A Low Cost Tester of Glaze Melt Fluidity
A One-speed Lab or Studio Slurry Mixer
A Textbook Cone 6 Matte Glaze With Problems
Adjusting Glaze Expansion by Calculation to Solve Shivering
Alberta Slip, 20 Years of Substitution for Albany Slip
An Overview of Ceramic Stains
Are You in Control of Your Production Process?
Are Your Glazes Food Safe or are They Leachable?
Attack on Glass: Corrosion Attack Mechanisms
Ball Milling Glazes, Bodies, Engobes
Binders for Ceramic Bodies
Bringing Out the Big Guns in Craze Control: MgO (G1215U)
Ceramic Glazes Today
Ceramic Material Nomenclature
Ceramic Tile Clay Body Formulation
Changing Our View of Glazes
Chemistry vs. Matrix Blending to Create Glazes from Native Materials
Concentrate on One Good Glaze
Cone 6 Floating Blue Glaze Recipe
Copper Red Glazes
Crazing and Bacteria: Is There a Hazard?
Crazing in Stoneware Glazes: Treating the Causes, Not the Symptoms
Creating a Non-Glaze Ceramic Slip or Engobe
Creating Your Own Budget Glaze
Crystal Glazes: Understanding the Process and Materials
Deflocculants: A Detailed Overview
Demonstrating Glaze Fit Issues to Students
Diagnosing a Casting Problem at a Sanitaryware Plant
Drying Ceramics Without Cracks
Duplicating Albany Slip
Duplicating AP Green Fireclay
Electric Hobby Kilns: What You Need to Know
Fighting the Glaze Dragon
Firing Clay Test Bars
Firing: What Happens to Ceramic Ware in a Firing Kiln
First You See It Then You Don't: Raku Glaze Stability
Fixing a glaze that does not stay in suspension
Formulating a Clear Glaze Compatible with Chrome-Tin Stains
Formulating a Porcelain
Formulating Ash and Native-Material Glazes
Formulating Your Own Clay Body
G1214M Cone 5-7 20x5 Glossy Base Glaze
G1214W Cone 6 Transparent Base Glaze
G1214Z Cone 6 Matte Base Glaze
G1916M Cone 06-04 Base Glaze
G1947U/G2571A Cone 10/10R Base Matte/Glossy Glazes
Getting the Glaze Color You Want: Working With Stains
Glaze and Body Pigments and Stains in the Ceramic Tile Industry
Glaze Chemistry Basics - Formula, Analysis, Mole%, Unity, LOI
Glaze chemistry using a frit of approximate analysis
Glaze Recipes: Formulate Your Own Instead
Glaze Types, Formulation and Application in the Tile Industry
Having Your Glaze Tested for Toxic Metal Release
High Gloss Glazes
How a Material Chemical Analysis is Done
How desktop INSIGHT Deals With Unity, LOI and Formula Weight
How to Find and Test Your Own Native Clays
How to Liner-Glaze a Mug
I've Always Done It This Way!
Inkjet Decoration of Ceramic Tiles
Interpreting Orton Cones
Is Your Fired Ware Safe?
Leaching Cone 6 Glaze Case Study
Limit Formulas and Target Formulas
Low Budget Testing of the Raw and Fired Properties of a Glaze
Low Fire White Talc Casting Body Recipe
Make Your Own Ball Mill Stand
Making Glaze Testing Cones
Monoporosa or Single Fired Wall Tiles
Organic Matter in Clays: Detailed Overview
Outdoor Weather Resistant Ceramics
Overview of Paper Clay
Painting Glazes Rather Than Dipping or Spraying
Particle Size Distribution of Ceramic Powders
Porcelain Tile, Vitrified or Granito Tile
Rationalizing Conflicting Opinions About Plasticity
Ravenscrag Slip is Born
Recylcing Scrap Clay
Reducing the Firing Temperature of a Glaze From Cone 10 to 6
Single Fire Glazing
Soluble Salts in Minerals: Detailed Overview
Some Keys to Dealing With Firing Cracks
Stoneware Casting Body Recipes
Substituting Cornwall Stone
Super-Refined Terra Sigillata
The Chemistry, Physics and Manufacturing of Glaze Frits
The Effect of Glaze Fit on Fired Ware Strength
The Four Levels on Which to View Ceramic Glazes
The Majolica Earthenware Process
The Physics of Clay Bodies
The Potter's Prayer
The Right Chemistry for a Cone 6 MgO Matte
The Trials of Being the Only Technical Person in the Club
The Whining Stops Here: A Realistic Look at Clay Bodies
Those Unlabelled Bags and Buckets
Tiles and Mosaics for Potters
Toxicity of Firebricks Used in Ovens
Trafficking in Glaze Recipes
Understanding Ceramic Materials
Understanding Ceramic Oxides
Understanding Glaze Slurry Properties
Understanding the Deflocculation Process in Slip Casting
Understanding the Terra Cotta Slip Casting Recipes In North America
Understanding Thermal Expansion in Ceramic Glazes
Unwanted Crystallization in a Cone 6 Glaze

