When carrying out quality control on a large-format tile, measuring just a few points may not be sufficient.
The reason is that problems relating to compaction are not always evenly distributed. A tile may have an average density within the specified range whilst, at the same time, containing areas with significant variations in density, thickness or mass distribution.
In large-format tiles, these inconsistencies can affect the tile’s behaviour during drying, firing, grinding, cutting or handling.
Therefore, properly inspecting a large-format tile requires moving beyond spot measurements to a more comprehensive assessment:
How are the properties actually distributed across the entire surface of the tile?
Why large-format tiles are changing the requirements for quality control
The increase in the size of ceramic tiles and slabs has raised the technical demands of their manufacture.
With a large-format tile, it is not enough simply to achieve a specific average density. It is also necessary to ensure a sufficiently uniform distribution of compaction across its entire surface.
The behaviour of the atomised powder during feeding and pressing influences the porosity and bulk density of the piece. Studies on atomised bodies intended for large porcelain slabs indicate that uniformity during compaction affects the properties of the final slab. They also link density gradients to differential shrinkage during firing and to the possible generation of residual stresses.
On a large surface, areas with different behaviours may coexist:
- One area may be less compacted.
- An edge may contain less mass per unit area.
- The centre may achieve a higher density than the corners.
- A longitudinal band may have a different thickness.
- Two ends may respond unevenly during firing.
The problem is not merely that different values exist. The risk arises because Differences between areas can result in varying degrees of contraction, resistance and deformation..
The outer surface may appear uniform and the average value may fall within specifications. Neither of these two observations alone guarantees that the internal distribution is homogeneous.
The problem with measuring only part of the tile
Traditional methods for checking bulk density usually require the freshly pressed piece to be cut into small test specimens.
These samples are then analysed using techniques such as mercury immersion, volume measurement with vacuum membranes, or measurement using laser systems and weighing.
These procedures can provide accurate information on the selected points, but they have two significant limitations:
- They are destructive.
- They represent only a part of the piece.
According to the methodology described by Tekinn, the traditional process may require cutting blocks measuring approximately 50 × 50 mm and taking between 50 and 60 minutes to obtain a representative measurement of the pieces from a press run.
To prevent the test from taking even longer, the number of test specimens analysed is reduced. As a result, part of the surface always remains uninspected.
You can read more about these limitations in the article on X-ray absorption technology for verifying the compaction of ceramic parts.
The limitations of spot sampling
Let us assume that the technical team takes samples from the centre, the edges and the corners of a tile.
If a heterogeneity coincides with one of these points, it is likely to be detected. However, if it occurs between two sampling points, it may go unnoticed.
This is particularly relevant when deviations follow complex distributions, such as:
- Longitudinal or transverse bands.
- Differences between the front and back.
- Gradients from the centre towards the edges.
- Localised areas related to filling.
- Transitions between areas with different levels of compaction.
- Asymmetries between cavities or sectors of a pressed tile.
The larger the surface area, the more difficult it is to represent it using a limited number of points.
Point sampling is not necessarily incorrect. It can be useful for monitoring known positions or carrying out historical tracking. Its limitation becomes apparent when it is used to conclude that the entire piece is homogeneous.
Why average density does not tell the whole story
Average density reduces the entire tile to a single figure.
This value allows for comparing production runs, observing trends and checking whether the process is approaching the set target. However, it does not provide information on how the density is distributed.
Let us imagine two pieces with a similar average density:
- The first has relatively uniform values across its entire surface.
- The second combines a more compact central area with lower-density edges.
The average density may be virtually identical. However, the behaviour of both pieces during drying and firing may differ..
Areas with different initial densities may exhibit differences in porosity, mechanical strength or shrinkage. When they form part of the same piece, these regions do not evolve independently: they influence one another.
To explore this relationship in greater depth, you can consult our article on the importance of measuring bulk density after compaction.
Which variables should be analysed across the entire surface
To understand how a large-format tile has been formed, it is advisable to interpret three variables together:
- Apparent density.
- Thickness.
- Mass distribution or surface load.
Analysing just one of these may lead to an incomplete understanding.
Distribution of apparent density
Apparent density is used as an indicator of the porosity and the degree of compaction achieved by the atomised powder.
