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Have you ever wondered how bone china is made? How could a pile of kaolin, ashes develop the mugs we use every day? Have you ever wondered why the bone china products are more expensive than other ceramic products?
In this article, we are going to tell you how bone china products are made step by step. The main making process including the following steps.
the 8 main steps are Clay making, Mould making and Forming, Biscuit firing, Glazing, Glaze firing, Decorating, Decal firing and Packing.
When selecting bone china, many people focus on patterns and designs rather than the manufacture of luxurious porcelain. Learning details about the complex bone china manufacturing process can help you gain more appreciation for the fine dishes in your collection.
As bone china is a kind of porcelain, its manufacturing process is similar to porcelain. The main differences between bone china and other ceramics are in two aspects, bone ash content and two times firing in the making process. The bone ash gives the bone china products features like high whiteness, translucency and strength, which is clearly written in the article “All You Want to Know about Bone China”
From the bone china factory’s worker’s view, we write this article, hope it can help you have a better understanding of bone china making process.
Making bone china involves several steps, it requires complex machinery and skilled technicians and workers. Among the making process, the 8 main steps are Clay making, Mould making and Forming, Biscuit firing, Glazing, Glaze firing and Decorating, Decal firing, and packing. If it’s just for white bone china products, then there is no decoration part. The typical bone china manufacturing facilities are quite large, different machines are involved in different steps.
Now let’s explain the process step by step.
Bone china is the toughest of porcelains and contains bone ashes. In fact, bone ash consists the greatest part of the formula for bone china (about 40% to 45% of the mixture), it is the key ingredient that enhances whiteness and translucency, it is also the reason of its milky white appearance which is not seen in other porcelains.
The other raw material of bone china products are Kaolin, lime, feldspar, and clay. the higher percentage of the bone ash, the more difficulty the technology, also the brighter the products.
The first step is clay making, mix the raw materials mentioned above with water at a certain percentage and then blend at a high speed in the ball grinding mill machine(see below picture), the slurry was formed in this step. The grinding time depends on the size of the particle, usually around 24 hours, the whole making process involves the slurry.
(Ball grinding machine)
The next step is sieving. The slurry is sieved to eliminate the iron and other impurities.
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. After 3-4 times of blending, final mix left in a liquid state is referred to as slip. The slip will be sliced to a different size for usage in next steps. A giant slicer divides each pug into several clay discs.
(End product of first process—clay)
In this step, the clay is diluted and cast into the molds, here we skip the process of making the mould, which requires high skills, as the molds are used to form all the shapes, either mug, bowl, plates, teapots, etc, now we assume the mould shape is ready.
Now we need to use the mold shape to batch produce the products, which is called forming. Forming methods vary depending upon the shapes.
There are mainly two kinds of forming method. Jiggering and casting. The Casting method can further be divided to high-pressure casting and hand casting.
Regular round pieces like plates, bowls, mugs are generally formed on jiggers, the slip is used to fill the molds, excess slip is drained from the mold, then refining slip shape with the machines.
Each disc of clay is 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 green ware plate.
(Working jiggering machine)
Casting can further be divided into high-pressure casting and hand casting.
a Hand casting
Irregular pieces as pots and gravy boats are cast in plaster molds. Oval, square, and rectangular pieces are, moreover, cast in two-parted plaster molds to stabilize the shapes, which is called hand casting.
Casting. Liquid clay called slip is poured into each mold. As the plaster mold absorbs water, the wall of the cup is formed. When the wall thickness is correct the remaining slip is poured out and recycled back into the slip tank. The time the slip must remain in the mold varies according to the slip, how wet the mold is, and even the weather. After several hours the cup is dry enough to be removed from the mold without warping it.
b High-pressure casting
Other shapes like the oval plate and Rectangular plate are formed in the way of high-pressure casting.
In the high-pressure casting process, the following steps are involved.
1.Use the air hose to clean out the mold, making sure there’s no excess plaster or remaining slip from a previous fill.
2.Am the nozzle of the hose and fill the mold with the desired slip (white or ivory).
3.Wait for 10-15 minutes to allow the slip to forming and shaping.
4.Pour out the excess slip and let the mold sit for about another 10 minutes.
5.Release the mold.
The step followed is fettling.
For the irregular shapes, like teapot and mugs, handlers have to cling to the main body first after it’s dry, then sponge will be used to polish the surface.
Small finishing knives, water, and sponges are used by the workers to 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.
The formed bodies, now called “green wares” are left to dry and finished with water sponging to gently remove scratches and scars from the surfaces to casting and polishing, after sponge, the uneven and rough surface of the bone china greenware become smooth.
