Monthly Tech-Tip | No tracking! No ads! |
The very whitest porcelains are made from New Zealand kaolin. However, while Grolleg kaolin does not fire quite as white, it requires up to 10% less feldspar to produce a vitreous porcelain (it contains natural feldspar). That 10% less spar can be made up in kaolin, imparting better workability and dry strength to the body (and Grolleg is known for its dry strength). Assuming that 25% silica is needed for glaze fit, one only needs to discover what blend of feldspar and kaolin in the remaining 75% achieves the desired degree of vitrification (e.g. we like zero porosity just-reached at cone 6). We found 25% nepheline was too vitreous (pieces warped) and at 20% porosity was not yet zero. While the Grolleg version fires a little darker, the better workability imparted by the extra kaolin makes up for that. The plasticity needed for good throwing requires the addition of bentonite (4% for NZK and 3% for Grolleg). Both of these can be made into casting bodies by reducing the amount of bentonite (~ 1% for NZK, 0.5% for Grolleg). Do your testing to discover the % of bentonite needed for the leather hard to pull away from a mold without cracking but not take too long to cast.
URLs |
https://insight-live.com/insight/share.php?z=tgsPMxNsAP
Four Cone 6 (1200C) Casting/Throwing Porcelain Recipes (Grolleg and #6 Tile) Four recipes that can be adjusted to work for casting or throwing/hand building (by adjusting the percentage of bentonite). The more you are willing to spend on the materials the more whiteness, translucency and plasticity you will achieve. |
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Materials |
Grolleg Kaolin
A white burning kaolin from the UK, commonly used in porcelain bodies and as a glaze suspender. Sticky when wet, low plasticity. |
Materials |
New Zealand Halloysite
The whitest burning kaolin we have ever seen. It is very sticky when wet, suspends glazes well & makes super white porcelain (with help from a white bentonite). |
Glossary |
Porcelain
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. |
Glossary |
Deflocculation
Deflocculation is the magic behind the ceramic casting process, it enables slurries having impossibly low water contents and ware having amazingly low drying shrinkage |
Glossary |
Slip Casting
A method of forming ceramics. A deflocculated (low water content) slurry is poured into absorbent plaster molds. As it sits in the mold, usually 10+ minutes, a layer builds against the mold walls. When thick enough the mold is drained. |
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