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Translucency

A highly sought after property in porcelain, they are fired close enough to melting to pass considerable light. It can be very difficult to fire translucent ware without it warping.

Details

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 translucency when used in larger amounts (up to 4%) and much less feldspar is needed. 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 than 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.

The recipe for a cone 10 translucent plastic porcelain is really quite simple: It needs 20% silica for glaze fit (25% if your glazes craze). It needs 2-4% white bentonite (e.g. VeeGum, Bentone) for workability. 25% feldspar (high quality low iron) will give less fired warping and less translucency, 35% will give best translucency and the least stability in the kiln. The rest of the recipe is high quality white burning kaolin (e.g. New Zealand). For a casting version cut the bentonite to 1% or less (just enough to give it the plastic strength to pull away from the mold). For lower temperatures add more feldspar (at the expense of kaolin) or use a mix of feldspar and a little frit. Some white kaolins (e.g. English) contain significant fluxes so less feldspar is needed (up to 10% less), this enables higher kaolin and/or silica percentages.

Related Information

The translucency of Polar Ice porcelain

A slab-built porcelain mug with yellow glaze showing off its translucency in sunlight

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.

Translucent Porcelain Lithophane by Stephanie Osser

Made using porcelain fired to cone 11+. This is a splendid demonstration of translucency. Without a back-light it is just a white slab or porcelain. But the varying thickness in the porcelain determine the amount of light that passes at any given spot.

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.

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.

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.

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!

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.

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!

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.

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.

Now that is a translucent porcelain!

Polar Ice porcelain mug with a light inside to demonstrate its translucency

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.

Links

Glossary Porcelain
Standard porcelains used by potters and for the production of sanitary and table ware have surprisingly similar recipes. But their plasticities vary widely.
Glossary Bone China
A ceramic whose priorities are translucency, whiteness, fired strength and resistance to thermal shock failure.
Glossary Vitrification
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.
URLs https://en.wikipedia.org/wiki/Herend_Porcelain_Manufactory
Herend Translucent Porcelain
URLs https://insight-live.com/insight/sharesmanager.php?edit=tgsPMxNsAP
Develop and maintain your own cone 6 porcelain recipe

By Tony Hansen


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