All Glossary

Reactive Glazes

In ceramics, reactive glazes have variegated surfaces that are a product of more melt fluidity and the presence of opacifiers, crystallizers and phase changers.

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Variegated, or mottled, glazes are those that do not have a homogeneous solid color or character (i.e. like a ceramic sink or toilet bowl). They are often called 'reactive glazes'. They contain higher percentages of fluxes and additions intended to produce one or more variegation mechanisms. Variations in color and texture are highly prized by many ceramists. A variety of mechanisms are used to create the variegation. These include crystal growth, the addition of speckling agents, phase separation, layering, opacity variations (occurring with thickness variations) and multilayering.

You can make your own reactive glazes by adding variegators, opacifiers and colorants to your base glaze, especially one that has a fluid melt (of course you need to be mixing your own glazes). Rutile and titanium are the most common variegators. Zircon and tin oxide are the most common opacifiers. Metal oxides and stains are used for color. Click the link below for the article "Who Do I Start" to learn more about mixing your own glazes. Another method is to locate a reactive glaze at a store page online, then google the glaze name like this: "rutile blue glaze recipe cone 6". Most variegated glazes sold came from well-known recipes or are adjustments to such. Look at the ones you find with a critical eye, use a limit recipe approach to deciding whether to test them.

A catch glaze is often needed to arrest the flow of reactive runny glazes (so they do not run down on to kiln shelves).

Related Information

Cone 10R variegation and crystal magic

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This is an example of crystallization in a high MgO matte. MgO normally stiffens the glaze melt forming non-crystal mattes but at cone 10R many cool things happen with metal oxides, even at low percentages. Dolomite and talc are the key MgO sources.

Toilet bowl glaze vs. variegated glaze

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Most artists and potters want some sort of visual variegation in their glazes. The cone 6 oxidation 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.

Fluid melt glaze needs uneven surface to develop visual interest

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This is the same glaze on the outside of these two pieces. It develops the variegated deep blue character only when thick. But if it were applied thick enough on the left piece it would run off onto the kiln shelf. However the recesses in the texture-rolled surface of the one on the right have caught the flow, creating the thicknesses needed to get the color. Another factor is that the piece on the right is buff stoneware. Thus the clay contains some iron and it is bleeding into the glaze to help develop the color.

Layering glazes to get variegation

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Example of the variegation produced by layering a white glaze of stiffer melt (a matte) over a darker glaze of more fluid melt (a glossy). This was fired at cone 6. The body is a stoneware and the glazes employ calcium carbonate to encourage bubbling during melting, each bubble reveals the color and texture of the underlying glaze layer. It is also possible to get this effect using the same base glaze (stained different colors).

Titanium Dioxide in a cone 6 calcium matte glaze

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The glaze is G1214Z1 cone 6 base calcium matte on Plainsman M390 fired at cone 6 using the PLC6DS schedule. 5% titanium dioxide has been added. Titanium can create reactive glazes, like rutile, with no other colorants added. This effect also works well on matte surfaces, but the glaze needs good melt fluidity (that is good because functional mattes melt well). Calcium mattes host crystallization and work particularly well. Because titanium dioxide does not contain iron oxide lighter colors and better blues are possible compared to rutile (iron is still needed by it is coming from the body here). Like rutile, the effects are dependent on the cooling rate of the firing, slower cools produce more reactivity. Even application without drips is important (mixing as a thixotropic dipping glaze is best). This appearance also depends on using dark burning body or engobe.

Three visual glaze mechanisms make this piece unique

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"Mechanisms" are specifics about the glaze application or preparation process, the materials, the chemistry or firing schedule that produce a specific visual effect. This is fired at cone 10R. It is made from a buff stoneware, Plainsman H550, and has L3954N black engobe on the inside and part way down the outside. The transparent glaze on the inside gives the black a deep vibrant effect. The outside glaze is G2571A with 3.5% rutile and 10% zircopax added (the latter imparts opacity and the former produces the variegated surface). The powerful color of the black engobe wants to get through but it is only able to do so where the glaze layer is thinner (producing the varied shades of brown with differing thicknesses of glaze that occur because of the presence of the incised design).

The rutile mechanism in glazes

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2, 3, 4, 5% rutile added to an 80:20 mix of Alberta Slip:Frit 3134 at cone 6. This variegating mechanism of rutile is well-known among potters. Rutile can be added to many glazes to variegate existing color and opacification. If more rutile is added the surface turns an ugly yellow in a mass of titanium crystals.

Making Ravenscrag Floating Blue dance more at cone 8

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Here GR6-M it is fired to cone 8 where the melt obviously has much more melt fluidity! The photo does not do justice to the variegation and crystallization happening on this surface. Of course, it is running alot more, so caution will be needed.

Variegation and phase separation with about 5% rutile

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The glaze is a dolomite matte fired to cone 10R. High fire reduction is among the best processes to exploit the variegating magic of rutile.

Boron blue in low fire transparent glazes

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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.

