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Bone China

A ceramic whose priorities are translucency, whiteness, fired strength and resistance to thermal shock failure.

Details

True bone china is a special type of translucent porcelain. Instead of feldspar as a flux, bone ash is used (today available in synthetic form tri-calcium phosphate). A typical recipe is 50% bone ash, 25% Cornwall Stone and 25% kaolin. The quality of the porcelain hinges on the quality of the materials, especially the kaolin, it needs to be low in iron (for whiteness) and low in titanium (for translucency). With only 25% kaolin the body has extremely low plasticity, this limits manufacturing methods (casting is most suitable). How do they fit glazes on a body that has no silica? Lead glazes - they can have very low thermal expansion. How do they keep the thin pieces from warping during firing? Custom-made setters for each piece.

The process is completely different than what a potter would do: Bisque fire, glaze, high fire. Bone china is bisque fired to high fire and then glazed at a very low temperature. Since the porcelain has zero porosity, getting a glaze to stick and dry on it is not easy, the process needed goes well beyond what a normal potter would be willing to do.

The vitrification range of bone china is narrow so kilns need to be fired carefully. Traditional bone china is fired to temperatures that would make the average potter gasp: 1400C or more (that is about Orton cone 16!). No common potter can achieve anywhere near this temperature, even in a gas fired kiln. The refractory bricks in most common kilns would not be able to service this temperature either. Why do they fire so high? To achieve the extra strength and the better thermal shock resistance. So called "fine china" compromises this firing temperature to within reach of normal kilns (about 1250C), typically by the addition of fluxes (e.g. feldspar, frit). Fine china, fired at cone 10, is also very translucent and resistant to thermal shock failure.

It is also not easy to understand by it is worth firing ware to 1400C (or even 1250) only to put on a leaded clear glaze that is fired at less than 1000C. While the porcelain is incredibly hard and durable, that glaze is not! In addition, it is technically possible to make white translucent porcelain at almost any temperature, it is simply a matter of how much you are willing to spend on the materials. Fritware can mature all the way down to cone 06!

Since the temperature of bone china is likely well beyond your capability, consider this page as about ways to achieve white-porcelain-translucency at normal stoneware (and even lower) temperatures. Since you will likely bisque low and glaze high, the durability of your glazes will exceed real china. And it is not difficult these days, with the incredible materials available in ceramics, to achieve similar translucency.

A Lithophane exploits porcleain translucency to reveal its design

A Lithophane exploits porcleain translucency to reveal its design

Top: A thin porcelain tile with etched design. Bottom: The same tile with a back light. By Stephanie Osser.

Now that is a translucent porcelain!

Now that is a translucent porcelain!

These are two cone 6 transparent glazed porcelain mugs with a light bulb inside. On the left is the porcelainous Plainsman M370 (Laguna B-Mix 6 would have similar opacity). Right is a zero-porosity New Zealand kaolin based porcelain called Polar Ice (from Plainsmanclays.com also)! The secret to making a plastic porcelain this white and translucent is not just the NZ kaolin, but the use of a very expensive plasticizer, VeeGum T, to enable maximizing the feldspar to get the fired maturity.

Zero3 casting porcelain at cone 04, 03

Zero3 casting porcelain at cone 04, 03

Compared to a typical cone 6 porcelain, left, which has zero translucency, these are fired 10 cones lower. I am using the G3879 clear glaze and it is working very well.

Links

URLs http://home.howstuffworks.com/lenox.htm#
What is bone china and how is it made at howstuffworks.com
Glossary Fritware
Glossary Translucency
A highly sought after property in porcelain, they are fired close enough to melting to pass considerable light. It can be very difficult to fire translucent ware without it warping.
Glossary Tranlucency
Glossary Vitrification
The term vitrified refers to the fired state of a piece of porcelain or stoneware. Vitrified ware has been fired high enough to make it very strong, hard and dense.
Glossary Stoneware
To potters, stonewares are simply high temperature, non-white bodies fired to sufficient density to make functional ware that is strong and durable.
Glossary Terra cotta
The term Terra Cotta can refer to a process or a kind of clay. Terra cotta clays are high in iron and available almost everywhere. While they vitrify at low temperatures, they are typically fired much lower than that and covered with colorful glazes.
Glossary Plasticity
Plasticity (in ceramics) is a property exhibited by soft clay. Force exerted effects a change in shape and the clay exhibits no tendency to return to the old shape. Elasticity is the opposite.
Materials Bone Ash

By Tony Hansen


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