A homogeneous distribution promotes more stable behaviour in subsequent stages. Conversely, an uneven distribution can result in areas with varying:
- Porosity.
- Mechanical strength.
- Shrinkage capacity.
- Absorption.
- Response to mechanical stresses.
The average value indicates the overall degree to which the part has been compacted. The map shows where compaction is greater or lesser.
Thickness distribution
A comprehensive analysis of thickness allows differences to be identified between the centre, edges, corners or ends of the part.
It may reveal:
- Gradual changes across the tile.
- Variations between bands.
- Thicker or thinner areas.
- Asymmetries related to the geometry or the press settings.
Thickness is also necessary to correctly interpret X-ray absorption, as attenuation depends on both density and the amount of material traversed.
Surface mass distribution
Surface mass expresses the amount of material present per unit area.
Its distribution provides information on how the mould has been filled and helps to distinguish between potential feeding and compaction issues.
A combined analysis allows for more precise questions to be asked:
- Does the area have a lower density because it contains less mass?
- Is the loading uniform, but the compaction different?
- Does the variation in density correspond to a difference in thickness?
- Is the deviation concentrated at an edge, a corner or a band?
- Does the pattern recur in several pieces or cavities?
Research published in the Journal of the European Ceramic Society demonstrated that combining X-ray absorption and laser telemetry enables the creation of comprehensive maps of bulk density and thickness, as well as the calculation of surface mass distribution in large-format ceramic tiles.
How local heterogeneity can become a defect
A pressed tile does not always immediately reveal a defect that may appear later.
A heterogeneity may remain latent during transport or drying and become apparent when the tile is subjected to the thermal conditions of firing or subsequent mechanical operations.
Differential shrinkage
During firing, different areas of the piece become denser and contract.
When the initial density is not evenly distributed, some areas may contract differently from others. As they all form part of the same ceramic body, this difference in behaviour can affect the final geometry.
Possible consequences include:
- Variations in diameter.
- Loss of flatness.
- Longitudinal or transverse warping.
- Torsion or helical deformation.
- Misalignment.
In our article on apparent density and dimensional stability we explain in greater detail why the distribution of compaction determines the dimensional behaviour of the tile.
Internal stresses and subsequent breakage
When two areas behave differently during firing, internal stresses may be generated.
The tile may emerge from the kiln appearing stable but subsequently break when an external load alters the existing equilibrium:
- Rectification.
- Cutting.
- Drilling.
- Polishing.
- Handling.
- Internal transport.
Therefore, a breakage during cutting does not automatically prove that the cause lies with the disc or the grinding wheel. The mechanical operation may be the moment at which tension generated at an earlier stage is released.
You can explore this issue further in the analysis of the relationship between density variations in the press and subsequent breakages.
Differences in strength in the raw material
Less compacted areas generally exhibit lower cohesion between particles.
These areas may be more susceptible to:
- Breaks upon exiting the press.
- Damage during roller transport.
- Cracks before entering the drying kiln.
- Breaks during handling.
In a large-format tile, a weak area can compromise the whole tile even if it occupies a relatively small part of the surface.
Other defects related to the forming process are discussed in our article on defects in the manufacture of ceramic tiles.
Spot measurement versus inspection of the entire piece
The difference lies not only in the amount of data obtained. It lies in the type of questions the technical team can answer.
| Point measurement | Inspection of the entire component |
| Analyses selected positions | Analyses the entire surface of the unit being inspected |
| May require the cutting of test specimens | Can be carried out non-destructively |
| Provides independent values | Generates continuous maps |
| May fail to detect intermediate areas | Identifies the position and extent of the deviation |
| Allows previously defined points to be checked | Enables the discovery of unexpected patterns |
| Summarises the result in specific values | Correlates density, thickness and mass distribution |
| Makes it difficult to repeat the test on the same part | Allows the same part to be analysed before and after firing |
How full X-ray inspection works
TheX-ray absorption technologymeasures how much radiation passes through each area of the workpiece.
Attenuation depends mainly on:
- The composition of the material.
- The thickness being measured.
- The bulk density.
To interpret the measurement correctly, the system combines X-rays with high-precision laser telemetry.
