Techno File: Four Ways to Reliable Red Ceramic Glazes

Who can resist a beautiful bright red glaze? But red is also one of the most difficult colors to achieve in ceramic glazes. But it may not be as tough as you thought – as long as you choose the right method for your work.

In this post, Dave Finkelnberg explains four ways to get great red glazes and shares four fabulous red glaze recipes, from low-fire to high fire reduction! – Jennifer Poellot Harnetty, editor.


Chinese ceramic lore includes the tragic tale of a potter who became so frustrated with his many failures to produce a red glazed pot for his emperor that he finally threw himself into his kiln. When the kiln cooled and was opened, so the tale goes, the finest red glazes were found. Modern materials make it considerably easier to produce red glazes, although challenges remain. Knowing the chemistry and firing requirements of the types of red glazes will save you from throwing yourself into your kiln.

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Like to geek out on glazes?
Use The Ceramic Spectrum, the accepted standard for understanding glazes, as your guide.
Read more and download an excerpt!


 

Defining the Terms

Stain—Essentially a frit made of colorant chemicals, compatible fluxes, and possibly glass formers, which has been melted, cooled, and pulverized to a fine powder.

 

Encapsulated Stain (Inclusion Pigments)—A new generation of stable stains made by melting metallic colorants with zirconium silicate, cooling the melt, and grinding the result to a fine powder. Because zirconium silicate is refractory, stains containing it can produce brighter colors up to cone 10 using pigments that would otherwise fade at high temperatures. These colors are safe to use in the studio.

 

Flux (molar) Unity or Seger Formula—The chemical composition, commonly of a fired glaze, expressed as one mole of total flux to the number of moles of all other ingredients in the glaze. The term ‘unity molecular formula’ does not specify whether the flux, alumina, or glass formers are in unity. Each can be at different times for different purposes, but flux unity is used almost universally by ceramic artists.


 

Selenium/Cadmium Red

The easiest, most reliable path to red is to use relatively recently developed cadmium inclusion stains. These stains also contain selenium combined with sulfur, and they will produce the full range of colors in the red spectrum from yellow through orange to brilliant red. They work in both translucent and opaque glazes, in oxidation and reduction firings, and at all firing temperatures.

Historically, cadmium and selenium have produced glamorous red glazes but only at low temperatures. The colorants burned out at higher kiln temperatures and the resulting red glazes were pale. The discovery of the encapsulation process (the melting of the colorants into a zirconium silicate glass at high temperatures) has now made the many hues of yellow through red reliable at temperatures through cone 10 in both oxidation and reduction atmospheres. These stains are refractory at pottery temperatures and do not melt much, if at all. However, the manufacturers recommend that the stain not be ball milled.

As with lead, cadmium stains can produce food-safe colors. However as with lead, cadmium under certain circumstances can be leached from the fired glaze. A sample of any cadmium stain-tinted glaze used on potential food surfaces should be tested for leaching by a qualified laboratory.

Inclusion stains are suitable for use in a wide variety of base glazes. The amount of stain to use must be determined by testing, because the base glaze and application thickness will influence the fired results. Reds produced with these stains, while very reliable, tend to be flat and lack the variation produced when using oxides and/or atmospheric kilns.

 

 

 

Low-fire Satin Glaze
Cone 04
Ferro Frit 3195 50 %
Dolomite 30
EPK Kaolin 20
100%
Add: Encapsulated Mason Stain
#6025 Coral Red 15 %

 


 

Iron Red

Iron red glazes often have vibrant names like Tomato Red or Ketchup Red, and they are generally warm reds. The true reds are produced in oxidation around cone 5. By cone 10, they tend to turn toward orange or persimmon. High-iron glazes fired in heavy reduction will turn maroon to black.

Iron reds are mainly iron saturated, which means they contain between 5 and 10% iron oxide in the glaze recipe (most recipes use 7% or more). Iron reds with bone ash (calcium phosphate) as a source of phosphorous (phosphorous in general causes opalescence and brighter colors) typically contain on the order of 10%.

Even considering the above specifications, there is wide variation in iron red recipes. Traditional persimmon or kaki recipes, for example, are very high in both alumina and silica but contain no phosphorous.

The source of iron oxide is important to the color produced and is possibly the most variable colorant used in glazes. The percentage of iron, particle size, and amount of clay, silica or other contaminants may be dramatically different from one source of iron oxide to another.

