|Monthly Tech-Tip |
In ceramics, reactive glazes have variegated surfaces that are a product of more melt fluidity and the presence of opacifiers, crystallizers and phase changers.
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, addition of speckling agents, phase separation, layering, and opacity variations (occurring with thickness variations) and multilayering.
You can make your own reactive glazes by adding variegators, opacifiers and colorants to your a 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 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.
Rutile blue glazes are difficult, blistering and pinholing are very common. You must get it right on the first firing or pinholes and blisters will often invade on the second. The melt fluidity increases, it runs and creates thicker sections in which the bubbles just 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.
An example of variegation on a tile surface that occurred when using raw manganese dioxide (likely due to gassing)
The referred to surface is the outside of this large bowl. The base glaze (inside and out) is GA6-D Alberta Slip glaze fired at cone 6 on a buff stoneware. The thinness of the rutile needs to be controlled carefully, the only practical method to apply it is by spraying. The dramatical effect is a real testament to the variegating power of TiO2. An advantage of this technique is the source: Titanium dioxide instead of sourcing TiO2 from the often troublesome rutile.
The underglaze is G1214M cone 6 black (adds 5% Mason 6666 black stain). Overglaze left: GR6-H Ravenscrag Oatmeal. Overglaze right: GA6-F Alberta Slip oatmeal. Both produce a very pleasant silky matte texture (the right being the best). Both layers are fairly thin. In production it would be best to spray the second layer, keeping it as thin as possible. It is also necessary to adjust the ratio of raw to calcined Alberta or Ravenscrag Slips to establish a balance between drying hardness but not too much drying shrinkage (and resultant cracking).
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).
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 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.
This is Ravenscrag Slip Oatmeal over a 5% Mason 6666 stained glossy clear at cone 6. You have to be careful not to get the overglaze on too thick, I did a complete dip using dipping tongs, maybe 2 seconds. Have to get it thinner so a quick upside-down plunge glazing only the outside is the the best way I think. You may have to use a calcined:raw mix of Ravenscrag for this double layer effect to work without cracking on drying.
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, excessive running, susceptibility to leaching). As a final step the recipe will be adjusted as needed. We eventually chose G3806B and further modified it to reduce the thermal expansion.
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.
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. In the end I will pick one or two, fix the issues and provide instructions.
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).
It makes sense to maximize the percentage of wood ash. Since different batches and types of wood ash have drastically different chemistries how can you have a glaze have a high percentage? 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 stain it brown, this brings out the variegation better. 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 needed Al2O3 also). If that is not enough flux, I add a little gerstley borate or a borax frit. If crazing occurs silica is needed. In the end I got a recipe with about 50% ash.
The glaze is G1214Z cone 6 base calcium matte. 5% titanium dioxide has been added. This Plainsman M390 tile was fired at cone 6 using the PLC6DS firing schedule. Titanium can create reactive glazes, like rutile, even with matte surfaces (provided the glaze has good melt fluidity). Calcium mattes host crystallization and work particularly well. Because titanium dioxide does not contain iron oxide lighter colors and better blues are possible than with rutile. Like rutile, the effects are dependent on the cooling rate of the firing, faster cools produce less reactivity.
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.
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.
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.
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.
Phase separation is a phenomenon that occurs in transparent ceramic glazes. Discontinuities in the internal glass matrix affect clarity and color.
Understanding your transparent glaze and learning how to adjust its melt fluidity, thermal expansion, color response, etc is a base on which to build all your other glazes.
Random material mixes that melt well overwhelmingly want to be glossy, creating a matte glaze that is also functional is not an easy task.
GA6-C - Alberta Slip Rutile 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. It can be found among others at http://ravenscrag.com.
Where do I start in understanding glazes?
Break your addiction to online recipes that don't work. Get control. Learn why glazes fire as they do. Why each material is used. Some chemistry. How to create perfect dipping and drying properties. Be empowered. Adjust recipes with issues rather than sta
This is an overview of the various mechanisms you can employ to make glazes dance with color, crystals, highlights, speckles, rivulets, etc.