Volcanic Ash
What Determines a Glaze's Firing Temperature?
What is a Mole, Checking Out the Mole
What is the Glaze Dragon?
Where Do I Start?
Why Textbook Glazes Are So Difficult

Variegating Glazes

Description

This is an overview of the various mechanisms you can employ to make glazes dance with color, crystals, highlights, speckles, rivulets, etc.

Article

While industry often avoids so-called 'reactive glazes' potters do not like the 'porcelain sink look' of typical industrial glazes. As a general rule the latter are more practical because they fire consistently while the former are typically temperamental and difficult. Potters who work in reduction fired high stoneware come by glazes with interesting surfaces quite easily (although often inconsistent). This is mainly because the raw natural materials they employ melt easily at these temperatures and their unprocessed nature (with particles of widely varying size, chemistry and mineralogy) creates a melt concoction that solidifies into surfaces having all sorts of crystal, rivulet, speckle and color variations for light to dance on.

But can such glazes be made for lower temperatures in oxidation? Yes, and potentially better because there is a magic ingredient available there that is almost never used by reduction firing potters (more on that in a moment). However, while blindly mixing hundreds of recipes from books and web pages might scare up a good high fire glaze or two, at middle temperatures there are things to worry more about (e.g. Crazing, leaching) and much fewer materials melt well. Knowing the 'mechanisms' of the various kinds of variegation is the best way to achieve, enhance and control it. Another way of putting it: 'understanding' these mechanisms opens up more doors than the lower temperature conditions close.

Consider an example of the vase on the cover of the mastering cone six glazes book by Ron Roy and John Hesselberth. This glaze exhibits a number of different kinds of variegation (e.g. crystallization, rivulets, speckling, opacity variation). Look closer and you will see the reason this glaze 'dances': melt fluidity. Notice how it runs down to the shoulder. All kinds of things happen in glazes that melt this much. If you compare the chemistry of this glaze with a typical cone 6 transparent the difference becomes obvious: much higher boron, it melts big time at cone six. Add some rutile and iron to a fluid base like this and all kinds of things happen. As you can see, variegation in glazes does not have to be a mystery, it happens for some understandable reasons.

To review, an important part of understanding reactive glazes is being familiar with the term 'fluidity'. A fluid glaze is one that melts more than normal to form a fluid molten liquid. Like their high feldspar high temperature counterparts, lower firing fluid glazes like to run off the ware. However at lower temperatures highly fluid glazes dance for another price: crazing. This is because typical fluxes have high expansion and low expansion silica and alumina are less plentiful. However this price does not necessarily have to be paid. Why? Through the magic of boron. Boron is an oxide contributed by most frits and gerstley borate, colemanite and ulexite. The magic is that it melts like a super-flux yet forms a glass like silica and has a really low thermal expansion.

Try looking very closely at a heavily variegated glaze and you will be able to formulate some theories on the way in which these visual effects could have formed during melting and freezing. Consider some of the ways you can provide conditions for these effects to grow and ways in which you can physically create them.

