GLAZING CERAMIC DINNERWARE©

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Some clays can be dried, glazed, decorated and fired in a single sequence but many quality factories prefer to first apply and fire a transparent glaze (glost) prior to decorations for easier application of patterns. During a second firing, a hand-painted or decal decoration penetrates the glaze to become very durable.

If lead oxides are part of a glaze formula, they will leach
into food unless the firing temperature is sufficiently
high to burn out traces of lead.

The composition and firing temperature of each glaze must ‘fit’ each clay body because clay shrinks as it dries to the greenware state —and shrinks again an average of 12% or more when fired in a kiln.   Glazes must shrink at a comparable rate.                          

                                                   

Different colors or applied ornamentation may require firing at different temperatures so each piece may be fired several times before completion. Glazes seal the porosity of clay and protect decals or painted designs from wearing away. Labor costs increase every time a ceramic body is handled for decoration or firing.

EARTHENWARE GLAZES:

Opaque tin-oxide glazes, usually white, were applied to earthenware bisque to cover over the color of the clay —and then hand painted with colorful designs over the glaze. Well known indigenous names for opaque tin-oxide glazes include:

 FAIENCE:

Tin glazed earthenware has been made since 1100 in Faenza, Italy where even the street signs  are ceramic. When the French began to make similar wares, they named  it faience. It is still made in the oldest pottery factory in St. Clement, France.

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MAIOLICA:

Historians trace tin glazes and design influences on ceramics to the Moors who brought these techniques to Spain. By the 15th Century, tin-glazed pottery from the island of Majorca, made in Arabic tradition, inspired Italian ceramic wares named maiolica.

The name continues for soft-fired Italian earthenware covered with opaque glazes containing tin oxides which masque the color of the clay and allow for strong colors in a range of decorative motifs including original Renaissance pictorials painted by by hand in an earthy, robust style.

MAJOLICA:

Originated by Minton in the 1860’s, English earthenware with high relief designs and covered with brilliant colored shiny lead glazes is majolica (with a j)— borrowed from—but not to be confused with maiolica.

DELFT:  

Tin glazed earthenware made by Italian potters in the Netherlands mid-16th Century is delft.  In the production of Delft, an opaque white glaze is applied to earthenware and overglazed with a hand painted design, commonly monochromatic blue, inspired by imports of blue and white Ming porcelain.
These Dutch ceramics remain popular.

 CAVEAT:

Dishes used for preparing or eating food must be LEAD FREE.

For centuries, lead has been used in ceramic glazes to help glaze particles melt blemish free. Glazes must be formulated and fired at temperatures sufficiently high to destroy traces of lead.

Improperly fired, lead can leach into food. We’ve known for decades that severe  health problems have arisen from lead in paint, the same problem that arises from dishes that leach lead into our food.

Most domestic manufacturers have stopped using lead in their glazes. Dishes made before 1971, but not all,  are more likely to contain dangerous levels of lead although it is rarely found in plain white dishes.

The FDA recommends home test kits to determine if dishes are leaching lead. They’re available in most hardware stores–and detect only the presence of lead—not the amount.

Before buying ceramic dinnerware,
ask about lead content and require proof the dishes are safe.

 Pat Breen: EYEWITNESS TO QUALITY

My next post:   DECORATING CERAMIC DINNERWARE

FIRING CERAMIC DINNERWARE©

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FIRING CERAMIC DINNERWARE

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Clay shapes are fired in a kiln to become
stone-like, durable and functional
.

Clay, hardened by heat– is called ceramic.

Generally, the purer and finer the clay, the higher the firing temperature—which explains the strength and durability of some dinnerware that looks fragile.

Once fired, the composition of clay cannot be reversed.  Depending upon the clay and the decoration, ceramic dinnerware may go through several firings at different stages of production.

Because kilns require intense heat, dinnerware factories were usually located near forests until wood heat was replaced by coal, and much later gas, then electricity.

