Colorants for Glaze and Clay

[George Lewter]

Rather than reinvent the wheel when it comes to the large topic of ceramic colorants I am going to refer to the ceramics department manual from San Francisco State University that is generously posted with public access.

http://www.sfsu.edu/~ceramic/classmanual.htm  - the whole manual

http://www.sfsu.edu/~ceramic/manual/colorants.htm - the colorants section

It appears that the online materials were possibly scanned from paper documents and then run through OCR software to produce the digital content. I noted numerous formatting inconsistencies and a number of cases where quantities were not  given.

The following is extracted from http://seco.glendale.edu/ceramics/glazebasics.html, and is a very compact listing of common colorants and percentage ranges.

CHROME OXIDE - Usually added to glazes to produce greens in reduction firings. However, combined with tin, pinks can be produced. If zinc is present in the glaze, a brown will result. Browns are also produced in oxidation firings. Use in 2-3% amounts.

COBALT OXIDE or CARBONATE - Both are common cobalt sources and invariably yield blues in glazes, both reduction and oxidation. If magnesium is present in the base glaze, blue violet colors will result. Use in 1/2% to 2% amounts.

COPPER OXIDE or CARBONATE - Copper yields blue and green colors in oxidation glazes, and red to purple colors in reduction glazes. However, any barium present in the base glaze will cause copper to yield blues even in a reduction glaze. Use in 2-5% amounts.

IRON OXIDE - The most common glaze colorant, used to color clays, slips, washes and glazes. In high temperature, reduction glazes, it tends to develop into celadon colors in concentrations below 2%. In higher amounts, yellows, oranges, golds, brick, brown and black colors can result in reduction firings. Iron is a powerful flux also, so beware glazes running if you increase the iron concentration towards 8%.

MANGANESE DIOXIDE or CARBONATE - A relatively weak colorant which produces browns in reduction firing, and purples in oxidation firings.

NICKEL OXIDE or CARBONATE - A versatile colorant, producing many colors from blue to tan to gray to green depending on base glaze composition. Most often used to soften the effect of another colorant. Used frequently in crystalline glazes. Use in 1-3% amounts.

RUTILE - An impure oxide of titanium which contains iron oxide as well. Used alone, it produces tans and tends to make a glaze more opaque (see below). Most often used in combination with iron oxide and other oxides as a texturizer. Use in 3-5% amounts.

VANADIUM PENTOXIDE - A weak yellow colorant, often combined with tin oxide for stronger yellow. Used in oxidation only, as it produces dull grays in reduction firing. Use in 5-10% amounts.

A somewhat better presentation and layout is a pdf handout from Linda Arbuckle at:

• http://lindaarbuckle.com/handouts/colorants-and-opacifiers.pdf

(Linda Arbuckle's website has a whole list of handouts and articles stemming from her years as a prominent ceramics educator)

None of the forgoing delve into stains and encapsulated stains which can be used to very good effect where strong solid color is desired.

Anyone who finds better information on the use of colorants is encouraged to post it as commentary to this discussion.

Related content at:

• https://cone6pots.ning.com/forum/topics/clear-glaze-for-mason-stains
• https://cone6pots.ning.com/forum/topics/iron-glazes-and-achieving-r... an extended discussion of iron reds.
• Encapsulated stains, an overview http://digitalfire.com/4sight/glossary/glossary_encapsulated_stains...
• Vince Pitelka discusses coloring clay bodies http://iweb.tntech.edu/wpitelka/gallery/techniques/techniques.htm

[Norm Stuart]

My brightest Pink is produced by a glaze with Tin, but has not even trace amounts of elements which are reported to create pink when combined with tin, such as chrome or manganese.  The glaze does have a very high concentration of Boron, from the 3134 Frit which could be reacting with the tin to make pink? Typically high Boron levels are reported to inhibit pink colors.

Digitalfire Color Listing: http://digitalfire.com/4sight/properties/ceramic_property_glaze_col...

Powderbox Pink - the mystery is where the pink comes from

50.0%	Ferro Frit 3134
25.0%	Silica
15.0%	Ball Clay OM4
7.0%	Tin Oxide
5.0%	Calcium Carbonate (Whiting)

[Lyn Rapley]

Nice pink.  If you don't mind I shall try it and see what happens.  I'll let you know.

[George Lewter]

Many years ago I glazed a large pot with a white matte glaze in wide alternating stripes with a chrome green. It was reduction fired to cone 10. I believe some of the chrome fumed off and was absorbed onto the white, turning it to a beautiful pastel pink. Fume effects have been widely reported over the years with many effects possible.

[Norm Stuart]

This plate was the "Mastering Cone 6 Glazes" ^6 Majolica, but made with 10% Tin Dioxide rather than less expensive 16% Zircopax.

A vase glazed with a chrome glaze was fired on a half-shelf just above this piece, so I got a uniform pink rather than a white.

This piece is old enough that it's been through the dishwasher a few times after it collects dust.

116.00%	Majolica ^6
23.00%	Nepheline Syenite
23.00%	Ferro Frit 3124
23.00%	Silica
17.00%	Kaolin - EPK
14.00%	Whiting - Calcium Carbonate
10.00%	Tin Dioxide