Porosity is a dimensionless measure and represents the ratio between the void volume and the total volume of a substance or mixture of substances. It serves as a measure for the ranking of the actually existing voids. It is used in the field of materials and construction engineering as well as in the geosciences. Porosity has a major influence on the density of a material as well as on the resistance when it flows through a filler (Darcy's law).

Originally caused by natural and generally undesirable conditions, especially in the production of demanding castings [1], today there is also an artificially created and therefore desired porosity, especially in the production of lightweight construction materials. Metal foam and lightweight concrete are examples of porosity, but as such are not the subject of this paper.

Pores are the hollow spaces in the material between the individual components. The entirety of the cavities is called pore volume. Absolute porosity (total porosity) is defined as the ratio of pore volume to total volume by the porosity factor.

The volume of the pores depends on several factors, such as the distribution of the grains (grain sum curves), the storage density of the individual grains and the conformation (surface) as well as the non-uniformity of the grain mixture. The more distributed and uneven the grain mixture, the denser the storage density and the more uniform the surface, the lower the porosity. It is determined in the laboratory by weighing a water-saturated sample and the dry sample. Not the entire pore volume can be used for water movement. The part of the water that adheres to the grain and does not move is called adhesive water. With decreasing grain size, the proportion of adhesive water in the pores increases significantly so that the usable or effective pore volume is less than 5% for fine silt / clay.

Technical ceramics generally do not have open porosity and are therefore gas-tight. For some materials, however, porosity is desirable, e.g. because it makes it easier to control thermal stress or because large surfaces are required. Porosity can be selectively created by the choice of raw materials, the manufacturing process and, if necessary, additives. Thus it is possible to close and open pores of different sizes from a few mm to a few µm. The test methods for determining porosity are specified in DIN EN-623-2.

The characteristic values of ceramic materials for strength [MPa]¹² are statistically scattered as a function of the material composition, the grain size of the starting and filler materials, the manufacturing conditions and the manufacturing process. According to the so-called weak-point model, the resulting distribution of defects and the so-called critical defect (failure) determine the reliability of the component. For this reason, the scatter between the different batches is greater than that between the individual batches.

Units of measurement for ceramic porosity

The porosity can only be determined in the laboratory. When determining the water absorption - in boiling water - the buoyancy weight is compared with the dry weight using a special balance. The laboratory technician can then use the increase in weight to calculate the bulk density, which can be converted into porosity. The process engineering for determining porosity is regulated in DIN EN-1217 "Materials and articles in contact with food - Test methods for water absorption of ceramic products.

Porosity in the porcelain industry is measured by an absorption test (water absorption), i.e. a method in which the shear strength is determined by the volume of the absorbed liquid. If the absorption is 0%, the material is considered completely tight.

What does porosity say about porcelain?

The porosity of a body is an important criterion for

• the quality
• the stability
• thermal storage capacity
• the longevity
• the suitability 

of porcelain. As a general rule, the better the raw material and the higher the cooking temperature, the lower the porosity of the product.

It is known that "porcelain" varies considerably from country to country and from manufacturer to manufacturer. The composition of the body (raw materials) is often difficult to understand. Substitutes and pigments are becoming increasingly popular. In many places, new types of minerals and complementary substances are used to give porcelain all kinds of new properties. In particular these are

• higher strength against breaking
• clearer colors
• lighter weight
• higher density 

In our quality determination, we measure the porosity of porcelain based on the water absorption with an area of 1 qcm and 1 ml water on the unglazed body after hard firing. This method is sufficient to meet the standards of the ceramic industry and to evaluate the product. The amount of water absorbed by the cullet determines the degree of porosity. In order to keep the determination as accurate as possible, we work according to the weight method using a scale in milligrams - to be determined before and after the liquid absorption.

Porosity is usually expressed as a percentage or fraction (fractions of 1 = 100%) and is designated by the formula letter Φ. The total porosity of a sample is made up of the sum of the voids that are connected to each other and to the environment (useful porosity) and the voids that are not connected to each other (cemented or closed porosity).

Porosities of ceramic bodies in comparison

When evaluating the qualities for our collection, porosity is an important element in the assessment of porcelain and ceramic products. We measure according to the following criteria:

Porcelain owes its special properties as a heat accumulator to its specific density and low porosity. Neither tempered glassware nor metal can store pre-tempered heat as long as porcelain can. This makes it particularly suitable for serving hot food. The porosity of ceramic qualities depends on the raw materials and their firing temperature.