All Glossary

Opacity

Opacity of ceramics glazes is normally achieved by adding an opacifier like tin oxide or zircon. However, there are chemical profiles that can turn transparent glazes milky and make it cheaper to opacify them.

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Details

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 (like tin oxide or zircon) that do not melt and dissolve into the glaze with the rest of the oxides. 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 10-20% zircon opacifier is needed to get full opacity). Zircon materials are almost always 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 - other common ceramic material particles do not even come close to being able to do this.

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 glazes have depth, thus any colorants present produce deep and rich color (if the glaze does not crystallize of course). Opacification subdues color by reducing its depth. Thus a deep rich blue can turn into a dull pastel blue when the glaze is opacified. Partially opacified glazes are sometimes referred to as 'milky'. People testing opacification quickly learn how many shades of white there can be (depending on opacifier used, its percentage, kiln atmosphere and impurities present). Zircon opacifiers tend toward yellower whites whereas tin oxide produces bluer ones. Unlike opacified glazes that exhibit the same color regardless of thickness, the intensity in a colored transparent varies according to thickness (twice as thick will be twice as dark).

There are a number of mechanisms of opacity. These include the simple dispersion of refractory micro-particles (zircon or tin for example) that reflect and refract the light, the development 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 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 or rutile) are options (but more difficult to maintain). 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.

A good example of opacity occurring when it is not wanted is boron blue. When boron is too high, especially when there is plenty of SiO₂ and Al₂O₃, boron forms crystal phases that turn transparent glazes milky. However, since opacifiers are very expensive, this effect could be used to reduce the cost of opacification.

Colorants in transparent glazes produce the effect of varying color with varying glaze thickness. This is often a sought-after effect to highlight surface and contour variations in a piece. But on flat surfaces the phenomenon can detract from the appearance, highlighting thickness variations that occurred during application (e.g. runs).

Related Information

Does Zircopax only whiten and opacify a clear glaze? No.

This GLFL test for melt flow demonstrates how zircon opacifies but also stiffens a glaze melt at cone 6. Zircon also hardens many glazes, even if used in smaller amounts than will opacify.

Al2O3 in glazes make them durable and wear resistant

The cone 6 glazes on the left have double the boron of those on the right so they should be melting much more. But they flow less because they have much higher Al₂O₃ and SiO₂ contents. This effect renders them milky white vs. the transparent of those on the right. Why? Because G and H are trapping micro-bubbles because of the increased viscosity of the melt. In spite of this, the two on the left do fire almost transparent when applied to ware, they have enough fluidity to shed most of the bubbles when in a thin layer. The ones on the right are too fluid, they will run excessively on ware unless applied thinly. The sweet-spot is a little more fluidity than those on the left. But there is another very important factor: Durability. The increased Al₂O₃ in G and H make them fire harder, more resistant to abrasion. The added SiO₂ adds resistance to leaching.

The action of Zircopax vs Tin Oxide at cone 10R

On Plainsman H443 iron stoneware in reduction firing. Notice Tin does not work. Also notice that between 7.5 and 10% Zircopax provides as much opacity as does 15% (Zircon is very expensive).

Here is why you don't want an engobe to fire too vitreous

Too much frit in an engobe and it will lose opacity and whiteness. The white slip on the left is an adjustment to the popular "Fish Sauce" slip recipe (L3685A: 8% Frit 3110 replaces 8% Pyrax to make it harder and fire-bond to the body better). The one on the right, L3685C, has 15% frit. Although applied at the same thickness, it is becoming translucent, moving it into glaze territory. That means it will have a far higher firing shrinkage than the body (a common cause of shivering at lips and contour changes). This slip is basically a very plastic white body. Since white burning slips are made from refractory materials they are not nearly as vitreous as red ones, at low fire they need help to mature and a frit is the natural answer. With the right amount of frit the fired shrinkage of body and slip can be matched and the slip will be opaque. This underscores the need to tune the maturity of an engobe to the body and temperature. Although zircon could be added to the one on the right to opacify and whiten it, that would not fix the mismatch in fired shrinkage between it and the body. And it would increase the price.

The covering power and opacity of an encapsulated stain

This cone 6 porcelain bowl has a black engobe inside and half way down the outside. This inside glaze is a transparent (G2926B) but the outside is that same transparent with 11% added encapsulated red stain. Notice that the glaze is so opaque that you cannot see where the black engobe ends and the while porcelain body begins!

An example of how an opacity test is done in a tile lab

Strips of the opacified glaze has been laid over the dark burning body and over the white engobe.

Boron blue in low fire transparent glazes

This high-boron high-CaOcone 04 glaze is generating calcium-borate crystals during cool down (called boron-blue). This is a common problem and a reason to control the boron levels in transparent glazes; use just enough to melt it well. If more melt fluidity is needed, decrease the percentage of CaO in favor of a lower melting oxide, that will certainly help. There is a positive: For opaque glazes, this effect can actually enable the use of less opacifier.

The right amount of opacity highlights the incised design

The mug on the left is a commercial brushing glaze. The mechanism of this effect is that the glaze is much thinner on the edges of the design, thin enough that its opacity is mostly lost. The potter is attempting to mix her own equivalent (center and right). Her glaze adds 4% tin oxide to a transparent. However, as you can see, she has added too much. Further testing using lower percentages will find the right balance between the opacity needed to cover the brown body on the flat areas and the transparency needed to expose it on the contours.

A glaze is showing unwanted streaking. Why?

This is a fluid melt cone 6 glaze with colorant added and partially opacified. It runs into contours during firing, thickening there (notice the darkening around the logo), this is a desired visual effect. However, notice that drips and runs coming down from the rim, they are producing darker streaks. This is an application issue. Glazes that fasten-in-place too slowly will drain unevenly on extraction from the bucket (after dipping). This can be solved by making a thixotropic slurry. If bisque ware is too dense, glazes have a more difficult time fixing-in-place in an even layer, especially if they have no thixotropy. If glazes lack clay (e.g. less than 15% kaolin) they do not gel as easily. Slurries containing too much gum dry slowly and drips are almost unavoidable. If the problem is too much melt fluidity, choose a more stable base glaze can really help. Just because melt fluidity is less does not mean that it will be less glossy.

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