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Opacifier, Opacification


A glaze additive that transforms an otherwise transparent glaze into an opaque one. Common opacifiers are tin oxide and zircon compounds. Opacifiers typically work by simply not dissolving into the melt, the white suspended particles thus reflect and scatter the light. Since they do not participate in the chemistry of the melt, rather than being factored into chemistry calculations they are typically rationalized at the recipe level (their effect on thermal expansion, development of colors and glaze hardness are obvious factors).

Another mechanism of opacity is crystallization, this can occur when a crystallizing ingredient is super saturated into the mix (e.g. TiO2) or when slow cooling a glaze to encourage crystallization of less saturated oxides that crystallize easily (e.g. boron blue). This mechanism is more likely to produce a glass that cutlery marks.

A third mechanism is suspended micro-bubbles that cloud the glass. These are often candidates for being able to use less opacifer percentages.

Opacified glazes can be more subject to cutlery marking. This can be a product of the glaze simply not melting to as smooth a surface because of the addition of refractory opacifier, or it can occur because jagged sharp edges of the tiny particles protrude out of the surface.

When opacifiers are added to colored glazes, the depth of color is lost.

G2926B cone 6 transparent with 10% zircon and 5% tin oxide

G2926B cone 6 transparent with 10% zircon and 5% tin oxide

The body is Plainsman M390. The firing schedule is Plainsman PLC6DS.

G1214Z at cone 6 with 10% zircon and 5% tin oxide

G1214Z at cone 6 with 10% zircon and 5% tin oxide

The body is Plainsman M390. The firing schedule is Plainsman PLC6DS.

5% titanium dioxide in G2934 matte, G1214Z matte, G2926B glossy

5% titanium dioxide in G2934 matte, G1214Z matte, G2926B glossy

The body is Plainsman M390. The firing schedule is Plainsman PLC6DS.

G2934 Cone 6 Matte with 10% zircon and 5% tin oxide

G2934 Cone 6 Matte with 10% zircon and 5% tin oxide

The body is Plainsman M390. The firing schedule is Plainsman PLC6DS.

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

Does Zircon 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.

5% tin oxide in a transparent boron cone 6 glaze

5% tin oxide in a transparent boron cone 6 glaze

This is a 45:27:28 Gerstley Borate:Kaolin:Silica base on a dark firing clay body. But tin oxide will do that in almost any cone 6 clear glaze (use one that fits your clay body).

The action of Zircopax vs Tin Oxide at cone 10R

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).

Adding an opacifier can produce cutlery marking!

Adding an opacifier can produce cutlery marking!

G2934 cone 6 matte (left) with 10% zircopax (center), 4% tin oxide (right). Although the cutlery marks clean off all of them, clearly the zircopax version has the worst problem and is the most difficult to clean. To make the best possible quality white it is wise to line blend in a glossy glaze to create a compromise between the most matteness possible yet a surface that does not mark or stain.

A good matte glaze. A bad matte glaze.

A good matte glaze. A bad matte glaze.

A melt fluidity comparison between two cone 6 matte glazes. G2934 is an MgO saturated boron fluxed glaze that melts to the right degree, forms a good glass, has a low thermal expansion, resists leaching and does not cutlery mark. G2000 is a much-trafficked cone 6 recipe, it is fluxed by zinc to produce a surface mesh of micro-crystals that not only mattes but also opacifies the glaze. But it forms a poor glass, runs too much, cutlery marks badly, stains easily, crazes and is likely not food safe! The G2934 recipe is google-searchable and a good demonstration of how the high-MgO matte mechanism (from talc) creates a silky surface at cone 6 oxidation the same as it does at cone 10 reduction (from dolomite). However it does need a tin or zircon addition to be white.

What happens when you opacify a colored glaze?

What happens when you opacify a colored glaze?

Left: G2934 cone 6 matte glaze with 3% Mason 6300 blue stain. Right: An additional 4% tin added. Notice how an opacified color does not have depth and therefore is lighter in color. Also it does not break to different shades at the edges of contours the way the transparent color does.

Why is this transparent so full of bubbles?

Why is this transparent so full of bubbles?

An example of how a micro-bubble population in the matrix of a transparent glaze can partially opacify it. If this glaze was completely transparent, the red clay body would show much better. However this is not the fault of the glaze. On a white body it would be more transparent. The problem is the terra cotta body. This is fired at cone 02. As the body approaches vitrification the decomposition of particles within it generate gases that bubble up in to the glaze. A positive aspect of this phenomena that this glaze could be opacified using a lower percentage of zircon. This type of glaze responds better to opacifier additions.