Iron Red Glaze
Cone 10
Bone Ash 2.91 %
Pearl Ash (Potassium Carbonate) 10.68
Whiting 25.24
Custer Feldspar 6.80
Grolleg Kaolin 35.92
Silica 18.45
100.00 %
Add: Red Iron Oxide (Spanish) 9.71 %
From Pete Scherzer, CM Sept. 2003

 

Copper Red

Copper reds are achieved between cones 5 and 11 by reducing the copper (either copper oxide or copper carbonate) in the glaze. Only a small quantity of copper is necessary for this; 0.25% copper carbonate is sufficient, though more is often used. The red color is aided by the presence of a limited amount of tin. Iron can also help produce a red color in copper red glazes, but too much iron will lead to muddy reds.

The various hues of copper red are influenced by the amount of alumina, magnesium, and boron present in the glaze. High alumina tends to produce cooler reds, as does magnesium, while high boron produces warmer reds.

Copper red glazes tend to be somewhat fluid, so glaze runs should be guarded against in glaze application. Boron particularly enhances the fluidity at cone 10. Where copper reds flow off of rims or high points, they tend to turn white.

Oxidation copper reds in electric kilns are achieved by mixing a reducing agent, silicon carbide, with the copper in the glaze. Because silicon carbide can be a source of glaze blisters and pinholing, its use presents its own set of problems in the studio.

At cone 10, any combination of glaze ingredients that contains, in terms of flux (molar) unity, 0.3 moles of alkalis, 0.7 moles of alkaline earths (preferably most or all as CaO), 0.4 moles of alumina, 3.5 moles of silica, 0.15 moles of B2O3, 1% tin, and 0.5 % copper carbonate can produce a fine copper red if properly fired.

Understanding and controlling the reduction atmosphere in a kiln to achieve copper reds is usually by far the most difficult part of working with this family of glazes. The glaze above is best if the kiln is placed in moderate reduction at cone 010 and held there until cone 9 drops. The kiln can then be soaked in oxidation until cone 10 is down. A smoky fire, as used with carbon trap glazes, is never necessary to achieve copper reds. In fact, a sooty atmosphere in the kiln is likely to produce gray, dingy copper reds due to carbon trapping.

 

 

 

 

 

Copper Red #11 Glaze
Cone 10 Reduction
Colemanite 10.80 %
Whiting 15.73
Kona F-4 (sub Minspar 200) 15.57
Nepheline Syenite 20.43
English China Clay 1.48
Silica 35.99
100.00 %
Add: Tin Oxide 1.72 %
Copper Carbonate 0.42 %
A vibrant red that may turn blue, green, or purple where thick; runs when thick. From Andy Cantrell, CM May 2000.

 


 

Chrome-Tin Pink

Chrome-tin pink glazes are, as their name implies, a combination of chrome and tin that produces somewhat cool reds from a light pink to a deep burgundy. The combination works well from low fire into the cone 6 range, but poorly above cone 9.

According to Cullen Parmelee in his book Ceramic Glazes, the glaze chemistry necessary is fairly specific: calcium is the most important flux because it gives the color a greater stability and a more fiery red color while sodium promotes yellow shades. Boron should be limited because it tends to shift the color toward purple. Additionally, if your base glaze contains barium, the color effects will be stronger in the absence of boron. Zinc should be avoided because chrome and zinc can interact to produce brown. High alumina works against the red. Because a glaze can dissolve some of the clay body, changing the alumina and flux content of the glaze, these glazes require careful testing.

A good starting point for creating a chrome-tin glaze at cone 6, in terms of flux unity, is from 0.7 to 0.9 moles CaO, from 0.1 to 0.3 moles alkalis, 0.25 to 0.3 moles Al2O3, not more than 0.25 moles B2O3, 2.5 to 3 moles SiO2, up to 7.5% tin oxide, and not more than 0.5% of chrome oxide (0.15% is often enough).

A thin application of a chrome-tin glaze will tend toward gray rather than red. Close examination of the glaze with a magnifying glass will reveal the red is present in small islands within a matrix of clear glass. This explains why a thicker application will produce more vibrant red. Chrome-tin pinks can be produced more reliably from commercial stains than from the raw materials, but stains are not required to produce this red.

Chrome-tin pinks present special challenges when working at cone 5–6, because chrome (either from chrome green glazes or chrome-tin reds) can vaporize at the peak of the firing and give a pink blush to adjacent ware with white glazes containing tin. Since tin is occasionally used as an opacifier, this is not an uncommon occurrence. This can be especially problematic if working with commercial glazes where the full list of ingredients is not obvious.

