Why Is My Pottery Still Porous After Firing?

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If you’ve fired your pottery and noticed that it still absorbs water, you may be wondering, “Why is my pottery still porous after firing?”

The answer often comes down to the type of clay you used and how hot it was fired.

Some pottery is supposed to remain porous after firing, while other ceramic bodies are designed to become much less absorbent. Understanding vitrification, firing temperature, clay bodies, and porosity can help you understand whether your pottery fired correctly.

What Does Porous Pottery Mean?

Porosity refers to the amount of open space within the fired ceramic body that can absorb water or other liquids.

A highly porous piece can absorb water relatively easily, while a more vitrified ceramic body has fewer open pores and absorbs much less water.

Porosity isn’t automatically a sign that something went wrong.

The intended level of porosity depends on the clay body and the type of pottery you’re making.

Why Is My Pottery Still Porous After Firing?

One of the most common reasons is simply that the clay body is designed to remain porous at its intended firing temperature.

Different clay bodies mature at different temperatures.

For example, many low-fire earthenware clays remain relatively porous even after they have been properly fired.

Stoneware and porcelain generally vitrify more as they reach their intended higher firing ranges.

The key is to fire the clay according to the manufacturer’s recommended temperature range.

What Is Vitrification?

Vitrification is the process in which clay particles begin to fuse together during firing.

As the clay body matures, the spaces between particles become reduced and the ceramic becomes denser.

Greater vitrification generally means lower water absorption.

However, vitrification happens differently for different clay bodies, which is why you can’t simply fire every type of clay hotter to make it less porous.

Does Firing Temperature Affect Porosity?

Yes.

Firing temperature has a major effect on the maturity and porosity of a clay body.

As a clay reaches its appropriate firing range, it undergoes physical and chemical changes that can reduce its open porosity.

But hotter is not always better.

Every clay body has a recommended firing range. Firing beyond that range can cause problems such as warping, bloating, deformation, or excessive melting.

Always follow the manufacturer’s recommended firing temperature.

Could My Pottery Be Underfired?

Yes.

If your pottery is supposed to become relatively dense but remains unusually porous, underfiring is one possibility.

An underfired piece may not have received enough heatwork to reach the intended level of maturity.

Possible causes include:

• Incorrect firing temperature
• Incorrect firing schedule
• A kiln that didn’t reach the programmed temperature
• Worn heating elements
• Controller problems
• Electrical issues
• Incorrect clay firing range

If you suspect underfiring, review the firing record and consider using witness cones to help determine the heatwork achieved.

Does the Type of Clay Matter?

Absolutely.

Clay body is one of the biggest factors affecting porosity.

Earthenware, stoneware, and porcelain can have very different absorption characteristics after firing.

Two pieces fired in exactly the same kiln at exactly the same temperature can behave differently if they’re made from different clay bodies.

That’s why you should always check the technical information provided for your specific clay.

Is Porous Pottery Safe to Use?

That depends on what the pottery is intended for.

Some porous ceramics are perfectly appropriate for decorative purposes or specific traditional uses.

For functional dinnerware, mugs, bowls, and other pieces intended to hold food or liquids, you need to consider the clay body’s intended maturity, glaze, and the manufacturer’s recommendations.

A glaze can create a sealed surface, but the underlying clay body’s properties still matter.

If you’re making functional pottery, use materials specifically intended for that purpose and follow their firing recommendations.

Does Glaze Make Pottery Non-Porous?

A properly fired glaze can create a glass-like surface that significantly reduces water contact with the underlying ceramic body.

However, glazing the surface doesn’t necessarily mean the entire clay body has become non-porous.

The clay itself may still have some open porosity beneath the glaze.

This distinction is important when discussing ceramic absorption.

How Can You Tell If Pottery Is Too Porous?

One common way potters evaluate clay maturity is through a water absorption test.

The exact testing procedure and acceptable absorption level depend on the clay body and its intended use.

If you’re testing pottery for a specific purpose, follow the clay manufacturer’s technical guidance rather than relying only on how the piece looks or feels.

A piece can appear completely finished while still having significant internal porosity.

Can You Fire Pottery Again to Reduce Porosity?

Sometimes, if the piece is genuinely underfired and the clay body is designed to mature at a higher temperature.

However, don’t simply increase the firing temperature without checking the clay manufacturer’s specifications.

If the clay has already reached its intended maturity, additional firing may not provide a useful benefit and could create other problems.

Overfiring can cause the clay to become overly vitrified, deform, warp, or even partially melt.

What Happens If You Overfire Pottery?

Overfiring can be just as problematic as underfiring.

When a clay body is heated beyond its intended range, it can become excessively vitrified.

Depending on the clay, this can lead to:

• Warping
• Slumping
• Bloating
• Distortion
• Excessive shrinkage
• Deformation

This is why the goal isn’t to make pottery as dense as possible.

The goal is to reach the appropriate maturity for that particular clay body.

Why Is My Bisque Pottery Still Absorbent?

If you’re talking about bisque-fired pottery, some absorption is completely normal.

Bisque ware is intentionally fired to a stage where it becomes ceramic but remains porous enough to accept glaze.

This porosity helps glaze adhere to the surface during the glazing process.

So if your bisque ware absorbs water, that doesn’t necessarily mean the firing failed.

In fact, excessive vitrification during bisque firing can make glazing more difficult.

Why Is My Stoneware Still Porous?

If stoneware remains significantly more porous than expected after its intended glaze firing, investigate the firing temperature and clay body specifications.

Stoneware clay bodies vary, and not every stoneware behaves identically.

Check the manufacturer’s stated absorption rate and firing range.

If the kiln didn’t deliver the expected heatwork, the clay may not have reached its intended maturity.

Why Is My Porcelain Still Absorbing Water?

Porcelain is generally formulated to become highly vitrified at its intended firing temperature.

If a porcelain piece remains unusually absorbent, possible causes include underfiring, an incorrect firing schedule, or using the clay outside its recommended range.

Check the manufacturer’s technical information before changing the firing temperature.

Final Thoughts: Why Is My Pottery Still Porous After Firing?

If your pottery is still porous after firing, it doesn’t automatically mean something went wrong.

Porosity depends on the clay body, firing temperature, and degree of vitrification.

Some pottery is intentionally porous, while other clay bodies are designed to become highly vitrified and have very low water absorption.

Before firing your pottery again, check the manufacturer’s recommended firing range and absorption information. If you suspect underfiring, investigate the kiln’s performance and firing schedule rather than simply increasing the temperature.

Understanding how your particular clay body vitrifies will help you produce stronger, more predictable pottery—and prevent the common mistake of assuming that every ceramic piece should be completely non-porous.