Sampling of stockpiles maybe carried out by drilling cores through it after completion. Alternatively if a layered stockpile is created then a representative sample of each layer can be taken during construction, analysed individually and blended in the proportions of the stockpile to represent the stockpile for an overall analysis.

Carbon and Sulphur Analysis

Carbon and sulphur analysis is usually carried out using dedicated analysers. 

Eltra make analysers such as Oxygen / Nitrogen Determinator, Carbon and Sulphur Determinator and a range produced by Leco utilising infra red detectors. These analysers give results in percentages for total carbon and total sulphur.

Some laboratories give results expressed as SO₄ (calculated stoichiometrically) when requested by the customer. Care should be taken to avoid confusion with soluble sulphate analysis.

It is possible to distinguish between organic and inorganic carbon by a combination of wet analysis and the Leco Carbon analysis apparatus.

Chemical Analysis

Before the 1970’s a full chemical analysis was generally carried out classical wet analytical techniques. These were extremely time consuming requiring skilled analysts to obtain accurate results. Therefore simpler physical less time consuming techniques were devised to give an indication of clay chemical and mineralogical composition. One of these methods was Ignition Loss / Moisture Adsorption test which consisted of:-

Some of these tests have been developed into quick quality control checks used for both clay quarry and works testing procedures.

The carbonate test developed into a geological lime test and a lime index test. 

As chemical instrumentation techniques developed more of these were used within the Ceramic Industry  for accurate analysis. Such techniques included:-

Particle Size Analysis

Dry Analysis - Sieving

Both dry and wet sieve analyses are widely used for grading analysis of raw materials used in the heavy clay industry.

Dry Sieve analysis tends to be used for the assessment of sands, body additives and surface additives greater than 50 microns, usually using sieves with the following mesh sizes

This range of sieves was originally based on the Wentworth recommended sizes for analysis of sediments. B.S. 410 also specified a 4.75mm sieve to be used: however, this tends not to be in most test houses standard dry sieve analysis grading, unless requested by the customer.

Wet Analysis

Wet sieving technique is widely used for the quality control of clays, samples usually taken after the screening process or / and after the mixing stage in the production process. The material grading has an effect upon fired strength, water absorption and physical attractiveness (Crazing) of the surface of the finished products.

Most clay testing laboratories tend to use 2.36 mm, 1.00 mm and 0.212 mm sieve sizes for this analysis.

Material fraction less than 0.212mm tended to be assessed using either the Hydrometer or  Andreasen Pipette techniques. Both were based on Stokes Law. Over the years instrumental techniques were developed based on a variety of parameters; Sedimentation, (Sedigraph), Centrfuging (Joyce Loebl disc Centrifuge), Electrical Sensing Zone ( Coulter Counter) Laser Particle size ( Malvern Mastersizer). Malvern Instruments are leaders in this field, their apparatus takes surface properties of the grains, the density and the level of opacity into account during the analysis. UKAS accreditation can be gained for laser particle size analysis.

Plasticity of Clays

Several techniques can be used, the simplest is a needle penetrometer which can be pressed against the extruded column, a spring depressed and an indication in Newtons is given to check the relate plasticity.

Atterberg Plastic Limit test is used as part of Churchill Laboratories Plasticity Index test which gives an indication of the range of moisture contents that clays can be plastic formed into a heavy clay product. The Liquid Limit used within the test is measured using an ELE Cone Penetrometer.

Other techniques suitable for research and development purposes have been developed to assess the plasticity of extruded pieces by using mechanical compression apparatus such as those made by Instron and tension apparatus such as made by Hounsfield modified to work in compression.

Ceramic Properties

Assessment of clay raw material stockpiles and samples directly from the quarry face can undergo analysis for making moisture, wet to fired linear shrinkage, dry to fired linear shrinkage, wet to fired linear shrinkage, firing linear shrinkage under load, loss on ignition, water absorption after firing and fired colour.

These properties are important when choosing the appropriate die sizes and kiln firing profiles for producing clay products.

Production management tend to demand materials of a certain specification to avoid major plant changes. Most anomalies in shrinkage on extrusion plants can be taken up by a relatively cheap die change. Where soft mud process or where in tile production there is a separate pressing stage after extrusion then the cost of changing dyes becomes prohibitive.

Good control of raw materials enables the production process to run smoothly producing high quality product first time, with minimal waste.

Critical Moisture

To improve production efficiency at the drying stage the critical moisture content is very useful. The critical moisture content is the moisture content at which the shrinkage virtually stops. At this point the product can be dried faster.

Empirical Drying Test (EDT)

The EDT test is used to assess clays drying characteristics and give an indication of how easy the material is to dry when formed into a product. As a general rule a clay with an EDT result less 10 is relatively easy to dry, EDT between 10 and 15 is easy to dry with care, EDT above 15 and some production units may find there dryers are not capable of drying the product without cracks forming.