Opacifying a reduction dolomite matte

Opacifying a reduction dolomite matte

Opacifying a cone 10 reduction magnesia matte glaze. On the left: g2571a dolomite matte, a popular recipe (from Tony Hansen). Right: 10% Zircopax has been added. Both are on a buff stoneware (H550 from Plainsman Clays).

Two cone 6 matte glazes opacified with 10% Zircopax

Two cone 6 matte glazes opacified with 10% Zircopax

The clay is a buff stoneware. The upper two samples are G2934, an MgO matte. The one on the right has 10% zircon added to opacify. The bottom two are G1214Z (transparent version and opacified-with-zircon version).

How well zircon glaze can cover

How well zircon glaze can cover

This sanitary ware tank lid was made in China. Notice how thick the white glaze is being applied to cover the iron containing body below. This is a testament to how opaque a zircon opacified glaze can be. Of course, high percentages create a stiffer glaze melt and conditions can more often combine to produce crawling like this.

How do you turn a transparent glaze into a white?

How do you turn a transparent glaze into a white?

Right: Ravenscrag GR6-A transparent base glaze. Left: It has been opacified (turned opaque) by adding 10% Zircopax. This opacification mechanism can be transplanted into almost any transparent glaze. It can also be employed in colored transparents, it will convert their coloration to a pastel shade, lightening it. Zircon works well in oxidation and reduction. Tin oxide is another opacifier, it is much more expensive and only works in oxidation firing.

Tin Oxide is expensive, do not waste it by not mixing well

Tin Oxide is expensive, do not waste it by not mixing well

This is a cone 04 glaze on a terra cotta body. Two 300-gram test batches were made. Both have 5% tin oxide added. The one on the left was high-speed propeller-mixed for 10 seconds on a closed container. That was not enough, small agglomerates appear as white specks floating in the glass. The one on the right was mixed for 60 seconds. Now the tin particles, which are incredibly small, have been dispersed and can do their job of opacifying the glaze. Notice that 5% is not quite enough, more is needed.

Two stains. 4 colors. Will the guilty oxide please step forward.

Two stains. 4 colors. Will the guilty oxide please step forward.

We are looking at two pairs of samples, they demonstrate why knowing about glaze chemistry can be so important. Both pairs are the same glazes: G2934 cone 6 matte and G2916F cone 6 glossy. The left pair has 5% maroon stain added, the right pair 5% purple stain. The red and purple develop correctly in the glossy but not the matte. Why? The Mason Colorworks reference guide has the same precaution for both stains: the host glaze must be zincless and have 6.7-8.4% CaO (this is a little unclear, it is actually expressing a minimum, the more the CaO the better). The left-most samples of each pair here have 11% CaO, the right-most have 9%. So there is enough CaO. The problem is MgO (it is the mechanism of the matteness in the left two), it impedes the development of both colors. When you talk to tech support at any stain company they need to know the chemistry of your glaze to help.

Toilet bowl glaze vs. variegated glaze (at cone 6)

Toilet bowl glaze vs. variegated glaze (at cone 6)

Most artists and potters want some sort of visual variegation in their glazes. The mug on the right demonstrates several types. Opacity variation with thickness: The outer blue varies (breaks) to brown on the edges of contours where the glaze layer is thinner. Phase changes: The rutile blue color swirls within because of phase changes within the glass (zones of differing chemistry). Crystallization: The inside glaze is normally a clear amber transparent, but because these were slow cooling in the firing, iron in the glass has crystallized on the surface. Clay color: The mugs are made from a brown clay, the iron within it is bleeding into the blue and amplifying color change on thin sections.

White spots and blisters in a high zircon glaze at cone 6

White spots and blisters in a high zircon glaze at cone 6

This is also a common problem at low fire on earthenware clay (but can also appear on a buff stonewares). Those white spots you see on the beetle also cover the entire glaze surface (although not visible). They are sites of gas escaping (from particles decomposing in the body). The spots likely percolate during soaking at top temperate. Some of them, notably on the almost vertical inner walls of this bowl, having not smoothed over during cool down. What can you do? Use the highest possible bisque temperature, even cone 02 (make the glaze thixotropic so it will hang on to the denser body, see the link below about this). Adjust the glaze chemistry to melt later after gassing has finished (more zinc, less boron). Apply a thinner glaze layer (more thixotropy and lower specific gravity will enable a more even coverage with less thickness). Instead of soaking at temperature, drop 100 degrees and soak there instead (gassing is much less and the increasing viscosity of the melt overcomes the surface tension). Use a body not having any large particles that decompose (and gas) on firing. Use cones to verify the temperature your electronic controller reports.

