These are thermal expansion curves for body, engobe and glaze (from a dilatometer, a device that measures it against increasing temperature). The upper line is the body. The center line is the engobe. The lower line is the glaze. The ceramic tile industry is very conscious, not only of glaze-fit but also engobe-fit. Engobes (slips) are employed to cover brown or red burning bodies so they glaze like a porcelain. Typically technicians tune the formulation of the engobe to have an expansion between the body and glaze. The body is highest so that during cooling, as it contracts, it puts a squeeze on the engobe (the engobe thus never finds itself under tension). The glaze has the lowest expansion, it is under a state of compression by the engobe (and slightly more by the body). This equilibrium enables the tile to wear for many years without crazing or shivering. Chart courtesy of Mohamed Abdelmagid.

This picture has its own page with more detail, click here to see it.
Since white burning slips are made from refractory materials they are not nearly as vitreous as terracotta bodies, they need help to mature and a frit is the natural answer. With the right amount, fired shrinkage can be matched to the body while the slip remains opaque.
L3685A slip on the left: An adjustment to the popular "Fish Sauce" recipe (which is basically a super plastic white body). 8% Frit 3110 replaces 8% Pyrax to make it glass-bond to the body better while remaining completely opaque.
Right L3685C: It has 15% frit, moving it into glaze territory, with translucency replacing opacity. Although expensive zircon could be added to restore opacity, that doesn’t fix the excessive firing shrinkage it now has, mismatching it with the body (which becomes more eventful the thicker it is applied).

This picture has its own page with more detail, click here to see it.
Example of a glaze (G1916J) shivering on the rim of a mug. But the situation is not as it might appear. This glaze fits some bodies, crazes on others and shivers on yet others! This is a testament to how tricky it can be to fit glaze at low temperatures. But this is not a white glaze, it is a transparent over a white slip (or engobe). Alone on this red body this glaze appears to fit OK, but here it is exploiting the weaker body-engobe interface - this is where the release is taking place. This failure turned out to be a blessing, alerting us to the need to increase the expansion of the glaze a little to fit this body better (and enable its use with this engobe). This being said, the engobe likewise may be under too much compression, it may not contain enough silica.

This picture has its own page with more detail, click here to see it.
This artisan, Dennis Cuku, is the king of DIY tile, making product that he calls "actually HANDMADE". He uses a red-burding terra-cotta-like middle temperature clay body. He also makes glazes in-house and fires using 36 shapes. He mixes 129 glazes and produces about 50,000 ft.² of tile per year. Tile making presents many unique challenges, not the least of which is the need for consistency and predictability of surface character and color. This endeavour is made possible with data, a lot of it. Not just glaze recipes, but many forming, glazing and firing procedures and techniques that must be documented.
| Glossary |
Co-efficient of Thermal Expansion
The co-efficient of thermal expansion of ceramic bodies and glazes determines how well they fit each other and their ability to survive sudden heating and cooling without cracking. |
| Glossary |
Ceramic Tile
Tile manufacture is the largest sector of ceramic industry. Engineers overcome the very difficult technical challenges of drying and firing defect-free, flat and durable tile. Potters can do it too. |
| Glossary |
Engobe
Engobes are high-clay slurries that are applied to leather hard or dry ceramics. They fire opaque and are used for functional or decorative purposes. They are formulated to match the firing shrinkage and thermal expansion of the body. |
![]() PayPal | No tracking, No ads, No paywall, No transient content! Just organized, concise information constantly updated and improved. Was this helpful? Consider supporting me. |
Buy me a coffee and we can talk