|Monthly Tech-Tip |
A ceramic whose priorities are translucency, whiteness, fired strength and resistance to thermal shock failure.
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.
Top: A thin porcelain tile with etched design. Bottom: The same tile with a back light. By Stephanie Osser.
These are two cone 6 transparent-glazed porcelain mugs. On the left is the porcelainous Plainsman M370 (Laguna B-Mix 6 would have similar opacity), it is semi-vitreous and has no translucency. Right is a highly vitreous, New Zealand kaolin based porcelain, Polar Ice. The secret to making this porcelain super-white is the NZ kaolin. The secret of its impossibly-high plasticity is the very expensive plasticizer, VeeGum T. What about the translucency? Nepheline syenite is used as the feldspar, but it alone cannot deliver this kind of translucency at cone 6. Amazingly the 4% Veegum acts as a translucency catalyst, it is the secret. Commercial manufacturers could never use a sticky and difficult-to-dry porcelain like this, but a potter can do incredible things with it (e.g. throw thinner, lighter, bigger than any other clay he/she has ever used!).
What is bone china and how is it made at howstuffworks.com
A highly sought-after property in porcelain, fired close enough to melting to take on the glass-like property of passing light. Translucency implies tendency to warp during firing.
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.
To potters, stonewares are simply high temperature, non-white bodies fired to sufficient density to make functional ware that is strong and durable.
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.
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.