Monthly Tech-Tip from Tony Hansen SignUp

No tracking! No ads!

200 mesh | 325 mesh | 3D Design | 3D Printer | 3D Printing Clay | 3D Slicer | 3D-Printing | Abrasion Ceramics | Acidic Oxides | Agglomeration | AI in Ceramics | Alkali | Alkaline Earths | Amorphous | Apparent porosity | Artware | Ball milling | Bamboo Glaze | Base Glaze | Base-Coat Dipping Glaze | Basic Oxides | Batch Recipe | Bisque | Bit Image | Black Core | Bleeding of colors | Blender Mixing | Blunging | Body Bloating | Body glaze Interface | Body Warping | Bone China | Borate | Boron Blue | Boron Frit | Borosilicate | Breaking Glaze | Brick Making | Brushing Glaze | Calcination | Calculated Thermal Expansion | Candling | Carbon Burnout | Carbon trap glazes | CAS Numbers | Casting-Jiggering | Catch Glaze | Celadon Glaze | Ceramic | Ceramic Binder | Ceramic Decals | Ceramic Glaze | Ceramic Glaze Defects | Ceramic Ink | Ceramic Material | Ceramic Oxide | Ceramic Slip | Ceramic Stain | Ceramic Tile | Ceramics | Characterization | Chemical Analysis | Chromaticity | | Clay body | Clay Body Porosity | Clay Stiffness | Clays for Ovens and Heaters | Co-efficient of Thermal Expansion | Code Numbering | Coil pottery | Colloid | Colorant | Commercial hobby brushing glazes | Cone 1 | Cone 5 | Cone 6 | Cone plaque | Copper Red | Cordierite Ceramics | Crackle glaze | Cristobalite | Cristobalite Inversion | Crucible | Crystalline glazes | Crystallization | Cuerda Seca | Cutlery Marking | Decomposition | Deflocculation | Deoxylidration | Differential thermal analysis | Digitalfire Foresight | Digitalfire Insight | Digitalfire Reference Library | Dimpled glaze | Dip Glazing | Dipping Glaze | Dishwasher Safe | Dolomite Matte | Drop-and-Soak Firing | Drying Crack | Drying Performance | Drying Shrinkage | Dunting | Dust Pressing | Earthenware | Efflorescence | Encapsulated Stain | Engobe | Eutectic | Fast Fire Glazes | Fat Glaze | Feldspar Glazes | Fining Agent | Firebrick | Fireclay | Fired Strength | Firing Schedule | Firing Shrinkage | Flameware | Flashing | Flocculation | Fluid Melt Glazes | Flux | Food Safe | Foot Ring | Forming Method | Formula Ratios | Formula Weight | Frit | Fritware | Functional | GHS Safety Data Sheets | Glass vs. Crystalline | Glass-Ceramic Glazes | Glaze Blisters | Glaze Bubbles | Glaze Chemistry | Glaze Compression | Glaze Crawling | Glaze Crazing | Glaze Durability | Glaze fit | Glaze Gelling | Glaze laydown | Glaze Layering | Glaze Mixing | Glaze Recipes | Glaze shivering | Glaze Shrinkage | Glaze thickness | Globally Harmonized Data Sheets | Glossy Glaze | Green Strength | Grog | Gunmetal glaze | High Temperature Glaze | Hot Pressing | Incised decoration | Industrial clay body | Ink Jet Printing | Inside-only Glazing | Insight-Live | Iron Red Glaze | Jasper Ware | Jiggering | Kaki | Kiln Controller | Kiln Firing | Kiln fumes | Kiln venting system | Kiln Wash | Kneading clay | Kovar Metal | Laminations | Leaching | Lead in Ceramic Glazes | Leather hard | Limit Formula | Limit Recipe | Liner Glaze | Liner glazing | Liquid Bright Colors | LOI | Low Temperature Glaze | Majolica | Marbling | Material Substitution | Matte Glaze | Maturity | Maximum Density | MDT | Mechanism | Medium Temperature Glaze | Melt Fluidity | Melting Temperature | Metal Oxides | Metallic Glazes | Micro Organisms | Microwave Safe | Mineral phase | Mineralogy | Mocha glazes | Mohs Hardness | Mole% | Monocottura | Mosaic Tile | Mottled | Mullite Crystals | Native Clay | Non Oxide Ceramics | Oil-spot glaze | Once fire glazing | Opacifier | Opacity | Ovenware | Overglaze | Oxidation Firing | Oxide Formula | Oxide Interaction | Oxide System | Particle orientation | Particle Size Distribution | Particle Sizes | PCE | Permeability | Phase Diagram | Phase Separation | Physical Testing | Pinholing | Plainsman Clays | Plaster Bat | Plaster table | Plasticine | Plasticity | Plucking | Porcelain | Porcelaineous Stoneware | Pour Glazing | Powder Processing | Precipitation | Primary Clay | Primitive Firing | Propane | Propeller Mixer | Pugmill | Pyroceramics | Pyrometric Cone | Quartz Inversion | Raku | Reactive Glazes | Reduction Firing | Reduction Speckle | Refiring Ceramics | Refractory | Refractory Ceramic Coatings | Representative Sample | Restaurant Ware | Rheology | Rutile Blue Glazes | Salt firing | Sanitary ware | Sculpture | Secondary Clay | Shino Glazes | Sieve | Sieve Shaker | Silica:Alumina Ratio | Silk screen printing | Sintering | Slaking | Slip Casting | Slip Trailing | Slipware | Slurry | Slurry Processing | Slurry Up | Soaking | Soluble colors | Soluble Salts | Specific gravity | Splitting | Spray Glazing | Stain Medium | Stoneware | Stull Chart | Sulfate Scum | Sulfates | Surface Area | Surface Tension | Suspension | Tapper Clay | Tenmoku | Terra Cotta | Terra Sigilatta | Test Kiln | Theoretical Material | Thermal Conductivity | Thermal shock | Thermocouple | Thixotropy | Throwing | Tony Hansen | Toxicity | Trafficking | Translucency | Transparent Glazes | Triaxial Glaze Blending | Ultimate Particles | Underglaze | Unity Formula | Upwork | Variegation | Viscosity | Vitreous | Vitrification | Volatiles | Water in Ceramics | Water Smoking | Water Solubility | Wedging | Whiteware | Wood Ash Glaze | Wood Firing | Zero3 | Zero4 | Zeta Potential

