior projects. The full breakdown is below.
Table of Contents
- The 4 Classifications
- Why It Matters for Pools & Outdoors
- How Absorption Is Tested
- Application Chart by Absorption Rate
- Water Absorption by Material
- How to Verify a Supplier’s Claims
- FAQ
The 4 Classifications
| Classification | Water Absorption | Description |
|---|---|---|
| **Impervious** | ≤ 0.5% | Essentially non-porous; the standard for pools, outdoor, and constant-wet applications |
| **Vitreous** | 0.5% – 3% | Low absorption; suitable for most indoor residential use, marginal for constant submersion |
| **Semi-vitreous** | 3% – 7% | Moderate absorption; indoor walls or dry-to-lightly-splashed areas only |
| **Non-vitreous** | > 7% | High absorption; restricted to dry indoor decorative walls; will harbor moisture and mold if exposed to water |
These four tiers, codified under ANSI A137.1, form the backbone of nearly every other technical decision in a mosaic tile specification — adhesive type, grout selection, and application suitability all flow from where a given product falls on this scale.
Why It Matters for Pools & Outdoors
Water absorption isn’t an abstract lab number — it directly predicts two real-world failure modes:
Frost wedging (outdoor freeze-thaw climates): water trapped inside a porous tile body expands approximately 9% in volume when it freezes. In a Non-vitreous or Semi-vitreous tile, this expansion generates enough internal pressure to crack the glaze or fracture the tile body itself — often after just one or two freeze-thaw cycles. Only Impervious-rated material reliably survives sustained outdoor exposure in a climate that drops below freezing.
Constant submersion failure (pools and spas): in a continuously wet environment, a porous tile body doesn’t just risk cracking — it can absorb pool chemicals, support mold or mineral growth beneath the surface, and gradually degrade the bond between tile and substrate as moisture cycles in and out of the tile body. This is why glass (0% absorption) and Impervious-rated porcelain are the only materials the pool tile industry considers standard for submerged applications.
For the complete buyer framework specific to pools, see: How to Choose Mosaic Tile for Swimming Pools.
How Absorption Is Tested
Water absorption is determined through a standardized laboratory process:
- A tile sample is dried completely and weighed (dry weight)
- The sample is fully submerged in water for a specified period (typically 24 hours under ASTM C373, or a boiling-water method under some ISO protocols)
- The sample is reweighed (saturated weight)
- Absorption percentage = (Saturated Weight − Dry Weight) ÷ Dry Weight × 100
This is a destructive, lab-based test — it cannot be reliably estimated by visual inspection, and marketing language like “water-resistant” is not a substitute for an actual absorption percentage from a recognized testing standard.
Application Chart by Absorption Rate
| Application | Minimum Required Classification |
|---|---|
| Swimming pool interior (submerged) | Impervious (≤0.5%) |
| Outdoor patio / freeze-thaw climate | Impervious (≤0.5%) |
| Shower floor | Impervious preferred; Vitreous marginal |
| Bathroom floor (outside direct shower spray) | Vitreous or better |
| Kitchen backsplash (dry, away from sink splash) | Vitreous or Semi-vitreous acceptable |
| Dry interior accent wall | Any classification, including Non-vitreous |
Water Absorption by Material
| Material | Typical Water Absorption | Classification |
|---|---|---|
| Glass mosaic | 0% | Impervious |
| Porcelain mosaic | ≤ 0.5% | Impervious |
| Marble (Carrara) | 0.2% – 0.4% | Impervious to Vitreous, quarry-dependent |
| Marble (Calacatta) | 0.75% or higher | Vitreous |
| Standard ceramic mosaic | 3% – 7% | Semi-vitreous |
| Unglazed low-fire ceramic | > 7% | Non-vitreous |
Note that natural stone absorption varies meaningfully by quarry source even within the same stone type — always request quarry-specific test data for marble rather than assuming a general “marble” absorption figure applies to a specific product.
How to Verify a Supplier’s Claims
- Request the actual test report referencing ASTM C373 or ISO 10545-3, not just a stated percentage without supporting documentation.
- Confirm the test was performed on the specific product/SKU under consideration, not a general material category claim.
- For pool and outdoor projects specifically, treat this documentation as a non-negotiable part of the purchasing decision, not an optional nice-to-have — see our full guide on Mosaic Tile Quality Certifications: What Every Importer Should Verify.
MosaicPeel provides water absorption test documentation for every product line rated for pool, outdoor, or wet-area use — request current test data through our Swimming Pool Mosaic Tiles or Outdoor Mosaic Tiles product pages.
FAQ
What is the difference between impervious and vitreous tile?
Impervious tile absorbs 0.5% or less of its weight in water and is the only classification reliably suited to constant submersion (pools) and outdoor freeze-thaw exposure. Vitreous tile absorbs 0.5%–3% — low enough for most indoor residential use, but not reliably suited to constant wet or freeze-thaw conditions.
Can semi-vitreous mosaic be used in a shower?
It is not recommended for shower floors or direct-spray shower walls. Semi-vitreous tile (3%–7% absorption) is better suited to dry or lightly splashed applications, such as a bathroom wall outside the direct shower spray zone.
How is tile water absorption tested?
A dried tile sample is weighed, then fully submerged in water for a standardized period (commonly 24 hours per ASTM C373), then reweighed. The percentage weight gain from water absorption determines the classification. This is a laboratory test, not something that can be reliably assessed visually.
Is porcelain always impervious?
Not automatically — “porcelain” describes a manufacturing process (high-temperature, high-pressure firing of refined clay), and while properly manufactured porcelain typically achieves Impervious classification, buyers should still request specific test data for a given product line rather than assuming the classification based on the material name alone.
This guide reflects ANSI A137.1 water absorption classifications and ASTM C373 / ISO 10545-3 testing methodology referenced across the tile industry in 2026. Always request product-specific test documentation before specifying material for wet, submerged, or freeze-thaw applications.
Related reading from MosaicPeel:
– How to Choose Mosaic Tile for Swimming Pools
– DCOF and Slip Resistance: What Importers Need to Know
– Mosaic Tile Quality Certifications: What Every Importer Should Verify
– Mosaic Tile for Outdoor Living Spaces
– Swimming Pool Mosaic Tiles — Full Product Range