ponder craft & design

Clay

Materials of the Trade

 

What are you using your crafted piece for?

 

What clay material best suits your lifestyle needs?

Where will your piece spend most of its time

– kitchen table, patio, bathroom counter?

Know what finishes will be kind to your table.

If you find a piece you like, be mindful of the relationship between aesthetics and function, for example, it looks great but doesn't pour right. 

Is the design, function and structure proportionate to all sides of the work - front, top and bottom?

Is it comfortable to hold? Are there cracks or warpage? Ceramics are made by the hand, for the hand. 

Check for secure handles, spouts and ridges. Surface decoration should be proportional, in scale and size.

Consider...

Is this piece functional or sculptural ceramic?

What process was used to create the piece?

Does the work comply with Health Standards? Glazes must meet Health Canada requirements. Become familiar with the Hazardous Products Act for more regulations. 

If the piece is for the kitchen, glazes should be lead-free and void of crazing.

...Ask the Artist About

The Material

Making ceramics is a long process and can take at least a couple of weeks to complete. Once the piece has been made, it needs time to dry completely. It is usually fired for the first time at a medium temperature (between 1720 and 1835 degrees F), this is called a "bisque". Then once it is glazed and decorated, it is ready to be fired again. 

 

Ceramic forming is not a perfect science. Even after years of practice, there are always some surprises. The potter can tell roughly how a piece will come out, but there is no way to be sure. Some potters try to control as much as they can through monitored electric kilns, while some others prefer the challenge and randomness of wood or gas firing.

SWEETS

Earthenware

Identifying Features:

Earthenware clay is either red-brown or white in colour, becoming opaque once fired. A fairly brittle and lightweight clay, earthenware is notably soft in that when left unglazed, it can be scratched with a knife.

 

Properties:

Earthenware is fired at the lowest temperature among clay types, at about 1000°C or 1830°F. However, this also means while it can withstand oven temperatures, after a few heatings, eventually it will crack. Due to earthenware’s coarse and porous texture (meaning it absorbs liquids easily), it requires a glaze to be appropriate for spaces such as the kitchen. Since it is non-vitreous, glazing provides both water and heat resistance, as well as overall strength and durability.

 

Common Uses:

Earthenware is most commonly recognized as the basic, rusty orange outdoor planting pot found in your garden. However, earthenware is also used for tiles and building bricks. Today, fine earthenware makes up most of your contemporary tableware, subject to proper glazing.

SWEETS

Stoneware

Identifying Features:

Stoneware is a fine-grained denser clay, often feeling dense for its size. While opaque, it can be any colour ranging from white, grey, brown or even black.

 

Properties:

Stoneware is fired to ‘maturity,’ between 1100-1300°C or 2010-2370°F – this creates a sturdy and chip resistant material. Due to stoneware’s extremely hot initial firing, the material is able to handle cast temperature changes making it oven, microwave, dishwasher and refrigerator/ freezer safe. Unlike earthenware, stoneware is non-porous, vitrifying after a single firing. This means it does not need a glaze to be functional but is often glazed anyways for further durability and ornamentation.

 

Common Uses:

Known for its durability, stoneware is perfectly suited for your kitchen tableware, serving guests or even in the garden. More traditionally, stoneware’s have been used as baking stones because of its dense, supportive nature and ability to withstand the rapid heating of the pizza oven. Stoneware is popular for functional pottery and utilitarian wares because of its high quality and versatility.

SWEETS

Porcelain

Identifying Features:

In addition to its whiteness, porcelain is known for its smooth surface. A great test for identifying porcelain is simply to hold the work up to a light, as it is notably translucent compared to other clay materials. Porcelain is sometimes called china or fine china, signaling its introduction to Western culture from China.

 

Properties:

Porcelain is fired at the highest temperature of all the clay materials, between 1200-1400°C or 2190-2550°F, resulting in a high resistance to chemical attack and thermal shock. Its primary material is Kaolin, a very fine, soft white clay. Porcelain is non-porous, having low permeability and elasticity even before glazing. It is completely vitrified, making it exceptionally hard and tough and thus able to produce very thin, delicate objects.

 

Common Uses:
Porcelain’s ability to combine well with paints and glazes and ease of modeling makes it a popular choice of material for decorative treatments in tableware, pottery and figurines – Chinaware being one of the most obvious.

For information about polymer clay, see 

 
SWEETS

Techniques

 

Casting: Moulds and casts are used to produce duplicates of a specific pattern, shape or detail. Casting is performed when the artist wants to limit any variance between products of a series. This technique is often used in fine factory porcelain or upscale china tableware.

