Digitalfire Ceramic Glossary

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Variegation, Reactive Glazes

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 sometimes called 'reactive glazes' because 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).


Cone 6 GA6-C variegated blue showing different thicknesses (4% rutile+ 20% frit 3134 in Alberta Slip)

Boron blue in low fire transparent glazes

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.

Variegation and phase separation with about 5% rutile

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.

A highly variegated cone 6 cobalt rutile ash glaze

Example of a variegated wood ash glaze at cone 6 oxidation. It contains a small amount of cobalt as well as some rutile.

Cone 10R variegation and crystal magic

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.

A variegated glossy blue ravenscrag slip glaze

Cone 6 black with a second layer of oatmeal glaze

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).

Layering glazes to get variegation

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.

Your boron glaze might melt alot earlier than you think

The porcelain mug on the left is fired to cone 6 with G2926B clear glossy glaze. This recipe only contains 25% boron frit (0.33 molar of B2O3). Yet the mug on the right (the same clay and glaze) is only fired to cone 02 yet the glaze is already well melted! What does this mean? Industry avoids high boron glazes (they consider 0.33 high boron) because this early melting behavior means gases cannot clear before the glaze starts to melt (causing surface defects). For this reason, fast fire glazes melt much later. Yet many middle temperature reactive glazes in use by potters have double the amount of B2O3 that this glaze has!

Variegating effect of sprayed-on layer of 100% titanium dioxide

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.

Making a wood ash glaze

Iron stained wood ash glaze fired at cone 6. These glazes need to be reformulated with each new batch of ash you get (since the ash chemistry changes). This one was formulated by quadraxial blending the ash with feldspar, silica and kaolin to get a sweet spot, then fine tuning and finally adding about 4% rutile to variegate it.

An example of variegation on a tile surface that occurred when using raw manganese dioxide (likely due to gassing)

Cone 10 Reduction firing is the home of the most magic oxide in ceramics: iron

It is a powerful glaze flux, variegator and crystalizer, a colorant of many characters in bodies and glazes and a specking agent like no other. And it is safe and cheap!

Variegation gone too far!

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.

Why does this glaze variegate like this?

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.

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.

A super glassy ultra-clear brilliantly glossy cone 6 clear base glaze? Yes!

I am comparing 6 well known cone 6 fluid melt base glazes and have found some surprising things. The top row are 10 gram 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.

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

These melt-flow and ball-melt tests 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.

Out Bound Links

  • (Materials) Rutile - TiO2 - Iron Titanium Mineral

In Bound Links

  • (Glossary) Phase Separation

    This phenomenon is responsible for some of the mos...

  • (Articles)

    Variegating Glazes

    This is an overview of the various mechanisms you can employ to make glazes dance with color, crysta...

  • (Properties) Glaze Variegation

    In contrast to the typical homogeneous surfaces of...

  • (Glossary) Matte Glaze

    A glaze that is not glossy. Of course, unmelted gl...

  • (Glossary) Base Glaze

    A base glaze is one having no opacifiers, variegat...

By Tony Hansen

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