Titanium as an opacifier

Titanium as an opacifier

This is a lithium glaze fired at cone 6. It has 6% titanium. This effect will also work in other types of transparent recipes. There should be more blue with slower cooling. Mixing some rutile in (e.g. 4 titanium, 2 rutile) should enhance also. 6% is pushing the edge of how much titanium should be in a recipe. Any more, or cooling too slow, could transform the surface into a mass of white crystals (which would be rough and non-functional). It is best to manually program firings, the up and down schedules? That would make the result consistent no matter how heavily or lightly loaded a kiln is.

The right amount of opacity highlights the incised design

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.

Our base glazes plus opacifiers on a dark burning body at cone 6

Our base glazes plus opacifiers on a dark burning body at cone 6

The body is Plainsman M390. These are commonly used base glazes. The top one is an MgO matte, next down is a calcium matte. They behave very differently to these additions. Notice also the difference when titanium dioxide is applied thickly. Tin oxide fires whiter than zircon (e.g. Zircopax). Each opacifier has issues. Tin is expensive. Titanium is difficult to mix into the slurry (screening required), not as white or opaque, variations in thickness produce more difference in results and it can turn blue. Zircon is more likely to cutlery mark, twice as much is required and it amplifies the color of any iron present.

Out Bound Links

  • (Materials) Tin Oxide - SnO2

    Stannic Oxide, Tin(IV) Oxide, Tin Dioxide, SnO2

  • (Glossary) Boron Blue

    Boron blue is the bluish haze or clouding in a transparent boron glaze that results from the crystallization of calcium borate in the glass matrix during cooling. This is a common problem in high boron glazes, the higher the CaO the worse it is. If possible, try cooling faster (of course there is...

  • (Glossary) 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 under...

  • (Glossary) Cutlery Marking

    In glazes with this fault, rubbing a metal knife or spoon on the surface will leave black marks that cannot be completely rubbed off. This is a common fault in glazes, especially mattes. Even commercial tableware often exhibits this problem. It usually happens because the micro-surface of the glaze ...

In Bound Links

  • (Materials) Titanium Dioxide - TiO2 - Anatase, Brookite

    TiO2

  • (Typecodes) 1: OPA - Opacifier
  • (Project) Ceramic Properties

    A property in this context is a created physical phenomenon in a glaze or body that can be achieved in a variety of ways (called mechanisms). For example, there are a number of ways to suspend a glaze...

  • (Properties) Glaze Variegation

    In contrast to the typical homogeneous surfaces of sanitaryware and most table ware, reactive glazes exhibit discontinuities in texture, matteness, reflectivity, color, etc that are called variegation...

  • (Properties) Glaze Opacifier

    Glaze opacity can be achieved using a number of different mechanisms (e.g. phase difference, crystallization, unmelted suspended particles, bubbles, matteness). Each mechanism or combination of mechan...

  • (Glossary) Colorant

    Although colorants are added to bodies, most people think of them as materials that transform a colorless transparent or opaque glaze into a colored glaze. Colorants can be raw metal oxides (e.g. iron oxide, chrome oxide), metal oxide containing materials (e.g. rutile) or man-made powders which are ...

  • (Glossary) Transparent Glazes

    A fully transparent glaze is simply one that does not have opacity. But there are degrees of transparency. For example, if a glaze is matte it will show the color of underlying body and decoration, but these will be muted (so it is actually translucent). Completely transparent glazes look like a gla...

  • (Glossary) Majolica

    Majolica is pottery, almost always decorative and non-functional, fired to a low temperature, employing a red-burning terra cotta clay, covered with a soft opaque white glaze and decorated with colored overglazes. Historically, majolica glazes (or tin glazed earthenware) were opacified using Tin Oxi...

  • (Glossary) Base Glaze

    A base glaze is one having no opacifiers, variegators or colorants. Thus it should be transparent if glossy and translucent if matte. Developing or adapting a base glaze for your ware is a very important first step in developing a manufacturing process that produces good quality. In fact, from a qua...


By Tony Hansen




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