KINDS OF CLAY©

dinnerware header

Rock metamorphoses into clay which is shaped, cooked and decorated by man to suit his needs. Despite the simplicity with which clays were dug, shaped and cooked by sun or fire more than 10,000 years ago, current ceramic technology includes locating,transporting and combining clays and other ingredients from all parts of the world, creating glazing formulas and techniques and kilns capable of firing clays at very high temperatures.

Granite/type igneous rock account for much of the earth’s crust. Natural hot gases decompose these hard rocks into soft rock containing feldspar minerals which are further broken down by rain, sun and ice into CLAY; a generic classification of fine grained earth and the raw material of ceramic dinnerware. Some clays contain minerals as mica and quartz, some contain kaolinite called china clay.

Each kind of clay has different degrees of porosity, plasticity and purity that contribute character and durability to dinnerware but they present challenges to manufacturers who require many specialists to manage technical variables as shaping, glazing and firing.

Clay that remains where it’s formed,  is primary or residual clay.

Clays carried thousands of miles from their primary source by wind, glaciers, rivers, and the sea are secondary or sedimentary clays.  During their journey, they’re ground, collect impurities and eventually settle.  These clays vary in color, are finer, more plastic, and less pure than primary clay. A significant impurity is iron. Clays are blended by manufacturers   to assure uniformity for production.

There is also variation in the minerals and purity of the water with which clays are mixed  and each manufacturer develops their exacting formulae to achieve uniformity of quality and exclusivity.  From hands of artisans to machine techniques of mass-producers, every culture has produced ceramic dinnerware shapes, sizes and patterns to enhance their culinary arts.

Dinnerware departments in retail stores sell dishes made of different clays as porcelain, bone china, stoneware and earthenware.

Each kind of clay has a maximum firing temperature established by the temperature that would cause a shape to lose definition; called its’ slumping point. Generally; the purer and finer the clay, the higher the firing temperature, which explains the strength and durability of dinnerware that looks fragile.

The following dinnerware categories are made of clays with distinctly different  characteristics:

POTTERY:
A general label for low-fire porous clay products.

EARTHENWARE:
Low-fire, opaque, porous clays requiring glaze to become non-porous.

STONEWARE:
High-fire vitreous, opaque clays.

PORCELAIN & BONE CHINA:
High-fire vitreous clays; translucent if thin.

NATURAL CHARACTERISTICS  OF CLAY:

POROSITY: Most clays are porous in the bisque stage and require glaze to seal porosity.

NON-POROSITY:   Some clays are vitrified (non-porous) in the bisque stage.  All elements are fused and the bisque is impermeable to moisture.


PLASTICITY:   Clays that can be easily modeled or shaped.


PURITY:   The purity of each clay determines its firing temperature. The purer the clay, the higher the firing temperature—which makes some dinnerware more durable. e.g. porcelain.

POROUS CLAYS:

EARTHENWARE :

EARTHENWARE clays are sedimentary clays. Some are more pure than others and range in color from light buff to dark sienna. These clays are fired at relatively low temperatures and although earthenware products look sturdy, they chip easily. Earthenware is porous and must be glazed to seal the porosity.

TERRA COTTA  (cooked earth) is red earthenware containing iron oxide. Unglazed terra cotta is used for flower pots because its’ porosity enables potted soil to breathe.

CREAMWARE  was an important development in making pale earthenware. It preceded glazing techniques that made earthenware resemble white porcelains from the Orient.

NON-POROUS CLAYS:

STONEWARE:
Stoneware originated in China but was not introduced for dinnerware until the late 17th Century in England.  Stoneware is a composition of clay and fusible stone.  In the bisque state some stoneware is vitreous; some is semi-vitreous.  Stoneware matures slowly as it’s fired at temperatures higher than earthenware. It becomes dense and durable when glazed and fired.

Natural stoneware colors vary from almost white to dark brown.
Transparent glazes are applied when a manufacturer wants the natural color of the clay to show through.  Opaque lead or earthy salt glazes are also commonly used.  Lead burns out in firing.

IRONSTONE is English earthenware containing slag of iron. It is called stone china because of its durability. Do not confuse ironstone with stoneware.

PORCELAIN: 
The Chinese defined as a “development of fine stoneware” containing fusible stone. True porcelain, hard-paste, is fired at a very high temperature enabling clay and glaze to mature together. Porcelain does not exist in nature as clay.  Diverse formulae include:

  • kaolin: The purest of primary clays; indispensable for producing translucent porcelain.
  • ball clay: Clays, once-removed from primary kaolin clays, which add plasticity.
  • feldspar: Ground rock which helps bind all ingredients.
  • flint (silica): Substances which turn to glass during firing and impart hardness and freedom from warping.

Porcelain is the most pure of ceramic wares–and the reason  it can be fired at high temperatures. It is very white and very strong.  Porcelain is vitreous (non-porous) in the bisque stage and glazed only to provide a glassy surface for food and avoid stains. Porcelain wares can be made so thin the wares are translucent, yet exceptionally strong, and resonant when struck with a pencil. Porcelain can also be made heavy enough to be oven-to-table cookware.

OTHER PORCELAIN WARES :

PARIAN:   Unglazed granular porcelain resembling marble; also called statuary porcelain.

BELLEEK:
 A highly refined translucent porcelain with a luminous glaze and made in Ireland’s first porcelain factory which opened in 1857 in Belleek.


SOFT PASTE PORCELAIN 
(pate tendré):  A mixture of white clay and ground frit but without kaolin.

BONE CHINA:

Bone china, introduced by Spode in 1800, is a form of soft-paste porcelain made to duplicate the appearance of Oriental true hard-paste porcelain.

Bone china combines 50% animal-bone-ash with clay to lower the firing temperature and increase translucency. It contains no kaolin. Glazes for soft-paste porcelain and bone china are fired at lower temperatures than hard paste porcelain.

WE NEED LABELING TRANSPARENCY

 Pat Breen: EYEWITNESS TO QUALITY

My next post is:  CERAMIC DINNERWARE PRODUCTION