Important Terms and Concepts
Wet clay: Mixed clay ready to be formed. This is clay in its most elastic state and is very sensitive to water variation.
Leather-hard: A stage in the drying process when a clay object can be carefully handled without danger of the shape being deformed, but the clay is still pliable enough so alterations can be made if desired.
Greenware: This is the period between formation of your clay object and its drying time before it can be safely bisque-fired.
Bone dry: Completely air dry clay. It is very hard but brittle, like a biscuit. It can still be transformed back to wet clay if exposed to water.
Bisque: This is an intermediate state achieved after the first kiln firing above 900oC. The vessel is still porous but can no longer be returned to the wet clay phase. Your piece will still have the ability to absorb water (porosity up to 3% water by weight) but this helps any glaze solution adhere to your object.
Feldspar and Fluxes: A key component of any clay, feldspars are naturally found and contain various minerals. These include fluxes (e.g. either potassium oxide or sodium oxide, 10-15% by weight), mixed with alumina (Al2O3, 17-25%) and silica (SiO2, 60-70%). So we have the base glass-former, silica, in abundance. But it is crucially aided by the alkali fluxes which lower the maturing temperature of the clay and promote the vitrification process.
Glaze: A glaze is a glass that, on its most basic level, has been assigned to melt at a compatible target temperature. Its silica and flux composition should be matched to your object to achieve comparable thermal expansion (glaze-fit).
In ceramics, glazes contrast with clay bodies in that all particles melt. No particles retain their crystal identity but transition to a hard, vitreous surface. Glaze contains additions of minerals to lower the melting point of the silica (fluxes like potassium, calcium or boron compounds) and minerals to add different colour (e.g. cobalt, iron and copper oxides) , opacity and finish.
Glazes are applied to bisque-fired ware by painting, dipping or spraying.
Grog: A sand-like substance that is added to a clay body to add workability and strength. It is ground from previously fired ceramic substances. It also helps to reduce shrinkage of the clay.
Maturation not Melting: This is the point in the clay firing cycle where there is just enough fusion of the clay particles along with bonding strength for durability - but not so much that melting or deformation of the ware takes place. This point is called the maturing of the clay.
Shrinkage: Clay shrinks as it dries - both as greenware and then again during the firing processes.
Different clay bodies shrink at different rates. Linear sizes can change as little as 4%, or as much as 15% in TOTAL across a clay’s transformative processes.
For example, a typical mid-range stoneware clay (such as that offered in the Claybot Stoneware Cartridge) is specified to shrink 11%. This occurs as 5% during greenware drying, 0.5% during bisque firing (to Cone 06 circa 1000oC ) and the final 5.5% during glaze firing (cone 6, 1225 oC).
And of course, given the variability in natural substances and environmental firing inconsistencies, ±2% on any figure should not be considered unusual.
Depending on your need to have accurately sized finished pieces, be aware that a small change in linear shrinkage percentage can have a significant visual effect. (11% linear translates into 30% volume!) Your designs, therefore, should always account for shrinking that will occur before and after firing. (See 11% Shrinkage example below).
Silica: also known as quartz. Silica (SiO2) is the most abundant material in the earth’s crust.
It is the basic glass-former which melts and begins to fill air-spaces in the clay structure during a bisque-firing or, forms a glass surface when used as a glaze. By itself, silica melts at 1715°C and would be useless in studio ceramics without fluxes – which lower the melting temperature and allow it to flow as glass in a usable range (1060-1300°C).
Sintered: When clay is fired to red heat (c. 900 oC), it becomes sintered – a heat which causes the particles to stick together even before the fluxes and glass-formers begin to interact. Once the
clay is sintered, it can no longer be slaked down and reused.
Slip: is a term which has more than one definition in ceramics.
Its principal role is when as a simple mix of water and clay it is used as a glue to attach leather hard or dry elements together (e.g. handles to mugs, spouts to teapots).
Slip is also the term used for a mix of clay and other minerals and fluxes that is applied to dry or leather hard ware to enhance the surface in the glaze firing. Its main objectives are to improve glaze coverage, add certain colour saturations and improve fired hardness.
And finally, it is a term used for the clay slurry when casting into mould shapes. This slip is deflocculated to minimize water content and optimize viscosity.
Plasticity: is a term referring to the ability of a clay body to assume a new shape without returning to the old (it being ‘elastic’ if it springs back).
Plasticity is mainly, but not only, a function of particle size. In addition, the electrolytic character of flat clay particles (they have opposite charges on the faces and edges) is just as important.
Normally clays of finer particle sizes are more plastic because they pack more closely together with electrolytically charged water molecules. But more water means more shrinkage at later stages. The ideal condition, therefore, is to produce a clay body with a variety of particle sizes for good plasticity, both dry and bisque strength, yet without excessive shrinkage. For example, a combination of kaolins, ball clays and bentonite enable bodies of more plasticity with less drying shrinkage and better drying performance than kaolin alone.
Plastic clays enable large, thin pieces to be made. Non-plastic clays are short and prone to water loss at all stages so better for casting but not larger pieces.
Additive minerals such as bentonite and veegum need only be included in very small quantities to achieve large increases in plastic behaviour.
Vitrification: As temperature in a clay body continues to climb beyond initial sintering, towards the high-fire range, the fluxes and glass-formers within the body start to form a glassy-phase (rather than crystallization). Vitrification is sintering in the presence of a fully developed glass-phase, where the air spaces between particles are almost completely filled. The filling of these air spaces, along with the closer arrangement of the sintered particles, accounts for firing shrinkage in vitrified wares.
If done correctly, this provides high density, a good degree of impermeability, and strength.
Wedging: Over time, effects of mould growth and particle electrical charge creates non-homogeneous stiffness across the clay matrix. Wedging clay is similar to kneading bread dough. It evens out the stiffness and returns it to a consistency found at time of production. It is not uncommon for clay to soften quite dramatically on wedging.