|Monthly Tech-Tip |
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.
Polar Ice is made by Plainsman Clays, it is by far the most expensive body they make (because of the use of New Zealand kaolin and VeeGum). I call these my "sunshine mugs". They are fired at cone 6 (2200F) with a transparent glaze on the inside (G2926B) and G2934Y yellow silky matte on the outside. This yellow glaze showcases the translucency in sunlight better than any other I have seen. The high plasticity and this "pie crust" method of making them enables thinner walls than any other method I know of, even casting. Although the walls of this piece are about 3.5 mm thick, I have achieved 2 mm using stiffer clay! Even with very thin walls the weight of the handle does not pull the lips of these into an oval-shape.
Made using porcelain fired to cone 11+. This is a splendid demonstration of translucency. Without a back-light it is just a white slab. But the varying thickness in the porcelain determine the amount of light that passes at any given spot, thus producing the design.
Top: A thin porcelain tile with etched design. Bottom: The same tile with a back light. By Stephanie Osser.
On the top you can see the color difference. The other porcelain is made from a low TiO2 mix of typical North American kaolins, feldspars and bentonites. Bottom with a light inside: Polar ice on the left is far more translucent. Yet it is not overly mature, it resists fired warping remarkably well. And it is also more plastic (which seems impossible). There is a secret to the translucency that goes beyond the fact that it employs New Zealand kaolin and the percentage of feldspar it has. But I cannot tell you. But if you read this site carefully you will discover it in the most unlikely place!
Polar Ice (Plainsman Clays) has been fired to cone 10R (left). This is beyond the recommended cone 6 range, but it worked well in this instance. The result is even more translucency and a translucency of a different character: blue! This looks much more like real blue polar ice.
A bowl cast from Polar Ice porcelain and fired to cone 6. I has a thinner wall than the thrown pieces made from Polar Ice throwing, yet it is much less translucent. This appears to be because the VeeGum is much less.
Three cone 6 mugs. All have zero porosity. Why is the middle one so translucent? Three reasons.
1. It has 10% more feldspar than the one on the left and reaches zero porosity already at cone 5.
2. It employs New Zealand china clay while the one on the left contains high-TiO2 #6 Tile kaolin.
But this is also true for the one on the right. The third difference is the key.
3. The center one contains 4% Veegum T plasticizer (while the other two use standard bentonite). This is surprising when I tell you one more thing: The mug on the right also contains 3% Ferro Frit 3110. That means that the frit does not have near the fluxing power of the VeeGum!
On the right is a porcelain used in China, renowned for its whiteness and translucency. On the left is a body made from Grolleg kaolin, this is commonly used by potters. They were fired in reduction. The tiny iron specks that potters do not even notice are enemy number for the blue-white porcelain like this. Although they might be small the reduction atmosphere makes them blossom out in full glory to ruin the piece. These specks come as contaminants in the materials (especially the silica) and they are easily picked up during fabrication. For very white bodies like this, it is incredibly difficult to prevent the specks. For a perfectly white flawless result, the entire factory must be dedicated to this one body; they use wet processing, magnets, filter pressing, stainless steel equipment and impeccable procedures.
Standard porcelains used by potters and for the production of sanitary and table ware have surprisingly similar recipes. But their plasticities vary widely.
A ceramic whose priorities are translucency, whiteness, fired strength and resistance to thermal shock failure.
Every glossy ceramic glaze is actually a base transparent with added opacifiers and colorants. So understand how to make a good transparent, then build other glazes on it.
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.
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.
Herend Translucent Porcelain