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•The secret to learning from testing is documentation.
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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
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Casting, Slip Casting
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By Tony Hansen