Monthly Tech-Tip from Tony Hansen SignUp

No tracking! No ads!

200 mesh | 325 mesh | 3D Design | 3D Modeling | 3D Printer | 3D Printing Clay | 3D Slicer | 3D-Printing | Abrasion Ceramics | Acidic Oxides | Agglomeration | AI in Ceramics | Alkali | Alkaline Earths | All-in-one case mold | 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 | Ceramic Transfer | Ceramics | | Chemical Analysis | Chromaticity | Clay | 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 API | Digitalfire Foresight | Digitalfire Insight | Digitalfire Insight-Live | Digitalfire Reference Library | Digitalfire Taxonomy | Dimpled glaze | Dip Glazing | Dipping Glaze | Dishwasher Safe | Displacer | 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 | Infill and Support | Ink Jet Printing | Inside-only Glazing | 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 | Mold Natches | 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 | Pour Spout | 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 | Side Rails | 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 Content | Water in Ceramics | Water Smoking | Water Solubility | Wedging | Whiteware | WooCommerce | Wood Ash Glaze | Wood Firing | WordPress | Zero3 | Zero4 | Zeta Potential

Characterization

In ceramics, this normally refers to the process of doing physical or chemical testing on a raw material to accurately describe it in terms of similar ones.

Key phrases linking here: characterization, characterizing, characterize - Learn more

Details

The first hundred pages of almost any textbook on ceramic manufacturing are on clay properties. Material knowledge is thus all important. When ceramic materials are "understood" it is possible to control the properties of the bodies, glazes and engobes used in traditional ceramics. Characterizing materials is about understanding them. It is about being able to describe what a material is in terms that enable the determination of its suitability (often compared to alternatives rather than in absolute terms). Product datasheets typically highlight material properties of interest to a specific market. But these are often focussed at purchasing agents who want absolute numbers to compare brand names. But to technicians, data sheets full of numbers and acronyms often seem inadequate. At times it even appears that companies do not really understand products they manufacture for ceramics!

The ceramic world functions on "recipes" and often much less effort is put into understanding them than should be. An example of this is the pursuit of substitutes for materials. These substitutes can be straightforward (e.g. switching one source of silica to another) but they often come with a complicated list of trade-offs. For example, switching from an English kaolin to an American one in a porcelain: These are quite different materials. One needs to consider impacts on body plasticity, stickiness, drying performance, degree of maturity (with associated fired hardness and durability, fired color, translucency, thermal expansion).

Substituting materials becomes more complicated for secondary clays. Although these have chemistries, the chemical makeup is more difficult to connect with the physical and firing behavior. This is often because the materials are not finely ground and their powders have populations of a variety of different mineral particles (which interact in complex ways). It is common for people to substitute materials in recipes simply because they have similar-sounding names! Consider red-burning stonewares: They depend on recipes that contain refractory red clays and a controlled amount of feldspar or high-feldspar clays. Using low fire red clays with less feldspar Fired color is achieved by finding a balance between vitrification (for fired density and strength) and refractoriness (for red rather than brown coloration).

Consider a good example of the value of characterization of clays: Suppose you need a plastic body that fires around 1200C (cone 6). You have a fine-grained silty clay that is somewhat plastic and fires more vitreous than required. You also have a second fine-grained clay is highly plastic and is too refractory for the required temperature. That makes it likely that a mix of these two materials will produce a usable body. Studying data sheets of these two materials would not make that evident, but knowing them, via your own characterization efforts, would make it so.

Textbooks treat the subject of characterization in an intimidating way: They list a range of lab equipment used. Terms like “photon scattering”, “RM diffraction”, “scanning microscopy” or “mass spectrometry” can seem pretty distant and expensive when all one needs is simple physical tests. Knowing about these things typically falls in the realm of people designing the process not using it. The test procedures I am about to recommend will not be listed or studied in any college course. That being said professors are pointing students here for practical testing methods.

The most practical way to characterize clay materials is by:
-Firing test bars at various temperatures to profile the color, fired shrinkage and porosity (e.g. the SHAB test).
-Measuring the dry strength, dry shrinkage and drying performance (e.g. the DFAC test).
-Measuring the particle size distribution (e.g. the SIEV test).
-Making ware using the material pure.

In glazes, the focus is often on the chemistry of the materials. Frits, for example, find their entire merit in their chemistry and switching from one to another is all about how similar that chemistry is or what oxides they source. Feldspars are a similar story.

