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There is a need to discuss water in ceramic production as it related to a number of natural phenomena and production processes:

Plasticity: Clays are plastic because water glues and lubricates the particles. The micro-dynamics of this are complex.

Rheology: Suspensions (solids:water systems) exhibit properties (like viscosity, thixotropy) that are a product of the way particles interact with the water, each other and the way electrolytes affect the system.

Solubility: Generally, in traditional ceramics, insoluble materials are employed to make body and glaze slurries. However, in actual practice, many materials are slightly soluble over time and thus introduce electrolytes that affect the pH in a linear or non-linear fashion (and therefore rheological properties) of the system. Solubility is much less of an issue when the water:solids mix is not stored (used immediately).

Particle surface area, surface charge and size: Almost all glazes and bodies contain 'live' particles (like clay) that interact with water. The degree to which they interact and the total surface area that does that interaction profoundly affects the physical properties and the amount of water needed.

Dehydration and Decomposition: Many minerals are hydrates and thus have water bound in the crystal structure itself. This has to be ejected during firing. For product quality and even survival in the firing, it is important to understand how and when they convert from one phase to another (and release water at each). At times multiples conversions occur as a firing proceeds. In addition, bodies and glazes are mixtures of multiple minerals each having its own dehydration profile.

Dewatering: Many raw materials and even bodies are wet processed for purification purposes and must be dewatered to either powder stage or production use stage (an energy intensive process).

Drying: Water must be removed from bodies in such a way that articles do not crack during drying or explode during firing (due to steam pressure). Even when a piece appears dry it still likely contains 5% or more mechanical water that only drying above its boiling point will remove. The smaller the finest particle sizes the longer it will take to dry and the greater the drying shrinkage will be (some clays may literally need ten times longer to dry than others). Glazes and engobes likewise need to be dried in such a way that they do not crack and they maintain their bond with the body.

Focus on the actual formulations that utilize water is also helpful:

Plastic bodies: Water is the vehicle that makes the clay plastic and the primary focus is having the amount of water needed to get the desired stiffness (typically 17-23%) and seeing that it is incorporated in such a way that all particle surfaces are whetted. Electrolytes enhance or degrade that plasticity immediately and over time (if they they dissolve in the water). In industry, water is typically filtered and processed before being put into bodies to minimize the unknown (and therefore unwanted) electrolytes. Water can also dissolve salts in the body and cause efflorescence during drying.

Glazes: The rheological properties of glaze slurries are a product of the amount of water (typically 40-55%) and the way that water interacts with the inert material particles (like feldspars and quartz), the charged particles (like clays) and the conditioners (like gums, deflocculants, electrolytes). Highly fritted glazes need less water, those with alot of clay (especially fine-particles clay) need more. Water quality is more critical than with bodies because electrolytes have a greater affect on the working presence. In addition, the viscosity and thixotropy of glazes is even more likely to change over time when glazes are stored.

Casting slips: The same as glazes but with a focus on minimizing the quantity of water (to around 25-30%) by introducing electrolytes that deflocculate the slurry while at the same time producing a thixotropy that prevents particle settling. Casting slips are a finely tuned water:solids system whose rheology is even more fragile to change than glazes (when extra and unknown electrolytes are entering it by slowly dissolving from materials in the slurry).

Out Bound Links

  • (Glossary) Rheology

    Rheology refers to the array of characteristics that a ceramic slurry exhibits: its density, flow, thixotropy, permeability, viscosity, stability, etc. Technicians seek to understand and control the dynamics of the slurries they use (to maintain consistency and optimize them for the product and proc...

  • (Glossary) Deflocculation

    In ceramics, when we speak of deflocculation, we are almost always talking about making a casting slip. Glazes can also be deflocculated (to reduce water content and densify laydown). Deflocculation is the process of making a clay slurry that would otherwise be very thick and gooey into a thin po...

  • (Glossary) Mineralogy

    In contrast to man-made materials (like frits), ceramic minerals have a highly ordered atomic structure and a specific range of crystalline manifestations. By taking the characteristics of these into account technicians can rationalize the application of glaze chemistry when recipes are mixtures of ...

  • (Glossary) Efflorescence

    A term describing the whitish or brownish dry or glassy scum (depending on iron content and firing temperature) left on the surface of a fired clay body (most often red earthenware or raw stoneware and fireclays). Many clays contain soluble sulphates that migrate to the surface with the water and ar...

  • (Glossary) Plasticity

    This term is used in reference to clays (or more often bodies which are blends of clay, feldspar and silica particles) and their ability to assume a new shape without any tendency to return to the old (elasticity). Plasticity is a product of the electrolytic character of flat clay particles (they ha...

  • (Glossary) Surface Area

    Surface area (or SSA - Specific Surface Area) is a physical property you will see listed on the data sheets and certificates of analysis of many materials. Some materials (e.g. sedimentary clays) can contain particles that have a wide range of sizes, shapes, densities, surface texture, reactivities...

  • (Glossary) Decomposition

    Decomposition is the breaking of inter-molecule bonds during melting in the kiln. To understand it we need to understand elements, oxides, compounds, solutions and mixtures (from the chemistry jargon point-of-view). "Elements" are one kind of atom which cannot be broken down any further (except b...

  • (Glossary) Suspension

    In traditional ceramics, glazes are suspensions, not solutions. They are mixes of insoluble mineral, frit and/or stain particles that have been added to water to form a liquid useful in the ceramic process. That suspension is what confronts us in the bucket or tank, learning how to assess and contro...

  • (Glossary) Viscosity

    The term viscosity is used in ceramics most often to refer to the degree of fluidity of a slurry or suspension (the term 'shear' is often used when discussing viscosity, theoretically engineers understand viscosity in terms of layers particles or molecules that exhibit a friction that resists latera...

  • (Glossary) Casting, Slip Casting

    Forming pottery by pouring deflocculated (water reduced) clay slurry into plaster molds. In the process the absorbent plaster pulls water from the slurry and over a period of minutes a layer builds up against the mold surface. The slurry is then poured out and within a short time the item shrinks sl...

  • (Glossary) Leaching

    Glazes are not as inert and stable as many people think. All are slightly soluble and will thus leach to some extent, even if minute, into liquids they come into contact with. However some glazes are dramatically more soluble than others. It is common for leaching glazes to suffer from more than one...

  • (Glossary) Water Solubility

    A host of water soluble materials are available to source most of the important oxides needed in ceramic glazes. However such materials cannot normally be used in glazes. Why? Because glazes are suspensions of particulate materials, not solutions of soluble materials. Such suspensions have a far low...

By Tony Hansen

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