How Bone China Works


Display of various china patterns in the Lenox lobby. See more Lenox pictures.

When selecting tableware for your house, you have a number of choices: earthenware, stoneware, and porcelain. Among porcelain products, you've got basic porcelain, fine china and bone china. Many well appointed homes stock at least one, if not a combination of two or more of these options. In fact, one of the oldest standing customs for a bride and groom is registering for a china pattern.

No matter how elaborate or lovely the place setting, when it comes down to it, you're usually more concerned about what's being served on the china, rather than the china itself. But if you ever stopped to consider how china is made, you'd be amazed -- it's actually fascinating.

In this article, we'll go behind the scenes at the Lenox factory in Kinston, NC, to see how bone china is made.

China Basics

People have been making and using porcelain products for a very long time. Around the end of the 18th century, an Englishman named Josiah Spode developed a new formula for china that incorporated the use of calcined bone ash. The addition of bone ash in china mixtures continues today at Spode as well as many other china manufacturers, including Lenox. Lenox is the only manufacturer of bone china in the United States. Considered by most to be the finest of porcelain products, bone china is stronger and more translucent than the basic porcelain and "fine" varieties.

­Creating bone china entails an involved series of steps carried out by a number of highly skilled individuals and some really impressive machinery. When you first enter the Lenox factory, you're immediately struck by the size of the facility (about 150,000 square feet, 14,000 square meters) -- it takes a lot of space to accommodate all the equipment and the 350-member staff. Although this factory only produces bone china, what we'll find here is generally useful information because other porcelain products are made in pretty much the same way.

There are four main processes involved in creating china:

  • Clay making
  • Mold making
  • Glazing
  • Decorating

Another thing that stands out as you tour the Lenox facility is that all of these processes rely on the four elements. As you'll see in the next several sections, earth (the raw materials), wind (there are air hoses everywhere), fire (the kilns), and water (used during every process) are all required to make china.

Now let's take a look at how the clay is made.­

­

Bone China: Clay Making

Supplies
Supplies

The clay making process begins in the batch-house. Pulleys hoist giant metric-ton bags of raw materials from pallets located on the factory floor. They are raised up to a platform about a story high. The bags are then left to hover over enormous hoppers into which their contents will be emptied.

Two bags of china clay hover over giant hoppers.

From the hopper, the dry raw materials move to a high-speed blundger, where the materials are mixed with water to form a slurry. These slurries, stored in colossal tanks, will later be blended together to form the final mix and slip.

There are five main dry ingredients that go into the final mix:

  • Bone ash
  • China clay
  • Ball clay
  • Flint
  • Feldspar

These raw materials are both domestic and imported products. For example, the feldspar used at this plant comes from North Carolina, while the bone ash comes from Holland and the United Kingdom.

Lenox creates two colors of bone china. The sale of white china products comprises about 80 percent of its total sales; ivory-colored china makes up the remaining 20 percent.

Although certain additives or pigments are necessary to get the ivory color in the finished product, in the slurry state, both clays appear to be slightly gray. To differentiate the final mix for the ivory from that of the white, green vegetable dye is added to the ivory mix. The vegetable dye will eventually burn out in the kiln. If the dye isn't added, there's no way to distinguish one clay from the other. It also helps to avoid getting the two clays mixed together.

During the clay stage, which is any time before glazing, all scrap clay that hasn't been contaminated by debris (perhaps it was dropped on the floor and picked up some lint) can be reclaimed. In fact, every finished product here is made from 80 percent virgin clay and 20 percent reclaimed (scrap) clay.

Slurry for ivory china (left) and a holding tank for ivory slurry (right)

After the final mix has been prepared, it is pumped to a filter press to remove air and water from the mix -- the moisture level is reduced to about 20 percent. Final mix left in a liquid state is referred to as slip. We'll talk more about slip later in this article.

The sheets of clay that are formed in the filter press are then passed through an extruder to remove more air and change the flattened squares to a tube shape. These tubes are referred to as pugs. Resembling a really huge piece of chalk, each pug weighs in at a little more than 30 pounds (13.6 kg).

Sheet of clay (left) and extruder (right)
Pug machine

A robotic arm called a pug stacker collects the pugs and stacks them in a pyramid on pallets covered with plastic sheeting. The sheeting looks a little like the plastic wrap you would use in your kitchen, except that it's super-thick. Rolls of plastic sheeting hang on a wall nearby. Pieces are torn from the roll to cover the pyramid of pugs. The bottom of the plastic is tucked along the underside of the bottom row of pugs, forming a protective tent to maintain the humidity level.

