Clay for Claybot

Various properties that occur in Clay are magnified under the conditions within the Claybot 3d-print system. In generating successful, different clay recipes the Claybot team aims to present educators with a system they can depend upon to deliver consistent results in the teaching environment.

The power of correct material choice is a combination of physical appearance, its mechanical properties and the solution it’s used for. Our favourite and most forgiving is stoneware clay.

Understanding clay

It is important to understand the key physical and chemical processes through which clay transitions during preparation, printing, drying and kiln firing. This knowledge will help you and your students plan clay projects where defects are minimized. The artistry and science of ceramic materials, knowledge accumulated over thousands of years, is now widely available to help you avoid cracking, breaking, exploding and glaze defects in your 3D objects. 

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.

Drying clay

Air Drying Clay - why it is important to let clay work dry slowly

Wet clay contains a minimum of 25% water. If clay is drying, water has started to evaporate. As this happens, particles of clay draw closer together resulting in shrinkage. If that occurs at different rates - at the interface between dry and moist clay – then stress cracks become likely. Many problems with clay are formed by uneven rates of drying. Sometimes stress appears immediately as visible cracks or warpage. Sometimes later - as cracks, bloating or explosion during kiln firing. It is so important to ensure drying is even. Uniform thicknesses throughout your pieces aid good drying. And controlled exposure in humid conditions, with the aid of polythene protection, helps well.

Clays which have very fine particle sizes will shrink more than clays with larger particle sizes simply because they have more water to lose. Porcelain clay has very fine particle sizes which, whilst being very plastic, also shrinks the most. However, these bodies once dry, have the most strength in the dry greenware state because their small particles are packed closely together. Clays with grog (large particles) are best for sculpture and shrink the least.

When almost all water between the layers of clay particles have evaporated, your object has reached a leather-hard/bone-dry stage. The particles themselves are still ‘damp’ (chemically bonded water molecules are still attached) but more drying will not cause any additional shrinkage.

Accelerated Drying

If clay objects are just damp or slightly wet, it is not unreasonable to speed-up drying using a fan or warm kiln room. But swings in environmental temperature must remain slow - evaporation of the remaining non-bonded water needs to occur gradually.

Kiln Firing

Key Temperature Points
During the clay firing cycle in a kiln, clay transforms from a totally fragile substance (air-dry clay!) to a stone like substance (ceramic). A typical kiln firing cycle is planned to ‘ramp’ temperature upwards at approximately 150°C per hour. The firing moves through several stages, outlined below.

+100°C Initial Kiln Drying
Complete drying doesn't take place until your clay piece is in the kiln and the boiling point of water has been surpassed (100 oC). This must happen slowly, or the formation of steam within the body of the clay may cause it to burst. For this reason, the thermal ramp at the start of the cycle is shallow and the kiln lid opened to allow steam to escape.

+300°C Burn Off – Carbon, Sulphur and organic impurities
Clay bodies all contain carbon, organic materials, and sulphur which will burn off between 300°C and 800°C (570°F and 1470°F). Note your kiln should be vented to the outside to prevent the danger of breathing the various oxides and sulphate fumes which generate in this period.

+500°C Chemically Combined Water Driven Off
After the clay is air dried, it still contains about 15% water which is chemically bonded. Clay is chemically defined as a molecule of alumina and two molecules of silica bonded with two molecules of water.

The chemically combined water bonds weaken during the same heating ramp for Carbon and Sulphur burn off (350°C and 800°C). The ceramic will become substantially lighter but there is no physical shrinkage. And it remains critical during this stage to ensure temperature rise is slow to prevent steam pressure build-up which can result in shattering.

This irreversible chemical change is known as dehydration.

572°C Quartz Inversion
Quartz, also called silica oxide, has a crystalline structure that changes at a temperature of 572°C (1060°F). This change in crystalline structure (Quartz Inversion) will cause the clay body to increase in size by 2% while heating but will lose this 2% when cooled. The ware is fragile during this change and kiln temperature must be raised and cooled slowly across this temperature point.

+900°C Sintering / Bisquing
Starting at about 900°C the clay particles begin to fuse. This process is called sintering that when completed, the clay has become ceramic. Once temperature is reached between 1000°C (cone 06) to 1060°C (cone 04) it is bisqued. At this stage, the ceramic is porous, somewhat fragile and not yet vitrified and is called Earthenware or Bisque. The Bisque allows wet, raw glazes to adhere to the pottery before Glaze firing.

+1000°C Vitrification and Maturity
Vitrification is the progressive fusion of a clay that makes the finished product harder and more durable. As vitrification proceeds with temperature increase, the proportion of glassy bond increases and the porosity of the fired ceramic becomes lower as the air cavities fill in with glass.

It is also during this stage that mullite or aluminium silicate crystals are formed that act as a binder, strengthening the clay body even further.

Bisque firing alone does not make clay bodies impermeable to water in most cases. However, Porcelain, which is among the most vitrified ceramic, is impermeable even without glaze. Stoneware is semi-vitrified and would not be impermeable without glaze.

220°C Cooling
The crystalline form of silica, as it cools past 420°F (220°C) must be cooled slowly as it moves through this critical temperature to avoid cracks. This is the final phase of your thermal ‘RAMP’.

Glaze Firing

When your piece is back in the kiln and put through a glaze firing cycle, it will undergo similar processes to the bisque fire but to a different heat ramp profile: impurity gases are expelled, sintering of the glaze particles take place above 800°C and at just above 1000°C (cone 06) needlelike mullite (aluminum silicate) crystals begin to form an interlocking structure. This glassy phase creates a combination of dense vitrification, great body strength, and a reinforced clay– glaze interface, all of which we want in high-fired functional ceramics. This is where glaze fit is crucial i.e. when the shrinkage characteristics of the glaze matches that of the clay vessel.

Around 1060-1240°C (cone 4-7) free silica in the clay and glazes that have not combined with fluxes to form a glass, begin to transform into cristobalite (crystalline quartz). This is an irreversible change. Note: Do not heat soak your piece above this temperature range because it creates pieces with low thermal shock resistance.

As your glaze-fired piece cools the molten glaze passes from liquid to thermoplastic to solid. Throughout this process of annealing, the physical volume of the glaze and body change constantly, and rarely at the same rate. This is due not only to differing thermal expansion of the various materials, but also to the inversions of quartz (572°C) and cristobalite (260°C).

Whilst cooling stresses through these stages are usually dissipated in all glass and ceramics without issue, there is a critical zone where cooling must be slow in order to accommodate differential shrinkage. For most glazes the critical zone extends from around 540°C down to 340°C, below the quartz inversion temperature. But note a porcelain glaze anneals at a temperature above quartz inversion and cooling ramps should be adjusted accordingly.