Opacity
Ceramic glaze opacity refers to the degree to which a glaze is non-transparent. Non-colored glazes can be either transparent, opaque or somewhere in between. Transparent glazes are glossy (matt glazes, by definition, are never completely transparent but they can be partly translucent to reveal underglaze decoration, for example). Opaque glazes are normally just transparent glazes with additions of light-reflecting opacifer particles that do not melt and dissolve into the glaze with the rest of the oxides (like tin oxide or zircon). Often, significant percentages of opacifier must be added to a transparent glaze to achieve complete opacity. Tin oxide is by far the most expensive, whiteness can be achieved with 7% or less (whereas at times 20% zircon opacifier is needed to get full opacity). But the bottom line with opacity is almost always zircon materials, they are the most practical. The finer the particle size the better they opacify. It is really quite amazing that such small particles can resist being dissolved into the glaze melt, this is a testament to how refractory they really are.
Opacity can be 'designed in' and a result of crystallization that is occurring as the glaze melt cools, it can be the product of a simple addition of opacifier or it can be a combination of both, or it can be a glaze defect (e.g. incomplete melting, devitrification). Different glaze bases respond differently to opacification mechanisms and a good knowledge and testing regimen is needed to produce a good opaque glaze that is not overly expensive and does not exhibit some of the common problems associated with opacity (cutlery marking, poor glaze melt fluidity and associated issues like blistering and pinholing). Opaque glaze frits are available, the opacifier is smelted right in during the manufacturing process, these work the best not only in firing, but assure a better dispersion of the opacifier particles.
The degree of opacity of a colored glaze determines its depth of color. Transparent glossy glazes normally have vibrant color, whereas opacification subdues the color by reducing its depth (see Zircon for more information). Partially opacified glazes are sometimes referred to as 'milky'. People testing opacification quickly learn how many shades of white there can be, white can be stained to a host of other colors depending on what else is in the glaze and body (yellowing due to iron presence is common, for example). Zircon opacifiers tend toward yellower whites whereas tin oxide produces bluer whites.
There are a number of mechanisms of opacity. These include the simple dispersion of refactory micro-particles (zircon or tin for example) that reflect and scatter the light, the develpment of opaque crystalline phases in the glaze during cooling (from high CaO for example), the surface smoothness (mattes are often more opaque partly because the surface is not flat and scatters light), the development of multiple phases within the glaze matrix (islands of differing glass composition and structure which scatter and refract light as it passes through the borders between phases). Suspended micro-bubbles in a glaze will also scatter light and can produce a milky effect. Of course, the degree of melting will also affect the completeness to which transparency is developed.
If whiteness and homogeneity are needed (e.g. toilet bowl white), the opacification options are quite narrow, usually only tin and zircon additions are feasible. But for colored glazes, opacifiers that yellow or variegate the glaze (like titanium) are options (but more difficult to implement).
Often opacity occurs when it is not wanted (boron blue for example). Titanium and rutile can produce a wide range of glaze effects, most of which are not transparent. CaO and ZnO like to crystallize and can do this to the point that the entire glaze surface is covered with micro crystals that are completely opaque. Out Bound Links
- (Glossary)
Boron Blue
The blue haze in a transparent boron glaze that re... - (Articles)
Identifying Glaze Mechanisms If you can look at a glaze recipe and pick out the materials add to produce the color, opacity and v...
In Bound Links
Pictures
Flow tester demonstrates how zircon opacifys and stiffens a glaze melt
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