Bloating Defect in Ceramic Tiles: Causes and Cure

Bloating appears as swelling or bulging in a fired ceramic tile body. When we cut the tile, the swollen area normally shows an internal bubble or hollow space. This means gas was trapped inside the body after the body had already started to vitrify.

We should treat bloating as a body and firing defect. It is not only a surface glaze problem. The visible swelling may come after firing, but the cause is usually connected to organic matter, carbonates, body permeability, pressing density, fast preheating or over-firing.

What Bloating Looks Like

On the tile surface, bloating can look like a raised dome, local swelling or distorted area. In severe cases the tile also loses flatness. If the tile is broken through the swollen area, we may see a hollow bubble inside the body.

This cross-section check is important. If the bulge is hollow, trapped gas is the main suspect. If the body is dense and bent without a hollow, the problem may be more related to pyroplastic deformation or over-firing.

Why Bloating Forms

During firing, organic matter, carbonates, pyrite and other gas-forming materials release gases. If the body still has open porosity, these gases escape. If the body forms a low-viscosity glassy phase too early, the gas is trapped behind the vitrified structure.

As temperature rises, the trapped gas expands. The body is already soft enough to deform, so the gas pushes the tile outward. This creates the swelling we call bloating.

Bodies That Bloat More Easily

• Vitrified bodies — low porosity makes gas escape more difficult.
• Fast-fired bodies — gas does not get enough time to oxidise and escape.
• Bodies with high organic matter — more carbon and reducing gases are created during preheating.
• Bodies with carbonate particles — calcite and dolomite release CO2 during firing.
• White-firing bodies with low iron — the source notes mention that gas pressure alone can deform these bodies.
• Over-fluxed bodies — too much liquid phase lowers viscosity and allows swelling.

How Bloating Connects with Black Core

Black core and bloating are related, but they are not the same defect. Black core is a dark centre caused by incomplete oxidation and iron reduction. Bloating is physical swelling caused by trapped gas.

In vitrified bodies, black core can come before bloating because the same oxygen-poor condition creates gas and reduction products. But we should not say every black core will bloat. A tile may show black core without swelling if the gas pressure and glass phase condition are not enough to deform the body.

Common Gas Sources

Correction Order

1. Cut the bloated tile first. Check whether the swollen area is hollow or dense.
2. Check black core in the same sample. A dark centre points to poor oxidation and trapped gas.
3. Review the preheat curve. The body needs enough time and oxygen before the surface becomes sealed.
4. Check oxygen in preheating. Increase primary combustion air where the kiln condition allows it.
5. Check raw material LOI and carbonate particles. High gas load needs either better raw material control or slower degassing.
6. Check pressing density. An over-dense green tile has less path for gas escape.
7. Check peak temperature and flux level. Over-firing or over-fluxing can lower body viscosity and allow swelling.
8. Check spray drier combustion. Oil, naphtha or carbon residue in powder can create local bloating that kiln adjustment alone cannot fix.

What to Check First

When bloating starts, collect samples from different press cavities, kiln lanes and production times. Break the swollen part and compare the inside. If it is hollow and dark, start with oxidation, preheat and gas sources. If it is dense and bent, check over-firing, flux level and pyroplastic deformation.

I hope I have provided enough information about bloating defect in ceramic tiles. If you want to know anything more or share your plant experience, please share it below in the comment section.