Color highlighting

Varying the thickness of a transparent (or partly opacified) colored glaze will vary the intensity of color with depth (especially where the layer thins on the edges of sharp contours). Thickness variations can be achieved by pouring, double-dipping, brushing, waxing, and incising techniques.

Surface Crystal Growth

I am referring here to normal stoneware glazes and use the term 'fluid' in that context. The extreme, to which I am not referring, is crystalline glazes, they have almost no alumina and are so fluid that a catcher is needed to capture the runoff. These can grow dazzling single crystals of huge size, but alas, they are impractical for functional ware (they craze like mad, are soluble, inconsistent, soft and very difficult to apply and fire) so let us return to the real world of stoneware glazes. Fluid glazes like to form micro crystals on the surface during cooling (low alumina, high flux). TiO2 materials like titanium dioxide and rutile seed crystal networks and encourage their growth (of course they do alter the color also, both because of their interaction with other oxides and, in the case of rutile, because of the presence of coloring oxides). A thin rutile wash applied to a glaze surface can even act as a crystal growth catalyst. High calcium and boron levels encourage the formation of calcium-borate crystals, high zinc stoneware glazes also crystallize when they are fluid. The addition of up to 4% tin in such glazes can magnify the effect. Slow cooling greatly enhances crystal growth. Small amounts of lithium (e.g. 1%) can have a remarkable variegating effect on rutile glazes, especially when colorants like iron are present. It is worth mentioning that unwanted crystal growth on glaze surfaces is termed "devitrification", typically it is seen as bad by industry.

Oxide Saturation

Certain colorants (like iron, copper, manganese) and stains can be added in higher than normal amounts to fluid glazes to produce a completely saturated and metallic fired surface. This phenomenon is akin to the precipitation of sugar crystals while cooling a sugar saturate water solution. In this example, 8% iron has been used. Note that such glazes are obviously not for food surfaces.

Specking Agent

You can add a coloring oxide that contains particulate matter that speckles the glaze surface. Manganese granular, illmenite, and granular rutile are examples. However these materials are heavy and tend to settle in glazes that are too fluid. Coarser grades of iron oxide and cobalt oxide often produce small specks in unmilled glazes. Using pure metal powder or filings is also also an option.

Multilayering

Layered glazes interact with each other chemically and mechanically. Two molten liquids will obviously diffuse into each other during melting. This diffusion does not occur evenly however since one glaze will be more fluid than the other; surfaces will be vertical, horizontal or in between; bubbles from body and glaze decomposition are rising through the melt stiring things up; the fluidity of the boundary zone will be variable, etc. In addition the changes in chemistry of the boundary zone will bring about changes in surface tension, fluidity, ability to dissolve unmelted particles, etc. Thus the result is a variegated visual appearance.

Double layering of different glazes produces variegation well when the lower layer is more fluid and the upper is stiffer (it will tend to break into islands revealing rivulets of the lower one). Or a much more subtle effect would be to put a thin layer of matte white over a glossy white, the effect will be something that no single-layer glaze could produce. Similarly, a contrasting colored fluid glaze over a stable one often has its own variegation surprises.

However be aware of the problems associated with double layer glazing (crawling during firing because of drying-induced cracks in the double layer during drying, these happen because the upper layer tends to compromise the body bond of the lower one when it shrinks during drying, especially if applied too thick or onto wet ware). Use glazes with lower or less plastic clay content for multilayer work, heat the ware before glazing so the glaze dries fast or bisque the first layer on.

Another mechanism of double layering is the use of a lower layer that releases gases during the melting phase. During melting the lower one will percolate up through the top layer creating molten craters that later heal leaving the visual after-effects of the process.

Phase Differences

The glass matrix in a fired glaze can separate (or fail to mix) during melting forming globules of different glass chemistry. These reflect and refract light differently and thus variegate the visual characteristics of the surface, especially where colorants that react differently to the different phases are present. As expected, the visual effects are greatest when the glaze is thicker. 'Techies' look for chemistries that encourage phase separation. However populations of particles that have widely varying melting characteristics will also encourage it (e.g. really fluid particles of frit or lithium in a mix of clay, silica, colorant, tin and rutile will supply a population of particles with little in common so they won't melt as a team, the result will be variegation).

