MAKING LARGE OBJECTS OF BONE CHINA BY DEVELOPING MANUFACTURING AND FIRING
Background of the Research
In manufacturing bone china casting clay
the basic raw material is calcinated cattle bones. A typical recipe is:
50% bone ash, 25% china clay and 25% feldspar. Bone china is more white
and more translucent than the standard china. Reducing oxygen during the
firing is not needed to obtain white colour whereas the normal china needs
it. In a reducing firing the flow of oxygen to the kiln is prevented at
a spesific stage of firing.
The Aim of the Research
The maximum temperature in bone china firing is
about 1220 °C, whereas in china firing it is higher, about 1300 °C.
The lower temperature enables to save energy. In industry the kilns are
the more expensive, the higher temperatures the kiln material withstands.
The equipment which endures firing at 1300 °C is ten times as expensive
as the equipment which withstands 1200 °C firing. The firing to 1300
°C essentially weaken the durability of the kiln equipment. In industry
remarkable savings could be achieved by using bone china instead of china.
Bone china has properties such as translucency,
lightness, impact resistance which do not characterize the materials used
in Finnish ceramics (red clay, earthenware).
The knowledge of and skills on bone china is lacking
in Finland. There is bone china production in Great Britain and in Sweden.
Not taking into account the few articles no written material concerning
bone china has been found. The industry has the knowledge on bone china
and is not willing to share it with others. Consequently, it is difficult
to get information on this subject. The preparation of bone china casting
clay and the properties of unfired bone china objects differ remarkably
form the standard china and stoneware. The main points which presented
difficulties in the bone china research for the master's thesis are summariced
- The adjustment of viscosity of the bone china casting clay was difficult,
the viscosity of totally deflocculated paste was very low, even under 100
cPs. The normal figure is about 500 cPs by using the Brookfield viscosimeter.
- The pastes had a high pH value (about 10.5, the normal pH of casting
clay is about 6.5). The alkaline paste corroded the plaster moulds quickly
to unserviceable. The casts got stuck in the plaster moulds and the shape
of the objects warped when detaching from the moulds. Due to low green
strength of casted test objects making large objects was not possible.
- Special kiln equipment was needed for firing bone china objects.
It was necessary to design a special firing stand for all the objects.
The bone china firing is different from the normal ceramic firing. Bone
china needs a long soaking time (up to 2 hours) at the maximum temperature,
whereas the normal china does not require any soaking at all. The soaking
exposes the bone china to distortion of the shape.
The research report is supplemented to the master's thesis of Anne
Kauppinen, that was completed in May 1994.
The aim of the research is to develop
the bone china casting clay so that its properties do not limit the shape
or size of the object:
1) To develop the manufacturing techniques (casting properties, moulds,
mould materials) of the bone china casting clay. To test the plaster selection
together with the additives of plaster to discover the ideal hardness of
plaster moulds. The moulds have to be hard because the alkaline paste corrodes
them. From another viewpoint in too a hard plaster mould the silt forms
the wall too slowly.
2) To determine the particle sizes of the basic raw materials and the
relation between the particles in the bone china casting clay. To examine
the effects of small changes in particle sizes to casting properties, to
viscosity, to plasticity, to green strength, to handling of raw goods and
to holding the form of the unfired objects intact.
3) To use the Finnish cattle bones in making the bone ash and also
to test a chemical calciumphosphate together with organic additives instead
of bones. In bone china one of the basic raw materials (about 50%) is calcinated
cattle bones. In calcinating 1-2 % organic matter remains in the bones,
which has an effect on plasticity, green strength and casting time.
4) To develop the firing properties. To design the kiln equipment;
the bone china objects have to be fired in casettes to make sure that the
thin object will get an even temperature during the firing. If the temperature
in the kiln is uneven also object to be fired will become warm unevenly,
which causes distortion to the shape. The bone china objects are specially
sensitive to distortion due to the soaking at the maximum temperature.
For that reason it is necessary to design a specific firing stand for every
5) To determine the maximum firing temperature, the firing diagram
and soaking time.
6) To measure the colour, the translucency and the colour of the light
of the fired bone china object. When measuring the colour of the light
the bone china object is illuminated with different kinds of lamps and
the colour of the light that is filtered through the object is measured.
The most serviceable paste is choosen by its manufacturing and aesthetic
properties (colour, translucency, the colour of the light).
7) To design objects / a series of objects on the basis of research.
To produce the prototypes.
The research will be carried out in three years,
1.1.1996 - 31.12.1998. A part of the work (developing the manufacturing
and the firing properties, determining the firing diagram, measuring of
the colour) will be carried out at the University of Art and Design, Helsinki
(Finland) at the Department of Ceramic and Glass Design and at Hackman
Designor Oy (Helsinki, Finland). Determining the particles and the measuring
of the mechanical properties will be made at theTechnical University of
Tampere (Finland) at the Department of Geotechnics. The product design
and the production of the objects will be done in cooperation with the
enterprises linked to the research: PiiPot Ky (Kuopio, Finland), Suomen
Valoisa Oy (Turku, Finland) and Thorn Lightning Oy (Kerava, Finland).
The results of the research will be published in
international publications and presented at conferences and also as the
doctoral thesis of Anne Kauppinen.
University of Art and Design Helsinki UIAH
Department of Ceramics and Glass
Hämeentie 135 C
FIN-00560 Helsinki, Finland
fax: +358 9 75630275