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What is soft porcelain, also known as bone china?
Soft porcelain, also known as soft-paste porcelain or bone china,
consists of 50-60% bone ash or phosphates in the paste and a lower
kaolin content than that used in hard porcelain. Because of the
relatively low kaolin content, the firing temperature for soft porcelain
is lower than for hard porcelain: soft porcelain is first fired
at 1240-1280o C. Due to this lower firing temperature, soft porcelain
does not have as high a tensile strength or resistance as hard porcelain.
What is bisque porcelain, also known as biscuit porcelain?
Bisque, or biscuit, porcelain is the name given to porcelain
that is fired without glazing. The finished, fired piece is impermeable
to water but has a rough surface finish.
How strong is porcelain?
Porcelain is pressure-resistant to 5000 kg per cm². Expressed
in another way, a fully loaded 10,000 kg railroad car can be placed
on a 2 cm² piece of porcelain without its breaking.
What does translucence mean?
Translucence is the distinguishing characteristic of genuine
porcelain. That is, diffused light will shine through a piece of
fine porcelain. The brighter and clearer the translucence of a piece,
the better the quality of the porcelain.
Does porcelain age?
Despite diligent studies and rigorous tests, it has not yet
been determined whether or not porcelain ages. It does retain its
properties of hardness, density, tensile strength, brightness, and
translucence - all of which are unaffected by time. Porcelain is
likewise resistant to corrosion.
What does porcelain consist of?
Porcelain (hard paste porcelain) consists of 50 parts kaolin
(porcelain earth), 25 parts quartz, and 25 parts feldspar. These
components are bonded into a paste by grinding, mixing, and fusing
with each other.
How are the different pieces of porcelain produced?
There are several basic techniques for producing porcelain pieces:
molding, shaping, casting, and turning. These techniques may be
used alone or in combination, depending on the size, shape, and
complexity of the piece being created. Artisans use plaster molds
to form the top of plates and other flat pieces; they use a template
or rolling tool to fashion the bottom of these pieces.
They also use a template or rolling tool to shape the inside of
cups and other hollow pieces, then rely on a plaster mould to form
the outside of those pieces. Craftsmen use a hollow casting technique
to fashion pots, bowls, and jugs. They use surface casting or solid
casting to form oval or square platters and salad bowls. The technique
of turning is reserved for pieces that are round. Casting is typically
used to create porcelain figurines.
However, if a piece is highly complex, artisans may cast it in smaller
component parts and later join the parts into a finished whole.
For instance, Kaiser's white-headed eagle is composed of 25 separate
parts. Plaster molds may be either cast or turned. For instance,
patterns for relief or raised designs, such as Kaiser's Dubarry
tableware or bisque vases, are inserted into molds.
The relief design is automatically embossed onto the plaster by
turning, overturning, or casting. The plaster draws out the water
from the moist compound in the mould, causing the porcelain body
to shrink so that the body may be easily removed from the mould.
When and why is porcelain annealed?
Before the annealing process, Kaiser artisans clean the porcelain
pieces by removing any sharp protruding edges, such as casting seams.
The artisans anneal the pieces by heating them to 900-1000o C and
then gradually cooling them. This process frees the porcelain pieces
from internal stress and removes their water content. The pieces
remain porous but become strong and leather-hard; they cannot be
reshaped. Next, workers use compressed air to remove dust from the
porcelain pieces. At this point, craftsmen stamp the pieces with
Kaiser's trademark and send the pieces on to the glazing shop for
the next step in the manufacturing process.
How is glaze applied to porcelain?
The thin, liquid glaze is carefully mixed in a large trough.
This milk-like glaze is continuously agitated to prevent separation
and settlement of its ingredients. Craftsmen dip and wash each porcelain
piece in the glaze, which remains on the surface of the piece like
a coating of flour. Artisans wipe the pieces with moist foam rubber
strips to remove glaze from those contact points which will rest
on a porcelain "setter". This setter supports the glazed
piece during the firing process and is then discarded.
How is porcelain fired?
Traditionally Porcelain is smooth-fired in tunnel kilns which
are approximately 80 m long. The porcelain pieces ride on special
cars through the firing zone and move slowly on the cooling zone.
Nowadays Chamber Kilus are predominantly used the same effect in
less than one third of the time.
Why does a piece of porcelain have rough patches?
Rough patches on porcelain pieces are inevitable because of
the nature of the firing process. Under the high heat of firing,
the porcelain glaze becomes highly viscous and sticks to the surface
of the disposable porcelain setter on which the piece is resting.
A fired piece cannot be lifted from the setter without leaving rough
patches at the points of contact between the piece being fired and
the setter on which it rests.
To minimize this problem, before firing, craftsmen remove as much
wet glaze as possible from these contact points. At high quality
porcelain factories, cups, for example, are fired with the opening
facing downward on the setter. This method produces a cup with a
smoothly glazed base but a slightly rough upper rim where the rim
rested on the setter. This slight roughness on the tiny surface
area of the rim is a comparatively minor flaw which would not likely
be noticed by the casual observer.
However, craftsmen take the extra step of painstakingly polishing
the cup's upper rim to remove any trace of roughness. This is an
optional step, but worthwhile because it results in a piece that
is perfectly smooth on all surfaces.
What is in-glaze?
In-glazing is a technique for permanently protecting decorated
porcelain pieces from normal mechanical and chemical stresses. Artisans
first decorate porcelain pieces by transferring or sliding screen-printed
designs onto smooth-fired porcelain bodies. Next they add appropriate
metal oxides and liquids to the glaze to achieve the desired colour
effect. After they apply the glaze to the porcelain bodies, craftsmen
fast-fire the pieces at 1200-1300o C for 60-120 minutes. During
this firing, the decorative design is sealed under the glaze, or
embedded within it; thus the term "in-glaze".
What is on-glaze?
On-glazing is a technique in which a smooth-fired porcelain
body is coated with a colour enhanced glaze, the decoration is then
applied onto the glaze, and lastly the piece is fired. The firing
temperature for on-glazed pieces is determined by the melting point
of the glaze's colours, which is 750-900o C, for quality control
of the colours. In this process, the decoration is fused onto the
glaze; thus the term "on-glaze".
How long does porcelain decoration last?
In-glazing permanently protects porcelain decoration from all
kinds of external mechanical and chemical stresses because the decoration
is sealed under or embedded within a hard, durable glaze. On-glazing
fuses the decoration onto the glazed piece. Because an on-glazed
decoration is more exposed to external stresses, it is more susceptible
to damage.
What does "staffage" mean?
"Staffage", from the German word "staffieren",
meaning to dress, trim, adorn, is the art of embellishing porcelain
by the skillful addition of hand-painted colours and metallic trim
(gold, silver, or platinum). "Staffage" can be used to
accent knobs, handles, and rims of tableware and to enhance either
smooth or relief-decorated period table services. The careful and
tasteful application of "staffage" highlights a piece's
shape and a variety of other design elements.
What makes porcelain the ideal tableware?
The ideal tableware is hard and smooth, has an impermeable surface,
with a finish that is highly resistant to mechanical and chemical
stresses. Porcelain meets all these important requirements. Because
porcelain is hard, it resists cracking, cutting, and scratching. It
is impervious to the acids and alkalis found in the normal household.
It has been proved that porcelain is even bactericidal. Porcelain
has no odour or taste of its own and absorbs neither from food or
drink. Fine porcelain is therefore extremely practical as well as
beautiful.
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