1st FIRING: CHANGING GREENWARE to BISQUE:   

Clay shapes— dried but not yet fired, are greenware.  Firebrick separates stacked  and loaded greenware on a moving framework which passes slowly through a kiln for a first firing to the bisque stage.  Bisque is the name of porous clay shapes that are fired —but not glazed.

In the bisque stage, foot rims and edges of all shapes can be ground and polished to eliminate roughness prior to being glazed—or they are tumbled with polishing pellets for extra smoothness. Each piece is examined before a transparent glaze is applied and fired  to the ‘glost’ stage.

 2nd FIRING: CHANGING BISQUE to GLOST:  

After greenware is dipped or sprayed with a transparent glaze, the shapes are dried and stacked on refractory firebrick to separate each piece as they move slowly through controlled zones of heating and final cooling temperatures, usually within a tunnel kiln. 

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 BISQUE shapes, now GLOST , await further glazes and decoration.

Tunnel kilns illustrate how capital and real estate intensive is the dinnerware industry. I visited several factories having tracks in the floor to guide loads of ceramic shapes in various stages of production—including their journey through a tunnel kiln.

Uniform thickness of clay is critical to assure successful firing; uneven clay bodies crack or explode when fired.  Firing temperatures must be lower than the deforming or slumping temperature for each variety of clay and they vary greatly for different clays and glazes. Different colors and applied ornamentation may require firing at different temperatures and wares may be fired several times before completion.  Glazes seal the porosity of clay and protect decals or painted designs from wearing away.

Additional firings depend upon the glazes, patterns and decorations required for each pattern.  Labor costs increase every time a ceramic body is handled for decoration or firing. 

Pat Breen: EYEWITNESS TO QUALITY

My next post is: GLAZING CERAMIC DINNERWARE

CERAMIC DINNERWARE PRODUCTION©

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 PREPARATION OF CLAYS:

Artisan producers purchase clay in lump, powder or plastic form. Wet clays are wedged,  meaning the lumps are banged together to eliminate air and to even the texture, or they’re kneaded, which consistently brings all of the clay to the surface to be compressed and assure all air is eliminated.

Mass-producers of dinnerware purchase clays in a dry form.  Each factory develops their own formulae but generally, all coarse materials are first ground very fine in a ball mill and blended with water in a blunger to control plasticity, grain size and moisture.

The result is SLIP, a clay and water suspension used for molding hollow shapes. Slip is passed over electromagnets to remove particles of iron and other impurities.

 ARTISAN SHAPING OF CLAY:

PINCHED:  PLASTIC CLAY is pinched into a shape.

COILED:      PLASTIC CLAY is rolled into coils and joined to create a shape.

SLAB:           PLASTIC CLAY is rolled into slabs of uniform thickness, cut into shapes that are ‘stuck’ together using a glue-like slip.

MOLDED:    LIQUID SLIP is poured into a plaster-of-paris mold. Water in the clay is slowly absorbed by the plaster.  When a clay wall of specified thickness develops, excess slip is poured out. The remaining clay wall shrinks from the mold as it dries.

THROWN:    A BALL OF CLAY is thrown and centered on a revolving potter’s wheelCentrifugal force and water assist the hands of the potter who simultaneously applies pressure on the outside and inside of the whirling clay to control shape and height…..and a pot is born.  Throwing is traced to the Middle and Far East as early as 3,000 B.C. 

Uniform thickness is critical for every shaping technique in order to assure successful firing. Uneven clay bodies crack or explode when fired.   Artisan dinnerware is relatively uncommon although mass-production factories employ artisan techniques for some of their production.
            

MASS-PRODUCTION:    FROM CLAY TO GREENWARE     

                                                MOLDED HOLLOW SHAPES:  

Plaster-of-Paris molds are integral for mass-production of dinnerware.

Every factory I visited had a MOLD SHOP where the non-mechanical method of forming hollow shapes using plaster-of-paris molds is called slip-casting.

Models of a finished shape are first made in plasticine, wax, or plaster by skilled artisans who must consider the shrinkage of clay that occurs in both the green stage and bisque firing.  A negative mold is made from the model and in a sequence of positive and negative castings, plaster working molds are made for mass-production.

