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50 years KELLER ceramic laboratory


The crucial factor for the quality of a heavy clay product is the quality of the raw materials used. For the design and layout of an ideal production plant is necessary to receive information as precise and conclusive as possible regarding the raw materials that will be used to manufacture the brick or tile product. For this reason, the raw materials/factory blends sent to us by our customers undergo comprehensive analyses in our own ceramic laboratory already in the project planning phase. A plant in the planning phase can then be optimally laid out for requirements defined by the analysed raw materials. The determined ceramic data also supply valuable information when our customers intend to upgrade their existing older plants. Upon order, we also carry out all analyses described below to provide you with valuable data for the operation and optimisation of your plant.

What this means for you:

  • Optimised product characteristics
  • Minimised energy consumption
  • Shorter cycle times
  • Higher productivity

Our range of services

Exploring and sampling of new clay pits intended for development/sampling of existing clay pits

When building a new brickworks at a new site it is necessary to explore the new clay pit intended for development and to take representative clay samples.We make these examinations together with geologists and, if necessary, a drilling team. On the basis of geographic and geologic conditions encountered on site, we then make proposals for the extraction of clay from the pit, the dewatering of the pit and the optimum storage and mixture of the extracted raw materials. The extracted core samples or prospected samples are analysed as to their ceramic and technological behaviour. This allows us to develop an optimal factory blend and to configure the planned brick plant.

Development/optimisation of factory blends

For various reasons it may be necessary to develop new factory blends or to optimise existing ones. Whether you want to develop new products with special properties, to improve the quality of existing products, or when existing pits are exhausted, or you want to make the existing factory blend more cost-efficient while maintaining its properties: in any of these cases we will be able to help with respective analyses and with our long-standing experience.

Analysis of existing blends

Even when you already have an existing factory blend, analyses made in the KELLER ceramic laboratory can give valuable hints how to optimise the manufacturing process or the product quality. In any case, we will determine the ceramic and technological data of the blend for the shaping, drying and firing processes. An optimisation of the manufacturing process as a whole can then be made on the basis of these data.


The KELLER ceramic laboratory uses a variety of devices for clay preparation. The aim is to reproduce the existing or future preparation line as accurately as possible on a laboratory scale. Hard materials can be pre-crushed with a jaw crusher. Fine preparation can either be a dry preparation with a cross hammer mill with various screen outlets or a semi-wet preparation with roller mills with adjustable roller gaps of up to 0.5 mm. The samples are blended and moistened in a kneading mixer which ensures that variations of the grain size as a result of a high blending energy are eliminated. The preparation water content of the material to be extruded is kept at a level to obtain the same pressing stiffness (penetrometer value) that will be applied in the future production process. Penetrometer values between 1.0 kg/cm² and 3.5 kg/cm² are possible here.


The existing laboratory extruder was modified in 2009 to determine the behaviour of the material during extrusion. This procedure requires measurements in the extruder cylinder and in a measuring cylinder of the torque necessary for extrusion, the auger revolutions, the clay column speed and the pressure build-up/pressure reduction of the material. These data are digitally recorded. They are the basis for a model calculation that is used to optimise the extruder geometry in relation to the auger gradient, the length of material afflux and the pressure buildup. Likewise, conclusions can be made as to the expected torque, pressing pressure and the required auger torque.

Drying / firing

Several test bodies for drying and firing tests are prepared in the course of the analyses. The analyses are largely based on practical tests. As a supplement to the results of the ceramic analyses, additional information concerning physical and chemical properties are determined on the basis of chemical and eluate analyses, particle size distribution and, if required, the determination of the mineral content.

The prepared test bodies then undergo a variety of tests to supply data for the shaping process, such as preparation water content, drying shrinkage, susceptibility to drying cracks, dry bending strength and dry bending strength at an equilibrium moisture content. Recorded at the same time are the course of the water extraction, the drying shrinkage, the so-called Bigot curve and the moisture conductivity of the respective capillary structure.

The test bodies are fired in electric kilns with a heating rate of 180 K/h and a holding time of 3 hours in the final firing zone.

The height of the final firing temperature depends always on the future product and the raw material features. It can be anywhere between approx. 840 °C and a maximum of 1,300 °C. As it is important to know the possible margins both for commissioning and for the operation of the future plant, firing tests are carried out at five different temperatures. The analysed data offer valuable information about the quality to expect, together with dimensional tolerances, colour variations, water absorption values and pressure and bending tensile strength properties.

The fired test bodies also supply information on possibly arising firing defects, such as bloating, melting spots, spalling or efflorescence . This is documented in order to be able to take appropriate measures, e.g. addition of barium. Though these values are determined on a small scale but with similar technical means, they nevertheless allow conclusions to be drawn for the planning and the future operation of the plant.

Test dryer / gas kiln

The laboratory-scale tests can be refined by further on-site tests in our test dryer which uses freshly extruded products taken from your current production. The test dryer is a mobile 20-foot container with a drying chamber of approx. 1.5 x 1.8 x 1.7 m (WxHxD) and is ready to be equipped with all air distributing devices that Keller HCW has to offer. Tests can be made with this dryer in our laboratory in Laggenbeck. If possible, however, the test dryer will be transported to an existing brick plant of our customer where it is placed near the shaping line (image10). The products used for the tests are not only products made of the customer's factory blend but they also show the typical tensions and textures resulting from the shaping process, and they are dried on the existing or future drying pallets. Moreover, fresh wet products that have just left the extruder can be taken for each drying test.