Clay

What is clay? How is it different than dirt? For ceramics, the answer lies on the microscopic level with the particle shape, size and how the surfaces interact with water.

Details

The term 'clay' is used in different ways. Potters often refer to their 'clays', these are typically recipes or mixtures of clay minerals, feldspar and quartz (more correctly these are clay bodies, or, just bodies). A lump of the material that has been mined from a deposit is also referred to as 'a clay'. However, in its strictest sense, the term 'clay' refers to flat microscopic particles from which that lump is composed. Actually, even more precisely, it refers to 'some of the particles' (since the lump will invariably have particles of many other minerals also). Clay particles have a surface chemistry that imparts an affinity for water (the other particles are just dead micro-rocks).

Clays have plasticity. This property is a product of the fact that the surface chemistry attracts water electrolytically. The water thus becomes a glue and a lubricant that gives billions of particles the opportunity to express that collective property of plasticity. Simplistically clay particles can be viewed as 'water magnets'.

Clays are born when parent rocks, referred to as 'clay-making minerals', break down physically (by weathering) and hydrate to form new mineral particles with new properties. This hydration involves insertion of complete water molecules into the crystal structure (whereas with most minerals oxides are converted to hydroxides). From a mineral point of view, clays are hydrous-layer silicates of aluminum. Many clay minerals exist. "Kaolin" is the purest clay mineral, its theoretical chemistry is Al2O3.2SiO2 (one part alumina oxide, two parts silicon dioxide). Other clay minerals include ball clay (the most common), illite, montmorillonite, smectite, halloysite. These vary by particle size, shape, surface electrolytics and mechanism of plasticity. Clays used in ceramics have a wide range of purities, they are usually mixes of the above (unless highly purified).

The wide range of particle sizes, shapes and mineralogies are related to the identity of parent rocks, mode of conversion and whether they are primary (on site of alteration) or secondary (moved by water or wind). These factors produce different tactile properties, plasticities, drying rates and shrinkages, dry hardness and strength, behavior in slurries, etc. (physical properties).

Primary clays, found within eye-shot of the site of alteration, are often hiding in what appears to be gravel or sand (or a mix). Screening out the sand and gravel (e.g. at 200 mesh) can reveal very pure clays (where most particles are clay itself). There are thousands of places on the planet where you can easily see mountains being worn away and sedimented on flatlands around them. The planet is actually a sand, gravel and clay making machine. Mountains constantly push upward and erosion wears them down. It is also a water-making machine, volcanoes release it, the more water the more explosive the eruption.

Sedimentary clays, by contrast, have been transported, often hundreds of miles. During the move mother nature grinds them to finer and finer particle sizes. She also mixes them with many other minerals particles. Each particle type has its own mineralogy, chemistry and physics. And its own solubility and thermal stability properties. It follows that an overall chemistry of a secondary clay (that, as noted, is a mix) does not really have much significance unless the clay is being melted in a kiln and thus decomposing and yielding its oxides to the melt. So, in ceramics, connecting the overall chemistry of a new and unknown clay material to firing behavior in a body, which does not melt, (e.g. temperature of vitrification, color, strength, efflorescence) can thus be quite misleading. It is much better to think in terms of the physics: properties that you can measure and rationalize. Often the datasheets of clay suppliers can even miss the point of what their material really is, they do this by providing only the chemistry. But this does not answer questions like: Is it plastic? Does it have a fine particle size? Does it have a high soluble salts content? What is the porosity and fired shrinkage at various temperatures? What is the particle size distribution? What are the drying properties? What does it look like when fired to various temperatures?

