What is a hollow-sounding brick?
A hollow-sounding brick is a fired brick, floor tile or roof tile with internal voids, left by a firing that went wrong, which rattles or sounds dull when shaken or tapped. English has no single settled trade term for it; it is described as a hollow-sounding or underfired brick. Dutch does have a dedicated word, rammelaar, literally a rattle, which is where this entry comes from.
How the fault arises
Firing clay requires the heat to reach the core as evenly as it reaches the surface. If the firing runs badly, the core lags behind the outside, or gases form pockets inside. Voids open up in the middle while the face may still look perfectly sound. That is what makes the fault awkward: there is often nothing to see, and only the sound gives it away.
The ring test
- Hold the brick free in one hand. Resting it on a surface deadens the sound and wastes the test.
- Tap it with a piece of steel or a second brick. A properly fired brick rings clear and short.
- Listen for the difference. Dull, loose or rattling means voids in the core.
- Shake tiles and pavers as well. Where the voids are large, loose material can be heard moving.
- Set suspect pieces aside at once. Keep a single reject stack rather than reassessing them later.
Where they can and cannot go
| Use | Objection |
|---|---|
| Fair-faced brickwork | Weakened inside, liable to break when cut or loaded |
| Loaded floor | Does not carry load reliably |
| Exposed to frost | Voids hold water, which expands on freezing |
| Dry internal work to be plastered | Sometimes usable if loading is light |
A brick with internal voids is weaker than it looks. Keep it out of exposed work, out of loaded floors and away from anywhere rain or frost can reach. Where the work will be plastered and lightly loaded it may still have a use, but only if it has been sorted deliberately rather than mixed in unnoticed.
Related terms
- Fire-cracked brick
- Clinker brick
- Firing process
- Reject brick
- Common brick
- Frost resistance
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