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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 temperatures. In recent years the availability of white burning plasticizers has made it possible to maximize feldspar content in even potter's porcelains, resulting in very translucent ware (in the past the added feldspar meant less room for clay in the recipe, and therefore less plasticity).

There are more factors at play to produce translucency in non-bone-china porcelain than it might at first appear. Of course it is well known that kaolins containing significant titanium impede translucency. Thus, the most sought after kaolins are those with body low TiO2 and low iron content (Grolleg and New Zealand china clay are examples). It seems obvious that bodies need to be fired to a point well past where they reach zero porosity to have really good translucency. But this is not necessarily true. It also seems obvious that the range within which good translucency happens and ware is stable enough to hold itself in shape during firing would be very narrow. Again, this is not necessarily true. Using frits as the flux also seems obvious to achieving really good translucency but this is not necessarily true either, feldspar-only porcelains can be very translucent. Also, contrary to what many people believe, high temperatures (cone 10) are not required for good translucency.

Here is one secret of how to achieve translucency: Veeum T. Although not advertised by its manufacturer and not known by them, Veegum imparts amazing degrees of translucency when used in larger amounts (up to 4%). Of course it is very expensive, that 4% will cost more than the other 96% of the ingredients. What is so great about this is that the really clean kaolins are not plastic and the Veegum more makes up for that producing a body workability that seems impossible for the whiteness and translucency.

Of course, glazes and engobes can also be translucent, depending on the degree of melt and the presence of opacifiers, colorants and crystallization.

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.

What are the two key causes of firing warpage in porcelain?

Here is an example of how a profile having no inherent strength can warp during firing (the one on the left is just bisque fired, the one on the right is fired beyond zero porosity to achieve translucency). Two key factors contribute to this failure: This porcelain is highly vitreous. This shape is vulnerable to warping. If the lip were flared out, for example, it would have much more strength to stay round. If the porcelain was less vitreous it would warp less. Of the two factors, which contributes more to the warping for this specific piece? The shape.

Reduction and oxidation porcelains

Left: Cone 10R (reduction) Plainsman P700 porcelain (made using Grolleg and G200 Feldspar). Right: Plainsman Cone 6 Plainsman Polar Ice porcelain (made using New Zealand kaolin and Nepheline Syenite). Both are zero porosity. The Polar Ice is very translucent, the P700 much less. The blue coloration of the P700 is mostly a product of the suspended micro-bubbles in the feldspar clear glaze (G1947U). The cone 6 glaze is fritted and much more transparent, but it could be stained to match the blue. These are high quality combinations of glaze and body.

How much porcelain flux is too much?

A porcelain mug has pulled slightly oval because of the weight of the handle. This happens in highly vitrified porcelains (e.g. translucent ones). The amount of feldspar or frit in the body determines the degree of maturity, the correct percentage is a balance between enough to get the maximum translucency and hardness but not so much that ware is deforming excessively during firing. This is Plainsman Polar Ice at cone 6, this degree of warp is acceptable and can be compensated for.

Cast to only 1mm wall thickness? NZ Kaolin+VeeGum can.

This cast bowl (just out of the mold and dried) is 130mm in diameter and 85mm deep and yet the walls are only 1mm thick and it only weighs 89 gm! The slip was in the mold for only 1 minute. What slip? A New Zealand Halloysite based cone 6 translucent porcelain. This NZ material is fabulous for casting slips (it needs a little extra plasticizer also to give the body the strength to pull away from the mold surface as it shrinks).

You will not believe the secret of translucency at cone 6

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!

The difference between vitrified and sintered

The top fired bar is a translucent porcelain (made from kaolin, silica and feldspar). It has zero porosity and is very hard and strong at room temperature because fibrous mullite crystals have developed around the quartz and kaolinite grains and feldspar silicate glass has flowed within to cement the matrix together securely (that what vitrified means). But it has a high fired shrinkage, very poor thermal shock resistance and little stability at above red-heat temperatures. The bar below is zirconium silicate plus 3% binder, all that cements it together is sintered bonds between closely packed particles. Yet it is surprisingly strong, it cannot be scratched with metal. It has low fired shrinkage, zero thermal expansion and maintain its strength and hardness to very high temperatures.

Polar Ice casting translucency

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.

Alberta Ravenscrag Cone 6 Brilliant Celadon

The magic of this recipe is the 5% extra frit, that makes the melt more fluid and brilliant and gives the glaze more transparency where it is thinner on edges and contours. The extra iron in the Plainsman P380 (right) intensifies the green glaze color (vs. Polar Ice on the left). The specks are cobalt oxide agglomerates that were made by slurrying cobalt oxide and bentonite, then crushing it to sizes large enough to make the specks.

A problem with super-white mugs

These are made from Plainsman Polar Ice translucent NZ porcelain. The one on the right was used in the coffee room of the plant and washed between uses in a common manner (which is: not very much!). The stains are obviously not nearly as visible on a stoneware mug.

Translucency of Polar Ice compared to another porcelain at cone 6

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!

Want to make a cone 10R super translucent porcelain? Think again.

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.

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.

Reduction Polar Ice vs. Oxidation Polar Ice

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.

Out Bound Links


By Tony Hansen




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