 

Crystalline Glazing: A speciality glaze that encourages visible crystalline growth in the glaze during the firing process. Crystals inhabit many glazes but are typically too small or invisible to see. The crystals are made visible when the firing environment allows them to grow large enough. This means the glaze must remain molten for an extended period, fired at a hotter temperature for a longer duration (1140C/2084°F), and has a chemically high percentage of zinc, titanium or lithium. The crystals form when the kiln is cooling and therefore requires great skill to precisely achieve the necessary temperatures. Because crystalline glazes are quite runny, the pot’s base typically has to be ground and polished after it's been fired.

Coil Method: Coil pieces can be almost any shape or size, the pieces are joined by layering and blending them together. 

 

Embossing/De-embossing: The process where textures and images are either raised or imprinted into the clay, which is then baked in the oven, fixing the design in the material. Some tools include rubber stamps, textured plates and sandpaper.

 

Glazing: Earthenware ceramics are glazed for both decorative and practical reasons – the glaze works to seal up its pores. Earthenware typically uses two types of glazes, either tin, a white finish, or lead, either clear or cream. Lead glazes are less common as they often are avoided for health concerns, especially when intended for food products. Earthenware’s low firing temperature allows for a wide range of glaze colours through the addition of metal oxides such as copper, iron or cobalt. The glaze used on hard-paste porcelain is solely for decorative purposes, as it is a non-porous material.

 

Hand Building: A technique that quite literally embraces the ‘handmade touch.’ Hand building is the earliest clay forming practice, where pieces are made from rolling coils and slabs of clay together, and then joining the various parts with one’s fingers and water. Designs and textures are hand-printed with stamping tools and the artist’s hands.

 

Raku Firing: Raku is a traditional type of Japanese pottery used for tea ceremonies. However, the term Raku has since taken on some Western modifications. Artists enjoy Raku for its unpredictability and vibrant colours. The pot is heated at an extremely high temperature, then rapidly cooled. It is this rapid cooling, by water, air or a combustible material, which stops the chemical reaction of the glaze, fixing the vibrant colours and patterns. Western artists have experimented by applying horsehair, feathers and sugar upon removal from the kiln, burning organic patterns onto the pot’s surface, similar to salt firing. Due to the extreme thermal shock involved, stoneware is typically the material used with Western Raku.

 

Salt Glazing/Firing: When salt or soda (sodium chloride) is added to the kiln during high-temperature firing to produce a pattern. The salt vaporizes and combines with the silica in the clay surface, forming an extremely hard sodium-silicate glaze. This technique gives the clay a glossy, translucent look with an orange-peel-like texture. Salt glazing is specific to Stoneware as it requires its durability and tolerance for high-temperatures to cultivate the desired effect. This glazing technique creates a finish that is food safe.

 

Slip Casting: When liquid clay is poured into an absorbent plaster mould. Typically used by industries wanting to produce large quantities of a piece, or potters wanting consistency in their product’s shape, thickness, etc. A technique commonly used for modern, fine factory porcelain such as sanitary wares (toilets and basins) or smaller, more intricately detailed figurines and teapots.

 

Throwing: A manual clay-forming technique involving a turntable (either electric or manual). First, the clay is centred by pressing the clay upward/downward, into perfect rotational symmetry. Second, the clay is opened, where a hollow is formed in the centre of the clay. Flooring is the flattening of the pot’s inner base while throwing or pulling shapes the sides and thickness of the pot. Once the piece has begun drying, excess clay is trimmed and a foot is created. Unlike slip casting, throwing results in visible variations between each piece, as it relies heavily on the artist’s own hands.

 
For Your Reference

Crazing: The phenomenon when a fine network of cracks forms on the surface of a ceramic material, typically in the glaze. Crazing is common when tension forms in the material during the heating/curing processes.

 

Electric Kiln: Ensures uniformity among pieces due to the even spread of temperature. The artist has more control over the finished results, making it a great choice for potters who want predictability, or those needing to produce exact replications of a single piece.

 

Vitrification: The transformation of a substance into glass, typically with the application of heat. In terms of ceramics, vitrifying a clay ensures its impermeability by water, for example making earthenware less functional for spaces like the kitchen.

 

Wood/ Salt Firing: This form of firing is a lengthier process than firing in a kiln – from 3 to 10 days. Wood/ salt firing has less predictability in the final product, making it a much more challenging technique. Heating varies depending on where the pot is in relation to the flame, increasing the chances of inconsistencies among pieces within a single firing.