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.
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, it 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 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).
Strips of the opacified glaze has been laid over the dark burning body and over the white engobe.
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.
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!
This high boron cone 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 a more melt fluidity is needed, decrease the percentage of CaO. For opaque glazes, this effect can actually enable the use of less opacifier.
The white slip on the left is an adjustment to the popular Fish Sauce slip (L3685A: 8% Frit 3110 replaces 8% Pyrax to make it harder and fire-bond to the body better). The one on the right (L3685C with 15% frit) is becoming translucent, obviously it will have a higher firing shrinkage than the body (a common cause of shivering at lips and contour changes). The slip is basically a very plastic white body. Since these 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 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.
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.
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.
Boron blue is a glaze fault involving the crystallization of calcium, boron and silicate compounds. It can be solved using ceramic chemistry.
Identifying the mechanism of a ceramic glaze recipe is the key to moving adjusting it, fixing it, reverse engineering it, even avoiding it!
Glaze opacity refers to the degree to which it is opaque. There is more than meets to eye to the subject of opacity control.