Potters sometimes express the sentiment: “What the clay is used for is entirely up to the clay, I'm just here to help it along.” That is partly true if one attempts to use a pure clay and a specific process to make ware. But clay bodies are mixtures that combine the strengths and dilute the weaknesses of the pure clays plus add non clay materials to further tune properties to an intended use, thus a specific clay can find itself used in many product types.

Related Information

Lab testing a clay for its physical properties

Tap picture for full size and resolution

It only takes a few minutes to make these. But you would be amazed at how much information they can give you about a clay! These are SHAB test bars, an LDW test for water content and a DFAC test disk about to be put into a drier. The SHAB bars shrink during drying and firing, the length is measured at each stage. The LDW sample is weighed wet, dry and fired. The tin can prevents the inner portion of the DFAC disk from drying and this sets up stresses that cause it to crack. The nature of the cracking pattern and its magnitude are recorded as a Drying Factor. The numbers from all of these measurements are recorded in my account at Insight-live. It can present a complete physical properties report that calculates things like drying shrinkage, firing shrinkage, water content and LOI (from the measured values).

Example of a certificate of analysis for a kaolin

Tap picture for full size and resolution

When companies ship materials they often include these with the shipment. The information reported is often very basic and properties important to ceramics are often not found.

Closeup of Halloysite particles

Tap picture for full size and resolution

Electron micrograph showing Dragonite Halloysite needle structure. For use in making porcelains, Halloysite has physical properties similar to a kaolin. However it tends to be less plastic, so bodies employing it need more bentonite or other plasticizer added. Compared to a typical kaolin it also has a higher fired shrinkage due to the nature of the way its particles densify during firing. However, Dragonite and New Zealand Halloysites have proven to be the whitest firing materials available, they make excellent porcelains.

Whole rock analysis shows some as percent, some as PPM

Tap picture for full size and resolution

The ppm items are not oxides, they are elements. Ba for example, is shown as 4276 ppm. We do not know the form. It could be barium sulphate, barium carbonate, barium nitrate, barium chloride. But altogether they supply this amount of Ba. The same is true of chrome, strontium, nickel and vanadium.

Sedimentary clays are a whos-who of the periodic table

Tap picture for full size and resolution

These are the results of a detailed elemental composition analysis of a sedimentary clay. The first column of numbers is ppm (parts per million), divide them by 10,000 to get percent. The Fe here, for example, is 50,868 or 5.1%. The second column is +/- error. Notice that this test does not detect boron or lithium, they require a different method. By contrast, the chemical analysis shown on the data sheet of a typical ceramic material shows only the principle ceramic oxides (less than a dozen), but all of these trace elements will still be present.

Links

Glossary Clay body
A term used by potters and in the ceramic industry. It refers to the earthenware, stoneware or porcelain that forms the piece (as opposed to the engobe and covering glaze).
Glossary Vitrification
The term vitrified refers to the fired state of a piece of porcelain or stoneware. Vitrified ware has been fired high enough to impart a practical level of strength and durability for the intended purpose.
Glossary Efflorescence
A common problem with dry and fired ceramic. It is evident by the presence of a light or dark colored scum on the dry or fired surface.
Glossary Plasticity
Plasticity (in ceramics) is a property exhibited by soft clay. Force exerted effects a change in shape and the clay exhibits no tendency to return to the old shape. Elasticity is the opposite.
Glossary Porcelain
How do you make porcelain? There is a surprisingly simple logic to formulating them and to adjusting their working, drying, glazing and firing properties for different purposes.
Glossary Native Clay
A clay that a potter finds, tests and learns to process and use himself. To reduce the costs of importing materials manufacturers, especially in Asia, often develop processes for clays mined in their locality.
Materials Kaolin
The purest of all clays in nature. Kaolins are used in porcelains and stonewares to impart whiteness, in glazes to supply Al2O3 and to suspend slurries.
Materials Ball Clay
A fine particled highly plastic secondary clay used mainly to impart plasticity to clay and porcelain bodies and to suspend glaze, slips and engobe slurries.
Minerals Kaolinite
The most fundamental clay mineral. This mineral is found in nature in its purest form as kaolin. How
Minerals Montmorillonite, Bentonite
A clay mineral of extremely small particle size and high plasticity. Raw bentonite is generally a pa
Minerals Smectite
A highly plastic clay mineral related to montmorillonite (bentonite), more correctly, the name of th
Minerals Halloysite
From a purely physical properties point-of-view, halloysite is a clay mineral similar to kaolin in f
Articles How to Find and Test Your Own Native Clays
Some of the key tests needed to really understand what a clay is and what it can be used for can be done with inexpensive equipment and simple procedures. These practical tests can give you a better picture than a data sheet full of numbers.
By Tony Hansen
Follow me on

Got a Question?

Buy me a coffee and we can talk



https://digitalfire.com, All Rights Reserved
Privacy Policy