What is vitrification (verglazen)?
The Dutch building term “verglazen” means that a material becomes, or is made, glassy. It has three meanings: giving a ceramic product a glaze, the glassy transformation of silica (SiO₂) in clay during firing (vitrification), and fitting new glass in a window or glazed screen. The context tells you which one is meant.
1. Glazing ceramics
A roof tile, wall tile or brick is coated with a slurry that melts during firing and turns into a smooth, glassy layer. There are two routes. With an engobe, this happens in a single firing: the coating goes onto the unfired product. With biscuit firing, the product is fired first, then glazed and fired again. In English this is to glaze.
2. Becoming glassy during firing
At high temperatures, silica and other constituents in the clay partly melt and, on cooling, solidify into a glassy mass. This happens, for example, to the sand in the clay when bricks are fired. The further this goes, the denser and harder the brick becomes and the less water it absorbs. That is why clinker bricks are fired at high temperature. In vitrified tiles the whole body of the tile is vitrified, because the tile as a whole was fired at a very high temperature. In English this is to vitrify.
3. Reglazing
In glazing work, verglazen simply means fitting new glass in a frame, window or glazed screen.
Application
- Choosing tiles: a tile that is vitrified right through absorbs little water. That suits outdoor use and wet rooms, where a glazed tile with a porous body may be less suitable.
- Facing bricks and pavers: highly vitrified bricks are dense and hard-wearing but hardly absorb water. That affects the choice of mortar.
- Repairs: you cannot touch up damaged glaze like paint; glaze is only formed in the kiln.
| Tile vitrified right through | Absorbs little water; suits outdoor use and wet rooms. |
|---|---|
| Glazed tile with a porous body | Glassy layer only on the face; once the glaze is gone, the porous body is exposed; may be less suitable for outdoor use and wet rooms. |
Tips and points of attention
- Glazed is not the same as vitrified: on a glazed tile the glassy layer is only on the face. Once the glaze is gone, the porous body is exposed.
- Ask about water absorption: the manufacturer’s data sheet states how much water a tile or brick absorbs. That tells you more than the product name.
- Use the right adhesive or mortar: dense, vitrified materials bond poorly with products intended for absorbent substrates.
Related terms
- Glaze
- Engobe
- Clinker
- Brick manufacturing
- Glazing
- Hoffmann kiln
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