During the inspection:
- The workpiece moves into the measurement zone.
- The emitter and detector record the radiation passing through each point.
- The telemetry system determines the thickness of the piece.
- The software processes the data together.
- Maps of density, thickness and mass are generated.
The method developed by Amorós, Boix, Llorens, Mallol and other researchers enables the distribution of bulk density in large tiles to be determined non-destructively. It also allows the same fired piece to be measured subsequently, facilitating comparison between its pre- and post-firing states.
At Tekinn, we apply this technology to obtain high-resolution maps of both unfired and fired pieces.
Our system can scan a complete piece measuring 135 × 135 cm in approximately seven minutes and generate over two million measurement points. These capabilities enable us to move from a partial scan to a detailed representation of the piece’s internal structure.
You can view the equipment’s specifications on our page for ceramic quality control solutions.
When is it advisable to inspect a large-format piece?
The frequency of inspection should be determined based on the product, the process risk and the historical stability of the production line.
A full analysis is particularly useful at the following stages:
Start of production
This allows you to check whether the initial settings are producing a uniform distribution before proceeding with the batch.
Changes in format or model
Each change can alter the filling behaviour, the distribution of the mass and the compaction conditions.
Changes to the feed system
After adjusting the carriage speed, the grid, the dosing or the filling sequence, the maps allow you to check the actual effect on the part.
Changes to the atomised powder
Variations in moisture content, particle size, flowability or composition can alter behaviour during filling and pressing.
Interventions on the press or mould
Maintenance, cleaning or component replacement tasks may alter the pressure or load distribution.
Occurrence of warping, misalignment or breakage
The map allows you to check for any non-uniformity consistent with a deviation arising during the part’s formation.
Regular checks
Even when there is no visible defect, regular inspection can help to detect trends and compare the performance of different production runs.
How to translate maps into process decisions
Obtaining more data does not automatically guarantee greater control.
Value is realised when the maps are interpreted within the production context and linked to manufacturing parameters.
Non-uniformity can guide the review of variables such as:
- Powder distribution and metering.
- Filling speed and sequence.
- Condition and cleanliness of the screen.
- Moisture content of the atomised material.
- Pressing pressure and cycle.
- Mould adjustment.
- Differences between cavities.
- Condition of punches and components.
The map should not be interpreted as an automatic diagnosis of the cause. A single distribution pattern may be influenced by several variables.
Its function is narrow down the scope of the search, formulate technical hypotheses and verify the effect of the adjustments made.
A working methodology might follow this sequence:
- Identify a variation.
- Pinpoint its location and extent.
- Relate it to production conditions.
- Review the variables potentially involved.
- Carry out a controlled adjustment.
- Inspect a new part.
- Compare the before and after maps.
- Validate the result in subsequent stages.
This process allows part of the trial-and-error adjustment to be replaced by decisions supported by quantitative information.
With large-format tiles, quality cannot be represented by just a few data points
The manufacture of large-format tiles requires an understanding of how their properties are distributed across the entire surface.
A single measurement may confirm that certain positions are within the target range. An average density may indicate that the process appears stable.
However, none of these measurements allows us to rule out the possibility of heterogeneity between the points analysed.
A comprehensive inspection allows us to move from an isolated figure to a map capable of showing:
- Where the deviation lies.
- How extensive it is.
- What pattern it follows.
- How it relates to thickness and mass.
- How it changes following an adjustment.
At Tekinn, we use X-rays and advanced telemetry to obtain distribution maps of density, thickness and mass across entire components, both green and fired.
The aim is not to generate more information, but to provide the technical team with a useful insight into how the component has formed and to help them decide where to take action.
Request a demonstration using your own components and see what information a partial measurement might be hiding.
Technical sources
- Amorós, J. L. et al. Non-destructive measurement of bulk density distribution in large-sized ceramic tiles. Journal of the European Ceramic Society, 2010.
- Soldati, R. et al. Powder rheology and compaction behaviour of spray-dried bodies for porcelain stoneware slabs, 2018.
- Amorós, J. L. et al. Study of the pressing process for large-format tiles using X-ray absorption. Bulletin of the Spanish Society of Ceramics and Glass, 2010.