Some pug stacks are marked "for cups only." The moisture content of these pugs is at about 15 percent rather than 20 percent.

Molds are used to transform the pugs and slip into plates, cups, bowls, pitchers and other china pieces. Let's take a look at how these molds are created.

Bone China: Creating the Mold

This metal mold is prepped and ready to be filled. Notice the finished production plate mold on the right.
This metal mold is prepped and ready to be filled. Notice the finished production plate mold on the right.

The mold-making process is an integral part of china production.

Large metal master molds and plaster are used to make production molds. For dinner plates, the metal master molds look something like two automobile hubcaps sandwiched together.

To create a production mold, large bags of plaster are mixed with water and then funneled into a big mobile bucket that hangs overhead. Meanwhile, the metal master molds are prepped by spraying a soapy mixture inside. This residue will help with the release of the production mold later. The metal molds are lined up, one after the other, on three rows of tables. Each mold sits on top of something that looks like a lazy Susan (a disc-shaped piece of wood that spins).

The aerial bucket is used to fill the metal molds with thick, creamy plaster. This takes two people. One person pours while the other person spins the metal mold to make sure the plaster is distributed evenly.

Pouring the plaster mold
Ready to be released

It takes about 20 minutes for the plaster to set. A rubber mallet is used to loosen the plaster production mold by tapping along the outside of the metal. The mold is released when the two halves are pulled apart.

Air hoses are used to spray plaster bits out of the metal molds and to clean off the plaster molds, removing any excess plaster dust or particles. At a nearby table, someone inspects each mold and stamps it with the day's date. This helps track the number of times each mold is used.

Something like 300 molds are made in this factory every day. These molds are used in one of two ways: either with the clay pugs or with the slip. Let's check out pug molding first.

Forming a China Plate

The forming operation, or pug molding, starts at the jiggering unit. Like most of the machinery here, there are two jiggering units: one for white china (gray pugs) and one for ivory china (green pugs).

The pugs are pushed through another extruder to remove any remaining/excess air. A giant slicer divides each pug into several clay discs.

Each disc of clay is then placed on top of a plate mold. The plate mold begins to rotate. A jigger head hovering above starts to rotate, too, and presses down upon the rotating pug slice and mold plate. A scraping tool cuts off excess clay from the rim of the newly formed greenware plate.

Pug slices are placed on production molds.
A plate is formed: Note the excess clay traveling down the conveyor belt to the yellow recycling bucket.

The excess clay shoots down a conveyor belt to a recycle bin. This reclaimed clay will be blended with water to create a slurry that will eventually go into a batch of final mix.

The greenware plates are passed through a mold dryer, then removed from the mold and passed through a second dryer -- at this point, the clay has gone from a 20-percent moisture level to a 0.5-percent moisture level. The plates move through a finishing machine so that damp sponges can finish, or smooth out, the edges of each plate.

Workers inspect each plate as it leaves the finishing machine. The plates are placed on setters. Setters are actually very similar to the plaster molds, except they're made of material that can withstand the extreme temperatures of the kiln. The setters, now carrying the newly made plates, are stacked on metal racks to be passed through the kiln. The setters play an important role. At this point, the plates are still fairly malleable (they can bend). The setters ensure that the plates maintain their form in the kiln.

A Lenox employee places a plate on a setter.

There are separate machines for molding cups, mugs and small bowls. The cups move through a jiggering unit similar in concept to the plate unit. As mentioned earlier, the pugs used for cups are at a 15-percent moisture level. Once the basic cup form is made in the jiggering unit, it is put through the profiling unit. A carbide blade forms the side and foot profile of each cup.

Handles are affixed, by hand, to each freshly profiled cup. These cups are then passed to a finishing area. Using small finishing knives, water and sponges, workers smooth out the lip and foot of each cup and make sure the handles are securely attached. The finished cups are placed on small setters, called chums, and stacked on racks destined for the kiln.

Now, let's take a look at the casting process: forming china from molds and slip.

Bone China: Casting

These four-piece molds are used to make small pitchers.
These four-piece molds are used to make small pitchers.