(Workers cleaning, fettling, and cling the bone china greenware)
After the green ware is dry, it is ready to be fired for the first time.
Differ from the conventional porcelain making, the Bone China’s first firing, called “Biscuit firing”, is carried out at a temperature of around 1,200 to 1,300 centigrade in the oxidizing atmosphere in the biscuit kiln, the products are fired over 15 hours.
(Biscuit firing kiln)
As in the firing process, the bone china products will shrink and out of shape, plates and bowls should put in special equipment to endure long time firing. About 20% shrinkage of the green wares occurring at this stage is apt to cause deformation and cracking. In order to overcome this, most pieces are placed on refractory setters, called “cranks” to let them follow the profiles of the setters.
The setters ensure that the plates maintain their form in the kiln.
(Bone china products in the setters ready for firing)
In this process, the green wares vitrified to make them durable, white, and translucent. However, in order to achieve the best results within such specific temperature range, the firing schedule must be carefully controlled.
Polishing/ sponging and selecting
Biscuit ware is formed after the first firing, after the biscuit ware is taken out 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 smooths away the rough exterior (pieces fresh from the kiln feel a little like very fine-grained sand paper).
After the stone bath, the Biscuit ware 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.
So, what makes bone china so strong? We’ve already mentioned one thing: bone ash. The bone ash content 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.
Bone China’s glaze is a mixture of bone ash and kaolin. Because the biscuit (the greenware after first-fired) is nonabsorbent, the glaze needs to be applied by spraying. The glaze is applied by hand dipping or machine, to protect and decorate the biscuit.
The irregular pieces are glazed with hand. The work requires patience and a very steady hand.
After glazing, the foot (bottom) of each piece of china must be wiped clean with a damp sponge so it won’t stick to the kiln.
Glaze firing, also called glost firing, is the next step in the bone china making process, which is also known as second firing.
Once the glaze is dry, plates or bowls are fired for a second time over 7 hours at 1080°C in the glazing firing kiln.
The glaze-coated biscuits are set in the saggers in order to avoid direct exposure to flames. The saggers are piled up on kiln cars and sent into a tunnel kiln. When the temperature goes up to around 1,000 to 1,200 centigrade in the oxidizing atmosphere, the glaze melts and fuses onto the biscuits.
All biscuit wares coming out of the kiln are strictly inspected, after cooling, the plates, bowls, mugs etc are inspected and any imperfections are ground off, only a few can be passed onto the decorating stage.
The qualified products in this process are called white wares, which are delivered to the whiteware house for further use, direct use or apply decoration.
Imperfect pieces may be destroyed or sold as “seconds.” The pieces are then packaged and sent to retail stores.
As bone china comes in a variety of designs, so the homeowner has an array of choices. Almost every decor imaginable is available to apply to bone china, this is also the charm of on-glaze decoration. Some designs may include flowers, fruit, geometric designs, lacy swirls, animals, or even holiday motifs.
(Bone china products with decoration)
Bone China’s standard decorating methods are spraying, decal application, gold finishing, and hand painting. Among them, decal application is the most widely used.
Decals can be applied by hand or machine. When a decal is to be applied by hand, the bone china products, like plates, need to be cleaned, as any dust would impact the effect of the decal.
Experienced workers will first apply the back stamp, then they flip over the plate and apply the decals. 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.
The process of applying precious metal (gold or platinum) to whiteware is referred to as gilding. Like the decal application, this is done by hand.
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.
(bone china products with gold rim and decal applying)
After the decoration, pieces are fired at 700 to 800 centigrade again to make the pigments fuse into the glaze-coatings. Among the mentioned decorating methods, hand-painting is applied only by proficient artisans. Also, gold or platinum finishing is hand-blushed onto the pieces by well-skilled artisans when necessary.
No matter what the application, the decoration needs to fire for the last time. If it is with decal and gold rim, then need to fire for two more times, different temperature were required for this process.
The pigments (and sometimes metal) of the decals are sealed into the white wares in a kiln running at 1,600 degrees F (871 C) for about two and a half hours. While the metal is permanently formed to the china in a decal firing kiln running at 1,400 degrees F (760 C) for about one and a half hours.
(Bone china cups ready for decal firing)
Now that the china is completely decorated, it’s ready for final inspection and packaging.
After carefully selecting the qualified products for the end products, then workers careful pack to ensure safety in transporting.
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 bone china products will then go to a boxing area and be passed on to shipping.
(Final packing for bone china products)
The above words simply explained how is bone china made. Hope you get what you are looking for. Anything more about bone china, please feel free to leave a message, let’s discuss!
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