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Hundreds of identical molds are required at the same time for production of hundreds of duplicate shapes and after every usage, molds must be thoroughly dry before being used again and again.

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Liquid slip (a prepared suspension of clay and water)  is poured into a mold. When a sufficiently thick wall  of clay clings to the mold, excess slip is poured out.  As clay dries, it shrinks, is removed and is now called GREENWARE.

The dried clay shape is inspected and perfected to meet rigid quality standards. e.g. excess clay can be trimmed from edges—which are smoothed against a wet sponge before firing.

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In shapes having a relief or embossed design, recesses of the plaster mold become clogged with clay residue.  After approximately 20 pourings of slip, molds are replaced in order to maintain uniformity of the relief. This cost is incorporated in the price of the pattern.

OTHER SHAPING TECHNIQUES: 

The production of plates requires a more solid form of clay.  Liquid slip is pumped into cloth bags for filter-pressing.  Water is pressed out and semi-dry cakes of clay are put through a pug machine to be well-mixed and extract air. This firmer and more plastic clay is extruded into ‘giant clay sausages’ called pug.  Sliced pug is used for jiggering and jolleying techniques.

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Jiggering is a mass-production technique for making plates.  Thick slices of pug clay are deposited on spinning upside-down plaster plate molds.  A template lowers to stretch the clay to a uniform size and thickness.  Making five to six hundred plates a day requires five to six hundred molds a day.  Molds must dry thoroughly before they’re used again. 

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Jolleying is a mass-production technique also for hollow shapes as bowls.  Slip is poured inside a revolving plaster-of-paris mold and forced against the interior walls of the mold by a metal template to assume the mold shape and determine the proper thickness.

Paste-Up Techniques:
Handles, spouts, lids, and other components are individually cast in molds and pasted’ by hand on the greenware body of each piece. Liquid slip performs as paste/glue. To the eye, these components must be attached without visible joining.

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QUALITY CANNOT BE ‘ADDED-ON’

 Pat Breen: EYEWITNESS TO QUALITY

 My next post is: FIRING CERAMIC DINNERWARE

KINDS OF CLAY©

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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

THE STORY OF DINNERWARE©

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Read how dinnerware originated and continues to be artisan and mass-produced today. Sequential posts explain specific generic standards of quality from raw materials to finished products —entrusted by experts in renowned international dinnerware factories. I share them so you can JUDGE FOR YOURSELF.  

Brand names change, styles change, prices change:
generic standards of quality for everyday home products do not change.

EVERYDAY HOME ESSENTIALS explains my concerns about quality.
ABOUT is about me. 

Early dinnerware was made of clay indigenous to a geographic area.

From the clays, glazes and decorations of Pre-Columbian, Egyptian, Greek, Roman, Middle and Far East earthenware pottery— circa 3,500 B.C., historians gained insight into their cultures.

We learn how early man not only cooked food in clay; he shaped, fired and decorated clay objects from which to serve and eat food–and we appreciate—primitive man wasn’t so primitive!

Porcelain surfaced in China.  The wonder is that it originated
at all—because porcelain is not found in nature.  By 1,000 B.C.,
the Chinese developed stoneware by adding fusible stone to clay.
Over centuries, stoneware clays were blended with kaolin and
petuntse, a fusible form of feldspathic rock.

By the 13th Century, a formula had been perfected to produce translucent white ceramic that Marco Polo described as alla porcella –because it resembled translucent sea shells shaped like a little pig—and was forever more known as porcelain.  

Intervening centuries and trade routes brought porcelain to the West. By the 17th Century, spying and poaching of talent proliferated as monarchs of European countries supported efforts to imitate Chinese porcelain.

Earthenware, low-fire porous clays ranging in color from light buff to dark sienna, were already made in Europe.  Experiments to make porcelain-like products consisted of glazing earthenware with lead glazes containing tin oxides, thereby producing a white glaze to hide the color of the earthenware.  Colorful designs were painted over the white glaze and diverse styles were identified by names we still use; Faience, Mailoica, Majolica and Delft.

