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In ceramics, reactive glazes have variegated surfaces that are a product of more melt fluidity and the presence of opacifiers, crystallizers and phase changers.
Key phrases linking here: reactive glazes, variegation - Learn more
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 on line, 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.
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.
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.
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.
"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).
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).
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.
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.
Rutile blue glazes are difficult, blistering and pinholing are very common. You must get it right on the first firing because pinholes and blisters will likely invade on the second. On the second firing the melt fluidity increases, the glaze runs and creates thicker sections in which the bubbles percolate and just do not heal well during cooling (even if it is slow). When finishing leather hard or dried ware do not disturb thrown surfaces any more than necessary. Make sure that ware is dry before the glaze firing. Do not put the glaze on too thick. Limit the melt fluidity (so it does not pool too thickly in any section). Do not fire too high. Drop and hold firing schedules can help a lot (coupled with a slow cool if needed).
Here it is fired to cone 8 where the melt obviously has much more 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.
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 more melt fluidity is needed, decrease the percentage of CaO. There is a positive: For opaque glazes, this effect can actually enable the use of less opacifier.
It makes sense to maximize the percentage of wood ash. This glaze was the product of preparing a large ash batch and a project to develop a glaze specifically from it. This one contains a little iron to brown it. Ash generally contains low percentages of Al2O3, a critical oxide needed for stable glass development. I added kaolin (about 20%), it suspends the slurry and supplies Al2O3. Ashes contain lots of fluxing oxides, but they still may need a little help to melt a glaze at cone 6, so I added feldspar (it also supplies more needed Al2O3). If better melting add some borax frit (like Ferro frit 3195). If crazing occurs use frit 3249 instead. Use whatever feldspar and kaolin you have.
This is a cone 10R copper red. First, it is thick. "Thick" brings it own issues (like running, blisters, crazing). But look what is under the surface. Bubbles. They are coming out of that body (it is not vitreous, still maturing and generating them in the process). The bubbles are bringing patches of the yellow glass below into the red above. Normally bubbles are a problem, but in this decorative glaze, as long as everything goes well, they are a friend.
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.
I am comparing 6 well known cone 6 fluid melt base glazes and have found some surprising things. The top row are 10 gram GBMF test balls of each melted down onto a tile to demonstrate melt fluidity and bubble populations. Second, third, fourth rows show them on porcelain, buff, brown stonewares. The first column is a typical cone 6 boron-fluxed clear. The others add strontium, lithium and zinc or super-size the boron. They have more glassy smooth surfaces, less bubbles and would should give brilliant colors and reactive visual effects. The cost? They settle, crack, dust, gel, run during firing, craze or risk leaching. Out of this work came the G3806E and G3806F.
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.
Variegation gone too far!
Cone 10 Reduction, the home of an amazing oxide: Iron
Variegating effect of sprayed-on layer of titanium dioxide
Cone 6 black with a second layer of oatmeal glaze
Phase separation is a phenomenon that occurs in transparent ceramic glazes. Discontinuities in the internal glass matrix affect clarity and color.
Understand your a glaze and learn how to adjust and improve it. Build others from that. We have bases for low, medium and high fire.
Ceramic glaze variegation refers to its visual character. This is an overview of the various mechanisms to make glazes dance with color, crystals, highlights, speckles, rivulets, etc.
Ceramic glazes are glasses that have been adjusted to work on and with the clay body they are applied to.
Random material mixes that melt well overwhelmingly want to be glossy, creating a matte glaze that is also functional is not an easy task.
MA6-C - Alberta Slip Floating Blue Cone 6
Plainsman Cone 6 Alberta Slip based glaze the fires bright blue but with zero cobalt.
GR6-M - Ravenscrag Cone 6 Floating Blue
Plainsman Cone 6 Ravenscrag Slip based version of the popular floating blue recipe.
Where do I start in understanding glazes?
Break your addiction to online recipes that don't work or bottled expensive glazes. Learn why glazes fire as they do. Why each material is used. How to create perfect dipping and drying properties. Even some chemistry.
A raw TiO2-containing mineral used in ceramics to color and variegate glaze surfaces.
|By Tony Hansen|
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