In the casting area, 18 small lazy Susans sit on top of a conference-sized round table. Upon each of the 18 discs rests a casting mold. Three hoses hang from the ceiling: one for air, one for white clay and another for ivory clay. These hoses are used to fill each mold with liquid clay or slip.

Slip and air hoses

The casting process basically works like this:

  • You use the air hose to clean out the mold, making sure there's no excess plaster or remaining slip from a previous fill.
  • You aim the nozzle of the hose and fill the mold with the desired slip (white or ivory).
  • You allow the slip to set for about 10 to 15 minutes.
  • You pour out the excess slip and let the mold sit for about another 10 minutes.
  • You release the mold.

To release the mold, the giant red bands are removed and the four mold pieces are gently pulled away from the clay piece inside.

Because the mold is in several pieces, there are seams along the greenware where the molds fit together. Workers use damp sponges and finishing knives to smooth out the surface and remove the seam marks.

Raw clay pieces that have not been fired in the kiln are referred to as greenware -- not to be confused with the green-colored clay pieces that indicate an ivory finished product. Fired pieces are called whiteware. Once the greenware is complete, it's ready to be fired in the kiln.

Firing Bone China

Racks of setters laden with greenware dinner plates enter the kiln.
Racks of setters laden with greenware dinner plates enter the kiln.

Once the greenware pieces (either from the pug molding or slip molding process) are finished, they're stacked on metal racks and sent to transportation lanes (holding areas). Eventually, they'll be moved to and passed along a conveyor belt through the bisque kiln. This gas-fired kiln, which is 135 feet (41 meters) long, runs in nine-hour cycles, meaning each piece of greenware is fired for exactly 9 hours. The temperature inside this kiln is 2,290 degrees Fahrenheit (1,254 C).

The temperature inside this kiln is a whopping 2,290 degrees F!

After the whiteware is removed from the kiln, each piece gets a bath. Smooth stones and water are used to polish each piece of china -- the vibration of the stones smoothes away the rough exterior (pieces fresh from the kiln feel a little like very fine-grained sand paper). This process is called the vibratory finish.

Stone bath
Shiny!

After the stone bath, the whiteware is run through a giant industrial dishwasher and dryer. At the other end, each piece of china is closely inspected for any damage or flaws. Pieces that pass muster move on to the glazing process.

China isn't something you think about every day. In fact, if you're like many folks, you probably only use the good stuff for special occasions. People often do this because it seems so delicate. As it turns out, china is actually a lot stronger than you would think -- a whole lot stronger. Some clever marketing people at a large department store in Canada decided to show folks just how strong the good stuff can be by supporting the entire weight of a Lola race car on top of four Wedgewood teacups!

So, what makes china so strong? We've already mentioned one thing: The bone ash in bone china makes it stronger than other porcelain products. Another thing is the glaze. Think of it as liquid glass that, once heated, forms an incredibly strong protective shell. The glazing process at Lenox is very interesting to watch. Let's take a closer look.

Glazing Bone China

Notice the glowing orange area to the right -- this is where the dishes are pre-heated prior to glaze application.
Notice the glowing orange area to the right -- this is where the dishes are pre-heated prior to glaze application.

Each piece of china must be pre-heated so the glaze will adhere to its surface. The glaze is pumped from giant holding tanks to the glazing area.

Pre-heat

Bearing a slight resemblance to an automated car wash, the glazing booth contains something like a dozen spray guns. For the dinner plates that are being glazed, eight of these spray guns are in use so that every part of the plates will be covered. The plates, resting atop metal stands, are pushed through the continuous stream of glaze. A wall of flowing water (it looks like a series of waterfalls) faces the row of spray guns.

Glazing unit

This series of waterfalls serves an interesting and thrifty purpose. The water catches the mist of glaze that doesn't fuse to the clay. The great thing about this is that it turns out the glaze is heavier than the water. So, after the water from the glazing booth is left to sit for a while, the glaze can be reclaimed and recycled for future use.

Prior to being placed in the gloss kiln, which is 185 feet (56 m) long, the foot (bottom) of each piece of china must be wiped clean with a damp sponge so it won't stick to the kiln.

This kiln runs for eight and a half hours at 2,020 degrees F (1,104 C). Unlike the first kiln, which is all gas, this one uses both gas and electricity.