Combinations

Use combinations of the above to variegate surfaces even more. The popular Floating Blue cone 6 recipe is a good example. Its color varies with thickness so it highlights irregularities in the surface. Phase separation in the translucent matrix makes the color 'swirl' in patterns of blue. Titanium crystals in the matrix make it sparkle. The growth of calcium-borate crystals on the surface appear to float over a deep blue background. However, while this is a popular glaze among potters it is understandably temperamental!

Some glazes greatly amplify thickness differences in a way that appears related to a number of the above factors. In these there is a dramatic change in color and character with thickness differences. The classic Albany:Tin:Lithium glaze is a good example. The effect is related to a large extent to a non-opacifying reaction between the tin and iron that needs a certain thickness to manifest itself fully. This glaze also works well using boron as a flux (instead of lithium).

Physical means

An often overlooked method of creating variegation is actual physical intervention.

Conclusion

Knowing how to variegate using some of these techniques may not get you a job in a porcelain factory, but it will get the admiration of people who see your work. Variegation is typically a microsurface and glass structure chaos, controlling it can sometimes be like controlling chaos.

Related Information

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.

A cone 6 fluid iron glaze has a completely different surface when cooled slowly

The mug on the left has been cooled slowly (and crystallizes). On the right it was cooled quickly (and is glossy).

A matte white glaze over a fluid dark colored glaze at cone 6

GA6-A Alberta Slip rutile blue on a dark cone 6 body

4% rutile with 80 Alberta Slip and 20 of Frit 3134. Plainsman M390. This recipe contains no cobalt yet is bright blue!

Reduction high temperature iron crystal glaze

This is what about 10% iron and some titanium and rutile can do in a transparent base glaze with slow cooling at cone 10R on a refined porcelain.

Crystallization of Rutile at cone 6 completely subdued? How?

These glazes are both 80% Alberta Slip, but the one on the right employs 20% Ferro Frit 3249 accelerate the melting (whereas the left one has 20% Frit 3134). Even though Frit 3249 is higher in boron and should melt better, its high MgO stiffens the glaze melt denying the mobility needed for the crystal growth.

Ravenscrag oatmeal layered over black at cone 6

This is GR6-H Ravenscrag oatmeal over G1214M black on porcelain at cone 6 oxidation to create an oil-spot effect. Both were dipped quickly. You can find more detail at ravenscrag.com.

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.

Ravenscrag Cone 6 Floating Blue on buff stoneware

The clay is Plainsman M340. Unlike Alberta Slip floating blue, this version does contain a little cobalt to help guarantee the blue color.

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.

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.

Links

Articles G1214M Cone 5-7 20x5 Glossy Base Glaze
This is a base transparent glaze recipe developed for cone 6. It is known as the 20x5 or 20 by 5 recipe. It is a simple 5 material at 20% each mix and it makes a good home base from which to rationalize adjustments.
Articles Concentrate on One Good Glaze
It is better to understand and have control of one good base glaze than be at the mercy of dozens of imported recipes that do not work. There is a lot more to being a good glaze than fired appearance.
Articles G1916M Cone 06-04 Base Glaze
This is a frit based boron base glaze that is easily adjustable in thermal expansion, a good base for color and a starting point to go on to more specialized glazes.
Articles G1947U/G2571A Cone 10/10R Base Matte/Glossy Glazes
These starting recipes use no frits and work in oxidation/reduction and are inexpensive to make. They can be used as bases for the whole range of typical cone 10 pottery glazes (celadon, tenmoku, oatmeal, white matte, brown crystal).
Articles Reducing the Firing Temperature of a Glaze From Cone 10 to 6
Moving a cone 10 high temperature glaze down to cone 5-6 can require major surgery on the recipe or the transplantation of the color and surface mechanisms into a similar cone 6 base glaze.
Glossary Glaze Recipes
Stop! Think! Do not get addicted to the trafficking in online glaze recipes. Learn how they work. Understand them. Then make your own or adjust/adapt what you find online.
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.
Projects Properties
Projects Recipes
Properties Glaze Variegation
Materials Rutile

By Tony Hansen


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