Open Porosity vs Closed Porosity in Process Ceramics: Why the Difference Matters

Porosity is one of the most widely used terms in ceramic engineering.

It is also one of the most easily misunderstood.

A datasheet may say that a ceramic contains:

10%, 20% or even 80% porosity.

That number alone does not explain how the pores interact with the process.

The critical distinction is between:

open porosityandclosed porosity.

Open pores are connected to the external surface and can potentially be reached by:

• gas;
• liquid;
• contaminants.

Closed pores are sealed inside the ceramic body and do not directly communicate with the surrounding environment.

This difference changes how ceramic behaves in filtration, chemical exposure, water absorption, insulation and leak-tight applications.

What Is Open Porosity?

Open pores form connected or partially connected pathways from the surface into the ceramic.

Liquid can enter them.

Gas may pass through them if the pore network is sufficiently connected.

Open porosity therefore contributes directly to properties such as:

• water absorption;
• permeability;
• accessible internal surface.

This is especially important for:

• foam ceramic;
• porous catalyst carriers;
• biological filter media.

In those applications, open porosity is often intentional.

What Is Closed Porosity?

Closed pores are isolated cavities inside the fired ceramic.

They contain gas but do not connect to the external surface.

Water cannot normally enter them during a simple immersion test.

Closed pores can reduce the bulk density of a ceramic without increasing water absorption.

This is why density and water absorption do not always move together in a simple way.

Why Water Absorption Mainly Reflects Open Porosity

When ceramic is immersed in water, accessible pores can fill with liquid.

Closed pores remain isolated.

Therefore a water-absorption test mainly provides information about:

accessible open pore volume.

It does not directly measure every void inside the ceramic.

This is an important limitation when interpreting water-absorption data.

Dense Chemical Ceramic

For many acid-resistant ceramic packing materials, relatively low accessible porosity is desirable because excessive open pores can:

• increase liquid penetration;
• expose more internal surface;
• weaken the structure.

However, zero porosity is not necessary for every tower-packing application.

A properly fired chemical ceramic can perform very well with controlled pore structure.

Foam Ceramic Is Completely Different

An open-cell foam ceramic intentionally contains very high interconnected porosity.

Its function depends on allowing gas or liquid to travel through the structure.

In this case, reducing open porosity too far would destroy the product's purpose.

This shows why “lower porosity is better” is not a universal quality rule.

The correct porosity depends on function.

Closed Porosity and Thermal Insulation

Trapped gas inside closed pores can reduce thermal conductivity.

This is one reason some insulating ceramics intentionally use significant closed or fine pore structures.

A process component designed for structural strength may have completely different pore targets.

Again, application controls the desired microstructure.

Open Pores and Chemical Attack

If an aggressive liquid enters open pores, it exposes a larger internal area.

Chemical reaction may therefore occur below the visible surface.

This can become important when the ceramic contains a phase that is vulnerable to the process chemistry.

A very dense ceramic generally provides less accessible internal area than a highly porous material.

But chemistry still matters more than porosity alone.

A dense material made from an incompatible composition can still fail.

Closed Pores Can Still Influence Strength

Although closed pores are not chemically accessible, they are still discontinuities in the ceramic structure.

Under mechanical load, pores can concentrate stress.

Large closed pores can therefore reduce mechanical strength.

Pore size and distribution matter—not only the total percentage.

Why Two Ceramics With the Same Total Porosity Can Behave Differently

Imagine Ceramic A and Ceramic B both have 15% total porosity.

Ceramic A contains mostly small isolated pores.

Ceramic B contains a connected open-pore network.

They can have very different:

• water absorption;
• permeability;
• chemical penetration;
• strength.

This is why a single porosity percentage is incomplete without knowing how the pore network is structured.

Apparent Porosity

Ceramic specifications sometimes use the term apparent porosity.

This usually refers to pores accessible to the test liquid under the defined measurement method.

It is therefore more closely related to open porosity than total porosity.

The exact definition should be checked against the test standard used.

Why Process Ceramic Buyers Should Care

Different products need different pore structures.

For example, a:

gas-tight sleeve should generally minimize connected porosity.

A:

foam filter requires high connected porosity.

A:

catalyst carrier may intentionally combine macroscopic flow pores with microscopic coating porosity.

The word “ceramic” alone tells you almost nothing about the desired pore structure.

Test Method Matters

Methods used to characterize pore structure may include:

• water absorption;
• Archimedes-type density measurements;
• mercury intrusion;
• gas adsorption;
• microscopy.

Each measures a different aspect of the pore system.

For routine industrial procurement, advanced pore characterization is not always necessary.

But engineers should at least understand what the stated number means.

Engineering Takeaway

Porosity is not just “empty space.”

The connection of that empty space to the external surface determines how it affects real process behavior.