For most kilns, there's a warm-up stage, a hot-zone or soak stage, and a cool-down stage. In the gloss kiln, the stages go basically like this:

  • Three or so hours for ramp-up
  • Two and a half hours of soak
  • Three hours of cool-down

During soak, there's a lot of turbulence in a gas kiln. Because an even finish is needed for the glaze, the soak stage of the gloss kiln is powered by electricity.

Stacks of setters and plates enter the glazing kiln.
Plates, soup bowls and tourines exit the glazing kiln.
Notice the shine on these freshly glazed pieces.

After the gloss kiln, the whiteware is inspected again. Any sharp or uneven edges on the foot of the whiteware are ground on a diamond wheel.

Grinding the foot of a plate

The whiteware is now ready for decals and decoration -- until then, it's kept in a holding/storage area.

Decorating with Style

The Lenox back-stamp is applied to the underside of a plate.
The Lenox back-stamp is applied to the underside of a plate.

With about 85 different patterns and something like 400 shapes to cover, decorating the china can be a time consuming and tedious process. Depending on the shape of the piece and the detail involved, some processes require a personal touch while others are done by machine.

There are three things that can be applied to adorn a piece of china:

  • Decals
  • Precious metals, either gold, platinum or a combination of the two
  • Enamel

Decals can be applied by hand or machine. When a decal is to be applied by hand, to a plate, for example, a thin, blue line is drawn around the edge. This line will be used as a guideline for the decal that comes next. The blue ink eventually disappears; it burns away in the kiln. The decals are soaked in water and then placed, by hand, on the plate. Using a slightly damp sponge, the decal is smoothed onto the plate. Once the decal is in place, the plate is flipped over so the Lenox back-stamp (actually a decal made of gold) can be applied to the bottom of the plate. The pigments (and sometimes metal) of the decals are sealed into the whiteware in a kiln running at 1,600 degrees F (871 C) for about two and a half hours.

This stack of decals and back-stamps will soon be applied to a nearby stack of dishes.
The metal rims shown here can be applied by machine.

The process of applying precious metal (gold or platinum) to whiteware is referred to as gilding. Like the decal application, this can be done by hand or by machine.

For pieces like creamers and cups, a human touch is best. The metal, which is in a liquid state, is painted on each piece with a delicate brush. For other pieces, such as dinner plates or platters that are to be edged in a wide rim of metal, a machine can handle the job.

No matter what the application, the metal is permanently formed to the china in a kiln running at 1,400 degrees F (760 C) for about one and a half hours.

When enamel is applied, the design will ultimately look embossed on the plate. Enamel is applied either by hand or by a machine that stamps an entire design on the china.

Painting on the metal

At first glance, the by-hand application looks pretty simple, sort of like writing with decorator icing.

Applying enamel by hand

It turns out, though, that it's a lot more difficult than it looks. The work requires patience and a very steady hand. For the plate shown below, close to 400 dots of enamel must be applied to complete the decoration.

With a decal and gilding in place, 390 enamel dots must now be added to this plate to complete the pattern.

The machine used to apply enamel decals is one of the coolest things we got to see at the Lenox plant. Unfortunately, its many moving parts are encased in a glass box, so photographing them was difficult. But here's how it works:

  • The decal, covered in a waxy residue, is heated.
  • A giant silicone rubber bomb (looks like a huge, red, rubber kickball, except it's cone-shaped on one side) is squashed onto the decal.
  • As the bomb is lifted, so is the decal.
  • The bomb is squashed against a plate, transferring the decal to the plate.
This pink enamel decal was applied in the heat-release decal machine.

After the enamel has been applied, the china must go through a 1,400-degree kiln for about one and a half hours.

Once the decorating is complete, the china is ready to be packaged.

Now that the china is completely decorated, it's ready for final inspection and packaging.

The finished pieces of china are moved out of the decorating area and are inspected one last time. China that passes muster will be bar-coded, wrapped in foam and bagged.

The bagged china will then go to a boxing area and be passed on to shipping.

Plates being inspected and bar-coded

For a piece that isn't perfect, one of three things can happen:

  • It will be destroyed.
  • It will be marked as a "second" and sold in a special outlet store.
  • If the imperfection can be fixed (maybe an area is missing a bit of gilding), it will be fixed and sent through the final inspection process again.

For even more information on bone china, fine china and other porcelain, check out the links on the next page.

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