|Monthly Tech-Tip |
Modified: 2019-12-26 12:32:32
Base 25x4 porcelain recipe
The starting point for a cone 10 plastic porcelain. Such porcelains require about 25% silica for glaze fit and about 25% feldspar to achieve near zero porosity. The remaining 50% is clay to provide plasticity and workability. Bentonite, or whiter burning smectite or hectorite, are often added to improve plasticity.
The percentages of the ingredients are tweaked to adjust the degree of vitrification (e.g. more feldspar and less clay for better fired density and strength; more kaolin and less ball clay for better fired whiteness; more ball clay and less kaolin for better plasticity). Minor amounts of other materials can be added to further adjust working and firing behaviour (talc or frit to improve maturity, stain to adjust color).
The actual properties will also depend greatly on the brand-name, particle size and purity of materials used. Translucent porcelains employ 50% low iron and low titanium kaolins and zero ball clay. Plastic porcelains employ plastic kaolins. Finer mesh silica is often used to control thermal expansion behaviour. Potassium and sodium feldspars and nepheline syenite are selected based on price, availability, purity, fluxing power and their effect on various firing properties.
Lower temperature porcelains increase the feldspar (up to 40%) and reduce the clay content to achieve maturity. They can also employ small amounts of talc or frit to help. Because the clay percentage is lower they either need to increase the ball clay at the expense of kaolin or add more bentonite.
There are no short-cuts to creating a good porcelain from the materials available to you. It is vital to do the testing work to determine the best mix to produce the needed working and fired properties. The final recipe will always be a compromise between the fired properties needed and the cost (if money is no concern it is easily possible to produce a recipe that costs five times more). Almost always the fired property that must be sacrificed to keep costs down is fired whiteness. Plasticity is almost always sacrificed (since the single most expensive ingredient by far are white burning plasticizers).
A term used in the ceramics industry to signify the degree of vitrification in a fired clay. Mature clays are dense and strong, immature ones porous and weak.
How do you make porcelain? There is a surprisingly simple logic to formulating them and to adjusting their working, drying, glazing and firing properties for different purposes.
Formulating a Porcelain
The principles behind formulating a porcelain are quite simple. You just need to know the purpose of each material, a starting recipe and a testing regimen.
Bentonite can make a clay body instantly plastic, only 2-3% can have a big effect. It also suspends slurries so they don't settle out and slows down drying.
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|By Tony Hansen|
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