Soft paste porcelain (pâte tendré), a mixture of white clay and ground frit (materials of glass making), but without kaolin  “became the vehicle of genius in the hands of the potters at the factory at Sevres, France” wrote Curator, David McFadden.

Kaolin, the ingredient that lends hardness and whiteness
to true hard paste porcelain (pate duré) was finally located
in Europe in the 18th Century. Experiments at the royal
fortress at Meissen led to the technology of combining
kaolin with silica and feldspar (petunse was not found in
Europe) and the making of porcelain wares soon spread
throughout Europe.

Josiah Wedgwood opened a model ceramic factory in 1767. He divided factory space to enable spatial flow of production— allowing labor to become highly specialized. His factory system became a model for the mass-production of dinnerware.

Bone china, introduced by Spode in 1800, is a form of soft-paste porcelain that resembles true hard-paste porcelain but contains no kaolin.  It is called bone china because 50% ground animal bone-ash is combined with clay to lower the firing temperature and increase translucency.

After the Industrial Revolution created a middle class, dining
rooms with china cabinets became the way to display culinary
skills as well as new possessions.  American ladies commonly
took china painting lessons using blank porcelain wares
imported from many factories in Limoges, France.

At factories, steam power facilitated the grinding and preparation of raw materials and ‘throwing wheels’ previously dependent on wind or water mills.  Kilns require intense heat (to make clay stone-like, durable and functional)—so dinnerware factories were usually located near forests.  Eventually, wood heat was replaced by coal—much later by gas —and eventually, electricity.

Technical advances in uniform production and decoration coupled with global demand have made dinnerware a successful international product since the 19th Century.

DINNERWARE TODAY:

At home or restaurant, we eat from diverse sizes and shapes of a hard, vitreous material that’s washed and used repeatedly—called DISHES.

We buy dishes in sets,
or by the piece,
or by the place-setting,
in retail Dinnerware Departments.

There are thousands of dinnerware patterns available today—each—only as good as the materials and methods used to make it.

Visiting several international dinnerware factories, I was amazed by the vast real estate required for the labor and capital intensive production of dinnerware. Because of the need for exact duplication of diverse shapes and sizes by the hundreds, dinnerware is mass-produced using diverse techniques that we’ll later examine.

It’s crtical to understand raw material differences
— because each kind of clay contributes different character and durability.

Many labor intensive production techniques are necessary for making diverse dinnerware shapes: each requires working with clay in different ‘wet’ stages—from ‘liquid slip to pug’.

A highlight at one factory was their final step:
inspection – rejection –destruction.
Cup or casserole—any piece not meeting standards of excellence was destroyed.

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When handed a stick to participate in the destruction,— I couldn’t!

Of course we want good dinnerware
—regardless of taste and income—
but how do we know if it’s good?

Many people equate high-price with high-quality but there is no price/quality ratio. Factors impacting the price of dinnerware are country of origin, availability and cost of raw materials; skilled vs. non-skilled labor, and design, sales, distribution and promotion costs.

DINNERWARE NEEDS LABELING TRANSPARENCY.

I am cynical about everyday essentials advertising—in any media!
In addition to brand-bias, dinnerware advertising perpetuates myths
.

Newspaper ads and catalogs push only color and pattern names:
most don’t even mention the name of the kind of clay.

Many retailers have advertised dishes made of ‘chip-resistant white clay’
—a specious phrase to create an impression of durability—
and meaningless because the specific kind of clay is not given.

ALL DISHES CHIP:
because each kind of clay has a different tolerance of durability.

Currently, because of rising labor and raw material costs, many prestigious dinnerware manufacturers have merged with– or been acquired not only by other dinnerware companies; but also by glass and flatware companies.

Many international dinnerware brand-manufacturers outsource production to save labor, manufacturing and marketing costs. Many designer and store brands of dinnerware are produced this way.

Once you can judge the quality of dinnerware for yourself.
Who knows?
Perhaps you’ll fall in love with your grandmother’s china.  

PAT BREEN: EYEWITNESS TO QUALITY

My next post is:   KINDS OF CLAY