An Introduction to Color Birdie Boone

Appears in the Jul/Aug 2015 issue of Pottery Making Illustrated.

As a student, I was taught that the surface of an object is just as important as the object itself. A compelling surface, one that has a clear connection to its form, is one of the things that makes a good pot a great pot. The moment I discovered I could manipulate glaze color, my life was forever changed. Every formal or aesthetic decision is made with intention as I design each form, so they deserve to have an intentional surface as well. My palette tends to be at the softer end of the spectrum, in order to evoke emotional responses. I use colorants in fairly low percentages, and often they are modified, or toned down, by certain other colorants.

Johannes Itten, who developed and taught the first color course at the Bauhaus in the early 1920s, thought of colors as “primordial ideas.” Indeed, one of the reasons color is so powerful is that it can trigger a visceral response that is at once both personal and universal. You don’t need to know glaze chemistry in order to develop color in glazes. Certainly, knowing the chemical properties of a glaze can be helpful for making considered decisions about which colorants to use and which not to bother with, but it isn’t necessary. As you test, you will learn it empirically by looking at the results and seeing how the colorants are reacting in combination with one another as well as how they are reacting to the materials in the glaze recipe.

Easy Blends for Big Results

The coaxial color blend is a great way to start an investigation of color. It provides a lot of diverse color information with fairly little work. Over the years, I have learned that the most difficult part for many people is understanding the coaxial blend diagram.

The coaxial blend consists of a top row of base colorants. In a standard coaxial diagram, there are seven common ones: chrome, iron, copper, cobalt, manganese, rutile, and tin (1). Base colorants can be added or eliminated and the colorant percentages can be adjusted to suit a palette you’re interested in creating. My custom coaxial blend consists of nine base colorants: neodymium oxide, cobalt carbonate, copper carbonate, avocado Mason stain, chartreuse Mason stain, crocus martis, nickel oxide, rutile, and manganese carbonate (2 and 3).

The colorant percentages in the top row of circles are high; this is so that the test results clearly show how the colorants are reacting to the base glaze and to one another. The remaining 21 circles, in a standard coaxial chart, are the blends; in effect, each colorant in the top row is mixed in equal parts with all the other colorants in the top row to create numbers 8–28. Note: The top/base row is mixed by weight, the blends are mixed by volume from the top row.

Choosing a Glaze Base

If you have a glaze base that works well for you, start with that. If there is a glaze you like that already has a colorant or colorants in it, simply remove them from the recipe and you will have your base glaze. You can even test commercial clear glazes in blends. Keep in mind that a transparent glossy glaze is going to give you the brightest color response. Satin and matte glazes are usually higher in alumina, which is what helps create that softer surface and a softer glaze color. If you’re up to the task, test one glossy glaze and one satin or matte glaze and compare. Before you do the whole coaxial blend, you might want to just test the single colorants to see how they are reacting to your chosen base glaze and then choose which ones you want to move forward with.

DIY Coaxial Color Blend

To create your first standard coaxial color blend, start with a clay body (either one you currently use or one you wish to test glazes on), your chosen base glaze, and seven colorants similar to the ones in figure 1. Make and bisque fire 28 tiles. If you typically use a slip under your glazes, be sure to apply it as you normally would.

1: Weigh out the dry ingredients for 1000 grams of a base glaze.

2: Add the same amount of water (either by weight or volume; 1 gram of water = 1mL of water, so either add 1000 grams or 1000mL of water.) This gives you a total of 2000 grams of material, including water.

3: Mix well and sieve (the glaze may seem thin, but this is okay; do not adjust.) The first seven cups will need to be large enough to hold 200g of wet material. The remainder can be smaller. Number all 28 cups with an indelible marker and arrange in the same manner as the diagram.

4: Add 200g of glaze base to each of the cups in the top row (because it’s a wet mix, you need 200g since only 100g of it is glaze material, the other 100g is just water, which has no bearing on the amount of colorant you add.) You will have some left over.

5: Weigh out and add the appropriate amount of colorant to each cup in the top row and mix very well (I use a stiff paintbrush with about an inch-wide brush head; it gets into the corners of the cup so everything mixes well.)

6: Begin the blends under the top row using a measurement of either 1 or 2 teaspoons (5mL and 10mL, respectively). I use a small medicine syringe; use anything you like as long as you are consistent. Add this chosen amount of glaze base with chrome oxide (cup #1) to each cup numbered 8–13.

7: Add the same amount of glaze base with red iron oxide (cup #2) to cups 8 and 14–18.

8: Add the same amount of glaze base with copper carbonate (cup #3) to the column that includes cups 9, 14, and the row that has cups 19–22.

9: Add the same amount of glaze base with cobalt carbonate (cup #4) to the column that includes cups 10, 15, 19, and the row with cups 23–25.

