Translucent Thoughts

More-Than-Meets-the-Eye-de002Translucency in porcelain is a great example of an overlooked aesthetic virtue. While being one of the hallmarks of porcelain (along with density and “tone” or “ring”), translucency is often misperceived as being unrealized transparency. This is no more the case than lingerie being unrealized nudity. Translucency suggests. Translucency implies. Translucency entices and encourages closer examination.

The primary purpose of my art is to embrace the illusive, emotional content of traditional beauty. Instead of precisely rendered images that describe but a moment in time, I aspire to communicate the feeling behind moments—-the mysteries within a cloud shrouded morning mist as opposed to the simple, glaring reality of a sun filled-afternoon.

Clearly transmitted light—transparency—excites the senses. Caressed light—translucency—sooths the senses. My aesthetic intention is to provide a visual sanctuary. Translucency is a vital tool required to realize that intention.

Some technical observations on Translucency in Porcelain
(with excerpts and paraphrasing from “Clay Bodies” by Robert Tinchane)

“Actually, making translucent porcelain is a complex situation. It requires a delicate balancing act. The body needs to be dense and not have many pores in it, and yet some porosity is needed to provide the correct degree of translucency. The body also needs to melt to a certain degree, but it should not melt too much and begin to sag. This is the nature of the paradox of making translucent porcelain. The job can be attacked from the standpoint of these parameters, but the key work in all of this is control.”

Important points for maximum translucency and minimum slumping are:
1. Viscosity of the glassy flux— The viscosity of the glass needs to be as high as possible so that there is the least possible distortion. Increasing the silica content helps, as does adding alumina in the form of clay.

2. Pores— A small number of bubbles can be tolerated as long as they are sealed off from each other and as long as they do not expand appreciably.

3. Color— Obviously any transition metal oxides which are colored will absorb light and diminish translucence.

4. Crystals— The crystallinity of porcelain provides the structure that supports the mass but excessive numbers of crystal boundaries and the wrong refractive index will limit translucency.

So_much_to_say_detail002Obviously, the maximum translucency that could be exhibited in a ceramic body would be the kind produced by a transparent glass. But of course, that wouldn’t be translucent! It would be transparent. Still this may be considered to be the limiting case on one side of the equation.

Starting with this glass base let us look at ways in which a transparent material could back away from the complete transmission of light and become translucent. One technique for doing this would be to have other phases present in the glass, with different refractive indices. The first phase that comes to mind is a gas, forming a bubbly glass, just as foam would make water translucent. The same can be done with glass by melting it with a gas producer and having it become full of gas bubbles, with the gas having a different refractive index from the glass. In this case, light passing from the glass through the bubble, and then back into the glass again would cause much refraction, diffraction and reflection and give a polarizing effect.

A second means of achieving the same end would be to have another fluid in the glass. In this situation light passing through the sample would pass from the original glass into the second glass and be reflected, refracted and diffracted, causing a loss of transparency and the desired translucency. An example of liquid – liquid dispersion would be oil in water (mayonnaise) an example of glass-glass dispersion could consist of a phosphate glass dispersed in a silicate glass (bone china.)

A third way of producing translucency would be by generating a crystalline precipitate in the glass. This would also result in dispersion of the light as it passes from the glass through the crystal and back to the glass, many times, leading to the diffusion effect called translucency.

Another problem that has to be considered though is that there will be a limiting effect on the opaque side too. If the material under exanimation has a large index of refraction and if there are too many fine particles, light will be reflected and refracted so much that in a normal thickness, no light at all will pass through. This effect happens in the case of low-fired pottery and also in some of the materials known as glass-ceramics. Corning Glass Works produces a glass-ceramics that is very white in nature but practically opaque to light in normal thicknesses. The circumstance occurs because the crystals or droplets are very small, they are very numerous, and the index of refraction of the particles is high compared to the accompanying glass. Thus in a short distance light is completely absorbed.

Obviously then, translucency in porcelain aims for an intermediate effect. Porcelain shouldn’t be a transparent as normal glass, or even as transparent as an opal glass. For example, the Corning Glass Works makes a trademarked material known as CORELLE that is an opal glass. It could even be considered a glass-ceramic if you wanted to stretch a point because it does have some crystals in it. This opal glass is so uniform and so translucent that it definitely doesn’t command the respect and high price that it would if it were a true translucent porcelain.

From a practical standpoint, the most important way to look at translucent porcelain’s evolution is from the standpoint of its being a crystalline ceramic first and then becoming more and more glassy and transparent. If a low-fired clay body were considered it would be found that it is almost totally lacking in translucency. On examining it to find out why it is lacking in translucency the following points show up. First, if it is struck it does not ring very true. The reason for this is that it is very porous and is not homogeneous. This is also the reason why it is not translucent. A pottery body by definition has about 10-15% porosity and this kind of non-uniform structure is detrimental to translucency because of scattering. Also, there are a large number of boundaries between the crystals in pottery. Thus, it is apparent that we want a porcelain body to be lacier and less porous than a piece of pottery.

Over time people have found that the higher one fires a silicate material the more it tends to melt and the more translucent is the resulting piece. This occurs, because when dealing with silicate systems with moderate amounts al alumina present (as with clays) the materials, on melting, are very viscous. When they cool there is not a high tendency for crystallization to take place.

Hence, if one took a piece of pottery and heated it higher and higher, it would become more translucent. The reason that this is not always practical is that most pieces of pottery have a lot of dissolved gasses in them and they would bloat after strong heating. Therefore, even though they would be more translucent, they would be unacceptable because they would be distorted by this bloating.

Also, the viscosity question is important. Not every pottery material has the requisite degree of stiffness to resist slumping when it becomes slightly glassy. Thus warping and slumping occur.

The long and short of this explanation is that translucency in porcelain is a very specific quality and one that is most illusive. To obtain translucency in porcelain the artist must realize a very precise balance between glassy transparency and ceramic viscosity. When this balance is struck however, the result is a material that whispers a very subtle and sublime beauty.