An iron red cone 6 reactive glaze up close

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G3948A is a cone 6 iron red. This sample is firing using the C6DHSC schedule. It is a reactive glaze in more ways than one. This closeup reveals just how much is happening on that fired surface. The recipe contains spodumene, an expensive material, but clearly it is worth it.

High melt fluidity is required to achieve the visual effect of this glaze

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This is G3948A, an iron red cone 6 reactive glaze (similar to several commercially available products). The reason for the variegated surface is the high fluidity of the melt. Adequate thickness is also important, enabling it to run downward to some extent. That means this is not actually over-fired. Using it thus requires consideration of the running behavior, accommodating it in the shape of the ware on which it is used. Obviously, using this on the insides of pieces would result in pooling at the base, which would likely produce glaze compression, cracking the piece during cooling. Use on the outsides may require a catch glaze.

In pursuit of a reactive cone 6 base that I can live with

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These GLFL tests and GBMF tests for melt-flow compare 6 unconventionally fluxed glazes with a traditional cone 6 moderately boron fluxed (+soda/calcia/magnesia) base (far left Plainsman G2926B). The objective is to achieve higher melt fluidity for a more brilliant surface and for more reactive response with colorant and variegator additions (with awareness of downsides of this). Classified by most active fluxes they are:
G3814 - Moderate zinc, no boron
G2938 - High-soda+lithia+strontium
G3808 - High boron+soda (Gerstley Borate based)
G3808A - 3808 chemistry sourced from frits
G3813 - Boron+zinc+lithia
G3806B - Soda+zinc+strontium+boron (mixed oxide effect)
This series of tests was done to choose a recipe, that while more fluid, will have a minimum of the problems associated with such (e.g. crazing, blistering, low run volatility, susceptibility to leaching). As a final step the recipe will be adjusted as needed. We eventually evolved the G3806B, after many iterations settled on G3806E or G3806F as best for now.

Cone 6 oil-spot glaze effect, what works and does not work?

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Simulating a white-on-black oil-spot effect at cone 6 oxidation proved to be a matter of repeated testing (that got me past some misconceptions). Stopping to think about the results at each step and keeping a good audit trail with pictures, in my account at insight-live.com, really helped. I had three black glazes: G2934BL satin (G2934 with black stain), G2926BB super-gloss (G2926B with black stain) and G3914A Alberta Slip black. Going on a hunch, I mixed up a bucket of the G3914A first (with some gum to help it survive second-coating without lifting). Rather than just try any white, I created G3912A by substituting as much CaO and MgO as possible for SrO in the G2934Y base. I later learned this to be an error, SrO reduces the surface tension, I should have used MgO (the G2934Y is a high-MgO glaze so it would have been fine as-is)! As you can see on the far right, this white still worked (at cone 5, 6, 7, 8). Why? There is another factor even more important. The effect only works on the Alberta Slip black. But its LOI is not higher than the others. And it worked even after ball milling. So I need to continue to work on this to learn more about why this works.

Reactive glazes are good. And bad.

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Reactive glazes don't melt into a homogeneous melt and they don't freeze as a typical glass. The physical nature of the material powders (e.g. their particle size and the individual nature of how they respond to heat, soften, melt and interact with their own kind and others) create a melt that does not solidify into a homogeneous glass. These glazes are said to be dynamic. And unpredictable effects often occur during firing, like color variegation, speckles, streaks, mottled and flowing textures, crystallization, pooling, etc. The outcome is influenced by factors such as the materials chosen to source the needed oxides, firing schedule, kiln atmosphere, cooling or heating cycle, etc. These glazes are at their best when each piece has a unique, artistic character. But, this is also their worst feature, making them "tipping point glazes", ones whose visual character is a product of fragile and not well understood features of the materials and process. Small changes typically produce big changes in fired appearance (often to the chagrin of the potter).

A pottery glaze so reactive it can eat through a firebrick. The fix struck boron-blue gold!

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The melt fluidity tester was fired at cone 6. The glaze on the left is G2826A2, a 50:30:20 Gerstley Borate base glaze recipe (historically used for reactive glazes). The one on the right is G2926A3, an adjusted version that cuts the B₂O₃ level and adds lots of SiO₂. The result is much more sane, although still very melt-fluid glaze. This is also a lesson in the chemistry that produces boron-blue: The one on the left does not and the one on the right does, producing a highly decorative boron-blue, especially on dark bodies. Why? High B₂O₃ is not the key, it is lower. CaO is lower but it was higher in the original 50:30:20 recipe and that had plenty of boron blue. The SiO₂ appears to be the enabler, it is much higher. And we are using 325 mesh silica, so it dissolves in the melt better.

Inbound Photo Links

Cone 6 black with a second layer of oatmeal glaze

An example of variegation on a tile surface that occurred when using raw manganese dioxide

Cone 10 Reduction, the home of an amazing oxide: Iron

Variegating effect of sprayed-on layer of titanium dioxide

Variegation gone too far!

Same iron red glaze on black stoneware and on porcelain

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