10: Add the same amount of glaze base with manganese dioxide (cup #5) to the column that includes cups 11, 16, 20, 23, 26, and the row with cup 27.

11: Add the same amount of glaze base with rutile (cup #6) to the column that includes cups 12, 17, 21, 24, 26, and the row with cup 28.

12: Add the same amount of glaze base with tin oxide (cup #7) to the column that includes cups 13, 18, 22, 25, 27, and 28.

13: Using stain or an underglaze pencil, label 28 tiles with numbers 1–28 or label the actual colorant percentages directly on each tile (if the glaze slurry is too thin to get a good application onto the tile, at this point, you can add a small amount of flocculant to each cup, such as an Epsom salts solution), or leave it uncovered for several hours to allow some water to evaporate. For accurate results, be sure the glaze coats are around the thickness of a dime.

14: Fire when ready in either a neutral electric kiln or an oxidation or reduction atmosphere in a gas kiln. Be sure to place cones in the kiln so you know exactly what temperature the kiln reaches. Colors can appear different at even slightly different temperatures.

15: Once fired, arrange tests in the same pattern as the diagram and examine your results.

I fired my custom coaxial color blend tiles to cone 6 in an electric kiln (2). In order to help you really see what is happening in my customized blend, I have also separated some of the results into lines for each colorant I used (4). 

Guide for Using Oxides, Carbonates*, and Stains

Chrome oxide (CrO):

• If your base glaze has zinc in any amount, chrome will turn an it an unattractive brown.

• Refractory, meaning it doesn’t melt well and can turn a glossy glaze to matte.

• Very strong, use in very small amounts, less than 0.5%. Higher percentages of chrome may also turn an unattractive brown because there is too much to dissolve into the glaze melt.

Cobalt oxide (CoO)/Cobalt carbonate (CoCO₃):

• A very little goes a very, very long way.

• Tends toward purple in the presence of magnesium in a base glaze.

• Very strong, use in amounts of 3% or less. Higher amounts may give metallic results.

• I typically use 0.1% in the top row of my blends.

• Expensive

Copper oxide (CuO)/Copper carbonate (CuCO₃):

• Tends toward blue or green, depending on the base glaze.

• Mixes well with almost everything else.

• Very strong, use in amounts of 3% or less

• I typically use 0.3–1% in the top row of my blends.

Red iron oxide (Fe₂O₃)/ochre/crocus martis:

• Examples of iron-bearing minerals with slightly different effects

• Very strong, use 2% or less as a color modifier. Amounts higher than 4% can give very

different results, including micro-crystalline growth and metallic brown colors. Iron is a

strong flux, so you may notice this glaze

is runnier with high amounts of iron.

• I typically use 1–2% in the top row of

my blends for soft yellows and to modify

other colorants.

Manganese dioxide (MnO₂)/manganese carbonate (MnCO₃ ):

• Good color modifier

• Very strong, use 1% or less

• I typically use 1–2% manganese carbonate

in the top row of my blends.

• Manganese carbonate is less volatile and doesn’t form specks like manganese dioxide does.

• Creates colors in the purple family

Nickel oxide (NiO)/ nickel carbonate(NiCO₃):

• Good color modifier

• Refractory, (doesn’t melt well and can

make a glossy glaze become matte)

• Very strong, use in amounts of 1% or less

• I typically use 1% NiCO₃ in the top row of

my blends

• Combining nickel and cobalt carbonate or

  oxide often creates grays

• Expensive

Neodymium oxide (Nd₂O₃):

• Rare earth oxide, from the lanthanide

family of elements

• Changes from blue to purple under

various light sources

• Mixes well with almost everything else

• Colorfast in any type of glaze base

• Use 4% or less for tints, 6–8% for

stronger color

• Expensive

Light or dark rutile/titanium dioxide (TiO₂):

• Titanium dioxide and rutile are good color

modifiers in small amounts, 1–4%

• Good for variegation and micro-crystalline

growth in glaze with higher amounts 8–10%

• Rutile is an iron-bearing mineral form of titanium. Use titanium if you don’t want the

added iron (dark rutile has more iron than

light rutile).

Stains/Inclusion stains (Mason, Cerdec, Degussa, Chinese):

• You can get just about any color you want with stains.

• Stains can be good color modifiers in

smaller amounts, use 2–4% for tints, 5–8%

for tones, 10–12% for strong colors

• I typically use 4% in the top row of my

blends, never more than 6%

• Expensive

• Utilize Mason’s reference chart

(www.masoncolor.com) for detailed

information on how to use each stain.

*Note: A carbonate tends to be about half the strength of an oxide.

Birdie Boone is a full-time studio potter currently living in rural Virginia. She holds an MFA in Ceramics/Artisanry from the University of Massachusetts Dartmouth. She is available for workshops on color development; for more information, contact her at birdieboone@gmail.com.