These "close-to-the-product" tests are significantly more precise in identifying the possible drying times and determining the exact drying curve. The drying curve, the shaping tools or even the drying pallets can then be adapted on the basis of product defects that developed during the drying tests.

As the transport of dried products generally presents no problems, firing tests are made in a stationary gas-fired chamber kiln in the ceramic laboratory in Laggenbeck.  

The kiln atmosphere can be modified during the firing tests to carry out both tests in oxidising atmosphere and reduction tests.

Firing tests in the gas kiln are also made with original dried products. Therefore, possible defects resulting from previous production steps, such as cores, cracks, colour stains, deformations, etc. can be better identified and eliminated, if possible, by modifying the firing regime accordingly.

For ceramic compounds that have a tendency towards water reabsorption, it is important to take the products destined for later firing out of the operating dryer before they are completely dry. Final drying then takes place in an electric laboratory dryer  immediately before firing.

Measurement of quality data and environmental relevant data in plants

Together with the process-engineering department many quality and environment-related values are measured during commissioning and later when the plants are in operation.

This includes recording dimensions and weight during various production steps, but also measurements of shrinkage, ambient temperature, humidity, air speed and brick temperature during drying.

A specially equipped measuring car (usually available in the plant) measures the temperatures of the bricks during their passage through the kiln and records the temperature homogenisation across the setting's cross section. Further measures to optimise the firing curve can then be derived from these recorded data.

Ordinary measurements of pollutants, such as sulphur, fluorine and chlorine in the hot air and flue gas pipes of the kiln are also possible.

Required sample quantities

  • The KELLER specific analysis of a prepared factory blend only requires about 20 kg of sample material.  This sample must be taken before the factory blend enters the extruder. Ideally, it is taken directly from where it enters the double shaft mixer at the extruder.
  • A sample quantity of approx. 120 kg is necessary for an extruder measurement, as this type of analysis requires a freshly prepared and not an evacuated compound.
  • Depending on the complexity of the works involved, we need between 50 and 200 kg of the individual raw materials to develop a factory blend. Before starting an analysis, the required scope and time and the information needed regarding the raw materials must be discussed and agreed on with the Keller HCW ceramic laboratory.
  • Drying tests with the test dryer are usually carried out in the plant at the customer's site. If this is not possible, the required quantities and transport issues have to be coordinated with us.
  • Firing tests in the gas kiln require approx. 50 to 100 dried products depending on the scheduled setting load. It is imperative to retrieve these products from the drying process before it is finished, i.e. the products still have a residual moisture >5 to 10% to avoid reabsorption of the products during transport and storage.

Taking samples

When taking clay samples it is extremely important to get a representative sample and to preserve as perfectly as possible the condition of the sample as of the time it was taken.

Samples from a pit area under development

One of the most difficult sampling tasks is taking a representative clay sample from a new pit area that is not yet developed. With a suitable drill, preferably with a coring tube, samples are taken from the area of the future clay pit following a defined grid pattern. These samples must be removed from a depth that is at least equal to the future mining depth of the pit. A profile of the layers in the pit can be created on the basis of the cores and a mining plan can be made. Samples of various layers of the clay pit can be taken from the cores. Impurities, such as for example stone layers are hardly recognised. It may therefore be necessary to additionally cut out a section with an excavator to identify the layers properly.  
In doing so it is extremely important to document the individual steps. A precise geographic designation (country, region, pit, etc.) and the exact position of the sampling spot (GPS coordinates) must be recorded. Important is also a photographic documentation of the sampling process

Samples from an active pit area

There are several possibilities to retrieve a representative sample from an active mining area.

  • When differences or layers are not visible to the naked eye and the thickness of the quarry face is not too high, it is an option to take a channel sample across the entire height of the quarry face. This should be made at several positions of the pit and the sample amount could again be reduced with a riffle sample splitter, if necessary.
  • If distinctly visible layers are recognised in the pit, channel samples should be taken from the individual layers.

As already described under item 1, it is extremely important to document the individual steps as precisely as possible. Layer thicknesses and any anomalities in the pit, such as stone layers must absolutely be documented and photographed. Smaller impurities, such as inclusions of lime, gypsum particles or the like should always be shown on a photo  together with an object with known dimensions, such as pens, lighters, etc. to get an idea of their size.

Samples from a clay stockpile or a clay storage

Sampling is generally easier as the samples are usually already mixed. Small samples are taken from various positions of the stockpile or clay storage and are combined to one single sample.

Samples from the circular screen feeder or a double shaft mixer

For this type of sampling in a machine environment make sure that the person taking the samples will be safe during sampling. Samples can be taken from the exit of the screen feeder or the mixer. The samples should be mixed in such a way as to form a representative compound.

Packaging and shipping:

The samples must be packed in airtight plastic bags to ensure that specific properties, such as the pit moisture will be preserved. Markings and inscriptions must be applied in such a way as to withstand the humidity of the clay material. Make sure that they cannot be covered with labels from the shipping agent. A sample without designation is worthless. The samples should be shipped as fast as possible. In winter, make sure that the properties of the samples will not change due to frost. The shipping documents, e.g. airway bill, should be sent by e-mail to the ceramic laboratory ( to be able to track the route of the sample.


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