Contents

1. 1. Residual humidity and clays’ performance
2. 2. Improper drying process
3. 3. Source of the problem
4.  4. Problem-solving actions
       a) The dryer
       b) Dispersants for ceramic mixture
       c) Process waters
       d) The ceramic mixture formulation

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1. Residual humidity & clays' performance

As is well known, the ceramic mixture after being atomized is subjected, during the forming phase, to very high pressures (over 400 Kg/cm2) that are necessary for the creation of the ceramic body. The raw tile, even after pressing, contains a water amount (residual humidity) that is around 5/7%. It is a mandatory presence that guarantees the clays the proper plasticity level needed to model the piece under the press.
However, if the presence of residual moisture allows on the one hand to give shape to the ceramic support, on the other hand it prevents the tile from having the proper mechanical features that are necessary for subsequent applications.

As it is well known, in order to increase the mechanical resistance of the raw tile it is therefore necessary to eliminate the water from the system by means of an appropriate drying cycle that allows a gradual expulsion, useful to avoid stresses that can, already at this stage, give rise to defects.

More specifically: the water molecules, by means of the drying process, must progressively migrate from the center of the ceramic body towards the outer layers and evaporate so as to allow the system to bind, therefore minimizing the possible stresses.  

2. Improper drying process

During the drying process there are no particular problems due to the tile’s shrinkage (which, like sunburn earth, partially shrinks when dried) when:

1. The atomized clay powder complies with the formulation
2. The atomized clay powder has the proper amount of residual humidity
3. The pressing action is uniform and without abnormalities

The drying process is carried out by means of dryers, mostly vertical, in which the raw tiles are placed on metal supports (roller stands) for a preset time, until the presence of residual humidity is reduced to values close to 0%.

The progressive heating to which the ceramic body is subjected inside the dryers produces the aforementioned migration of water molecules that therefore leave the ceramic body by evaporating. This process, however, may in some cases not be completely homogeneous over the entire surface of the tile. More precisely: the parts of the ceramic body that rest on the basket rollers may have different evaporation times and modes compared to the areas of the tile that are not in contact with the metal parts of the dryer. The different evaporation mode that takes place in both the lower and upper parts of the tile, in some rare cases produces longitudinal bands/stripes that may remain visible even after the drying cycle.

Basically, these are more or less brilliant haloes that, in extreme cases, despite the following applications of inks and glazes, may persist and be perceptible even after firing. The problem is particularly apparent in conjunction with technical ceramics production or in cases of non-massive applications along the glazing line

3. The source of the problem

What underlies the uneven surface gloss?

The different degree of brilliance is due to the presence of some particular ANIONS and CATIONS (particular chemical species formed by one or more atoms) that freely move within the mixture and that during the drying phase are transported by water’s migration process. Since they do not evaporate, these kind of anions / cations tend to concentrate unevenly on the surface, favoring some areas rather than others: the areas with a higher concentration show a different degree of gloss.

Generally speaking, all anions and cations that do not form insoluble salts within the system (or that have low mobility due to their conformation) do not chemically bind to the mixture and tend to be transported by water to the surface.
Let’s step back and deepen.

Who promotes the presence of this type of anions/cations?

Without oversimplify, among the various sources of the problem three are the most relevant:

1. Dispersant for ceramic mixture (sodium silicate-based)
2. Process water
3. Ceramic mixture’s inorganic raw materials

In the latter case, in general, the type of clays and their different plasticity can affect positively or negatively depending on their ability to retain or regulate the migration of water (and therefore of ions/cations) to the surface.

4. Problem-solving actions

A) THE DRYER

In order to prevent the problem, it is first necessary to make sure that the drying cycle is properly carried out in terms of times, temperatures and homogeneity of the heat inside the dryer. The good functioning of the plant is in this sense a necessary pre-condition for the prevention of the critical issue.

B) DISPERSANTS FOR CERAMIC MIXTURE, SODIUM SILICATE AND WATER SOLUBILITY: A STRATEGIC CHOICE

If the problem persists even after repairing the values of the dryer, it becomes strictly necessary to intervene on the chemical side, so as to eliminate the root of the problem. From this point of view, it is necessary, for example, to analyze the type of dispersant used in the grinding phase: most of these agents are, in fact, based on sodium silicate, a chemical compound completely soluble in water which, as we have seen, can in several cases cause the problem. The choice of the type of fluidifier is therefore crucial: selecting a product with low solubility means that the active ingredients it contains (i.e. ions and cations), in the presence of a residual moisture content of 5/7%, are only partially transported by water during its migration and evaporation process.

NOTE
Alternatively, it is possible to formulate dispersants using special polymers or molecules capable of limiting the migration of ions to the surface.

C) PROCESS WATERS

It is also important to pay attention to the chemical composition of process waters. After a careful study of their characteristics, it is necessary to check the excessive presence of those particular anions and cations that can be responsible for halos.

For these reasons, it is therefore advisable to limit the presence of anions and cations of inorganic origin that tend to create soluble salts in process water. Although the electrical conductivity of water is not a parameter able to precisely identify the type of ions within the water, the check of its values can in any case give indications about the possible massive presence of ions. In case of massive presence of ions, it is usually possible to intervene by cutting the process water.

D) THE CERAMIC MIXTURE FORMULATION

Although this happens - for obvious reasons - rather rarely, acting on the formulation of the mixture could finally be a useful way to contain the problem. For example, it could be possible to replace, introduce or increase some types of clays capable of retaining more water or at least slow down and standardize the ions and cations’ migration.

As just one example, more plastic clays (such as bentonites) absorb and retain much more water and tend to release it more slowly, promoting more regulated migration of water and ions.