Cupping and curvature are defects that directly affect the planarity of porcelain stoneware tiles. Although they become visible in the fired piece, their origin may lie at different stages of the process: body formulation, compaction, the relationship between the body, engobe and glaze, or the thermal conditions the tile experiences inside the kiln.
That is why, when deformation appears, the first step should be to understand how it has developed. Changing the temperature, modifying a formulation or adjusting the glaze without first diagnosing the cause may reduce the defect in a specific test while introducing new deviations elsewhere.
The more accurately we identify the origin, the more precisely we can determine which variable needs to be reviewed.
What are cupping and curvature in porcelain stoneware?
Cupping refers to a concave or convex deformation of the tile, similar to the shape of a dish or shallow bowl. Curvature, on the other hand, describes the deviation of the centre or edges of the tile from an ideal plane. Although both defects affect planarity, they should not be confused with warpage, which refers to the deviation of one corner from the plane formed by the other three.
These deformations may develop during firing as a result of several interacting factors. These include pyroplastic deformation of the body at high temperatures, thermal gradients, density differences and stresses generated between the body, engobe and glaze.
In the study Curvature Evolution of Porcelain Tile during Firing (Henrique et al., 2015), significant changes in curvature were observed between approximately 930 and 1200 °C, coinciding with stages in which the ceramic body undergoes important transformations during sintering.
Using hot-stage fleximetry, the authors identified significant positive deflection within this temperature range and associated it with permanent planarity defects in the compositions analysed.
Why do cupping and curvature occur during firing?
There is no single cause behind cupping and curvature in porcelain stoneware. The same deformation may be related to the behaviour of the ceramic body at high temperature, stresses generated between the different layers, variations in density within the piece or specific conditions of the thermal cycle.
For this reason, the visual appearance of the defect alone is not always enough to understand what is happening.
Knowing the main mechanisms involved helps us build an initial hypothesis before making changes to production.
Pyroplastic deformation of the ceramic body
Pyroplastic deformation is the tendency of a ceramic body to deform when it reaches high temperatures and its rigidity decreases.
During sintering, a liquid phase develops. Both its quantity and viscosity influence the body's ability to maintain its geometry while densification progresses.
When this liquid phase causes significant softening of the ceramic body, the stresses present in the tile have a greater capacity to produce deformation. For this reason, two bodies that achieve similar water absorption values after firing may behave very differently in terms of cupping and curvature.
Formulation plays an important role in this behaviour.
In Curvature Evolution of Porcelain Tile during Firing (Henrique et al., 2015), two porcelain stoneware compositions were studied, and the authors observed significant differences in the evolution of their curvature.
This shows that a composition should be evaluated not only in terms of the final properties achieved after firing, but also according to its behaviour throughout the heating process.
Stresses between body, engobe and glaze
A glazed tile consists of several layers that evolve together during firing. However, the body, engobe and glaze have different compositions, meaning that their shrinkage, expansion and thermal behaviour may also differ.
When these responses are not properly balanced, stresses can develop that alter the geometry of the tile.
Their effect becomes particularly important while the ceramic body is in a less rigid state, because at this stage it offers less resistance to the forces exerted by the different layers.
Compaction and density differences
Dimensional stability also depends on the conditions in which the tile reaches the kiln.
When green density varies across different areas of the tile, these differences may lead to uneven behaviour during the shrinkage associated with sintering.
Powder distribution, moisture content and pressing conditions all influence the uniformity of the piece. If one area has a different degree of compaction, its behaviour during firing may also differ, increasing the risk of loss of planarity.
Firing cycle and temperature differences
The firing cycle should be analysed as a complete thermal journey. Peak temperature affects sintering, but heating rate, the time the tile spends within specific temperature ranges and cooling conditions also play an important role.
In addition, uneven temperature distribution may cause different areas of the tile to evolve differently.
If one surface or region reaches a certain stage of shrinkage or softening earlier than another, stresses may develop that affect the final geometry of the piece.
How to identify the cause of curvature in porcelain stoneware
Once we understand the mechanisms that may be involved, the next step is to narrow down the origin of the defect.
Analyse the shape of the deformation and when it appears
The geometry of the tile provides an initial clue.
It is useful to determine whether the deformation is concave or convex, where it is concentrated and, above all, when it develops.
If curvature continues to evolve after the tile leaves the kiln, we may be dealing with delayed curvature.
The study Novel Approach to Ensure the Dimensional Stability of Large-Format Enameled Porcelain Stoneware Tiles Through Water Absorption Control (Pinter Junior et al., 2022) relates this phenomenon to residual stresses and identifies a correlation between water absorption and dimensional stability in the samples analysed.
Samples with lower water absorption showed a smaller amplitude of delayed curvature.
Under the conditions studied, the authors monitored the tiles for ten days and observed that some pieces tended to stabilise their internal stresses during the first 72 hours.
For this reason, when planarity changes after firing, taking measurements at different points in time can provide useful information.
Compare the unglazed body with the glazed tile
Comparing the fired body without applications with the fully glazed tile can help locate the source of the defect.
If the ceramic body already shows curvature, the body formulation and its pyroplastic behaviour should be investigated.
If the deformation appears or changes after applying the engobe and glaze, greater attention should be paid to the interaction between the different layers.
Use thermal testing to study the evolution of curvature
When we need to determine at which stage of the firing cycle the deformation develops, hot-stage fleximetry allows us to monitor the evolution of curvature as temperature increases.
This enables us to compare formulations and application systems under equivalent conditions and identify the temperature ranges in which the most significant changes occur.
Every formulation and every production process presents its own challenges.
At Kerafrit, we develop ceramic solutions around those specific needs and work alongside you throughout the process, combining close collaboration with technical expertise.
Because understanding what happens beneath the surface is often the first step towards improving what we see on it. If you would like to explore what we can bring to your next development, let’s talk.