While the chemistry of glaze materials is their most important characteristic, it is also important to consider other properties. Consider some examples. Feldspar and kaolin source Al2O3, but the kaolin suspends the slurry and hardens the dried glaze (at least 15% of it is normally needed). Calcined alumina also sources Al2O3 but its physical form is highly refractory and it does not dissolve into the melt readily. Talc and dolomite both source MgO, wollastonite and calcium carbonate both source CaO, but the first (respectively) have lower LOIs.

Related Information

The top pile of clay can make one million coffee mugs. The bottom one can glaze ten million!


Two piles of raw clay behind the Plainsman Clays factory

These are raw clays behind the Plainsman Clays plant. The top one is a middle temperature stoneware. All it needs is a little bentonite (about 2-3%) to be a plastic, smooth, vitreous throwing body. If it was not mature at cone 6, that would be easy to fix by the addition of a little feldspar. Any fired-speck-producing impurities can be removed by using a propeller mixer to slurry it and then putting it through a screen (e.g. 60 mesh). After dewatering on a plaster table I am ready-to-go. And that bottom pile? That is the main ingredient in Ravenscrag Slip. All it needs is some feldspar and frit to be a base glaze at cone 6. It is non-plastic and easy to screen (although not really needed since it has few particulate impurities). Likely there are clays in your area you could use to make your own clay bodies and glazes also. The key is to characterize the material first so you know what type of body it would be best for and what to add to get it there.

How to decide what temperature to fire this clay at?


Test bars of a terra cotta clay fired from cone 06-8

This is an extreme example of firing a clay at many temperatures to get wide-angle view of it. These SHAB test bars characterize a terra cotta body, L4170B. While it has a wide firing range its "practical firing window" is much narrower than these fired bars and graph suggest. On paper, cone 5 hits zero porosity. And, in-hand, the bar feels like a porcelain. But ware warps during firing and transparent glazes will be completely clouded with bubbles (when pieces are glazed inside and out). What about cone 3? Its numbers put it in stoneware territory, watertight. But decomposition gases still bubble glazes! Cone 2? Much better, it has below 4% porosity (any fitted glaze will make it water-tight), below 6% fired shrinkage, still very strong. But there are still issues: Accidental overfiring drastically darkens the color. Low-fire commercial glazes may not work at cone 2. How about cone 02? This is a sweet spot. This body has only 6% porosity (compared to the 11% of cone 04). Most low-fire cone 06-04 glazes are still fine at cone 02. And glaze bubble-clouding is minimal. What if you must fire this at cone 04? Pieces will be "sponges" with 11% porosity, shrinking only 2% (for low density, poor strength). There is another advantage of firing as high as possible: Glazes and engobes bond better. As an example of a low-fire transparent base that works fine on this up to cone 2: G1916Q.

Clay lab report. Is this really what you need?


Lab testing report for a clay

If you are trying to use local clays for brick or tile or even pottery production, characterizing the available materials is the first step. But how? This is the kind of data a lab might return - perhaps you wonder about its value? We feel traditional ceramics technology is fundamentally relative. A history of many reports like these, in context with other data, might be good for mining companies to determine if new stockpiles have any shifts in certain specific properties. Or a tile company evaluating a new ball clay. But as a way to understand the utility of a clay for a specific ceramic purpose, this contextless report is of little use. For example, the physical properties, the whole reason for using a clay, are unrelated to the chemistry. This is also a tunnel vision view, looking at only one temperature. On the other hand, simple procedures, like the SHAB test, provide a hands-on way to understand what a clay actually is.

Inbound Photo Links


Simple tests being done on a found clay
Testing your own native clays is easier than you think

Two warping pottery mugs
I notice small details about the clays I test? Want to know why?

SHAB test bars for casting
Historical data on drying and firing casting slip performance

Links

Articles Low Budget Testing of Ceramic Glazes
There is more to glazes than their visual character, they have other physical properties like hardness, thermal expansion, leachability, chemistry and they exhibit many defects. Here are some simple tests.
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.
Articles Formulating a body using clays native to your area
Being able to mix your own clay body and glaze from native materials might seem ridiculous, yet Covid-19 taught us about the need for independence.
Glossary Physical Testing
In ceramics, glazes, engobes and bodies have chemistries and physics. To fix, formulate and adjust their relative importances in each situation need to be understood.
Glossary Glaze Chemistry
Glaze chemistry is the study of how the oxide chemistry of glazes relate to the way they fire. It accounts for color, surface, hardness, texture, melting temperature, thermal expansion, etc.
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 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.
Projects Tests
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