 

Raspberry
Cone 6
Whiting 20.0 %
Nepheline Syenite 18.0
Ferro Frit 3134 14.0
OM-4 Ball Clay 18.0
Silica 30.0
100.0 %
Add: Tin Oxide 7.5 %
Chrome Oxide 0.2 %
This glaze often benefits from a controlled slow cooling. From Mastering Cone 6 Glazes by John Hesselberth and Ron Roy.

 

**First published in 2011
Comments
  • Sorry for the silly question, but would I be adding the Selenium/Cadmium Red stain to say, a clear glaze? Like Duncan clear glaze? If so, what’s a rough percentage of stain to liquid glaze to start? OR…Am I actually dipping the piece in the stain? Then clear glaze? If so, am I adding water to the mixed dry ingredients? To arrive at what specific gravity? So confused.

  • Jonathan G.

    I find that this type of “thumb” test is of limited usefulness because when dipping the thickness of the glaze is dependent on the absorption rate of the pot. Thick walled pots adhere more glaze than thin pots; lower-temp bisque adhere more glaze than higher temp bisque. So, it’s better to check the actual thickness of the glaze on an applied test which is representative of your work and firing.

  • Kathleen H.

    Mandy and Elizabeth,
    For green glazes, just remember, when using copper in amounts 5% or more the glaze should be tested for food safety. Larger amounts of copper are usually not food safe.

  • This is why we produce glaze test pieces, for each new glaze you should produce up to a dozen test pieces. The glaze on different clays, differen temperatures, different dipping times; double dipping, over slip, over another glaze as a double dip and of course under a different glaze as a double dip-and remember to leave a decent margin to allow for dripping. Good luck

  • Genevieve N.

    Since tin oxide is so expensive, would it work (meaning, wouls it give as good a result) if we substituted it with zircopax ?

  • Elizabeth L.

    For Mandy: In oxidation simply try testing several clear glazes with a tiny bit of copper in them-.5- 2.5% for a clear pale green.It depends on the composition of the glaze.if you get one that pleases you and you will, try increasing the copper to see what happens. With increased copper, you can end up with a green glaze with black metallic areas. This has worked for me at cone 9/10 and cone 6.

  • Beth T.

    Re: glaze application, I’ve been teaching for many years in community studios which glaze fire in electric kilns to cone 6. We buy several different brands of powdered glaze which is mixed with water to a hydrometer reading around 50. Like many who commented above, I was taught to use a knuckle test when I learned glazing at cone 10 in gas reduction. BUT the cone 6 commercial glazes are all over the map in terms of this test…Laguna glazes seem thin and Coyote glazes seem thick. Now I only trust the hydrometer…and I know other potters who only trust the specific gravity test. Also, our dipping times are about 3 seconds in any glaze, sometimes less, and that time is shorter for each dip if glazes are layered.

  • Mandy Q.

    We all know that red glazes are tricky but this is a very helpful post. I have been trying to get a good green at stoneware temperatures and that is also difficult especially in an oxidising kiln. Has anyone had any success?

  • Nigel C.

    The liquid glaze should have a consistancy of single cream, I usually dip a finger into the glaze for a second or two to test this, if the glaze is too thin (like fresh milk) there is too much liquid and it needs to stand for a few days to decant the extra water. I also test the dryness of the bisque ware by placing my lower lip to the pot,if it is not sticky to the lip then the glaze needs to be less fluid (thicker)to adhere to the pot.You can test the thickness of the glazed pot by using a simple pin, ideally it should be about 1 to 2 mm. The pin hole disappears in the firing.

  • Viva J.

    Ditto the above for a thickness test. I have always found a dip time of 5 seconds a good rule of thumb. The glaze on the pot should be the thickness of a dime after dipping. This allows for a second dip into another glaze for fun and contrast. Be sure the first glaze is dry before dipping into another glaze. The second dip is about 3 seconds. But remember test, test, test, and keep good records. A folder taped to the top of the bucket lid can hold all pertinent records on each glaze, and is a time saver.

  • Subscriber T.

    Your instructor should direct you best as they will know the glazes. My personal rule of “thumb” is: test the thickness of th glaze (viscosity or specific gravity) by dipping your thumb into the well mixed glaze bucket and hold for 3 seconds (1001,1002,1003) and remove it. If you can still see the knuckle lines that is a good dip time, if not, the dip is too long. Make sure you pot bottoms and edges are cleaned of glaze so they don”t run on the kiln shelves. Good luck.

  • Subscriber T.

    I have just started to do ceramics at the local tech collage and have found that, like above, we have glazes but they don’t have the dipping time on them. It would be most helpful to have this info on the test tile/recipe. Is there a reason for this?

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