•The secret to cool bodies and glazes is a lot of testing.
•The secret to know what to test is material and chemistry knowledge.
•The secret to learning from testing is documentation.
•The place to test, do the chemistry and document is an account at https://insight-live.com
•The place to get the knowledge is https://digitalfire.com

Sign-up at https://insight-live.com today.

Bone China


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.

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.

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.

Out Bound Links

  • (Glossary) Stoneware

    Most often the term "stoneware" refers to a high fired (about 1200C+) ceramic clay:feldspar:quartz blend that is semi-vitreous (not translucent and not zero porosity). To appreciate the scope that stoneware can encompass it is helpful to contrast it with porcelains (this description is for people wh...

  • (Glossary) Terra cotta

    'Terra Cotta' (Italian for 'cooked earth') is red burning earthenware. It has been made for thousands of years by indigenous cultures, most often unglazed. If glazed, high lead content mixtures have been traditional. It is fired at much lower temperatures than stoneware so, not surprisingly, it is n...

  • (Glossary) Plasticity

    This term is used in reference to clays (or more often bodies which are blends of clay, feldspar and silica particles) and their ability to assume a new shape without any tendency to return to the old (elasticity). Plasticity is a product of the electrolytic character of flat clay particles (they ha...

  • (URLs) What is bone china and how is it made at howstuffworks.com

    http://home.howstuffworks.com/lenox.htm#

  • (Glossary) Translucency

    Translucent porcelain enables the passage of light through the wall of the item. Bone china is translucent. However it is so vitreous that pieces cannot support their own weight during firing, this requires bisque firing in setters to hold the shape at high temperature followed by glazing at low tem...

  • (Glossary) Fritware

    Fritware is the crow-bar method of making porcelain at super low temperatures. Frits are used where much higher material costs can be accepted to produce a body that fires glassy, white, at a very low temperature and rapid rate-of-rise. Traditional porcelains are made from feldspar, clay and quar...

In Bound Links

  • (Materials) Bone Ash - Ca5(OH)(PO4)3

    Calcium Phosphate

  • (Glossary) Vitrification

    Vitrification is the solidification of a melt into a glass rather than a crystalline structure (crystallization). Glass, clay bodies and glazes vitrify, but in ceramics use of the term focuses most on clay bodies. Vitrification is a process. Bodies do not have specific vitrification points. As cl...

  • (Glossary) Tranlucency

    Translucent glazes are neither opaque or transparent. A good example is a matte glaze that contains no opacifier. Glazes that are opacified will of course have varying degrees of translucency according to the amount of opacifier present. A colored glaze can be transparent, translucent or opaque and ...


By Tony Hansen




Feedback, Suggestions

Your email address

Subject

Your Name

Message


Copyright 2003, 2008, 2015 https://digitalfire.com, All Rights Reserved