|Monthly Tech-Tip |
Also called "middle temperature" by potters, cone 6 (~2200F/1200C) refers to the temperature at which most hobby and pottery stonewares and porcelains are fired.
Key phrases linking here: cone 6, 2200f - Learn more
This refers to the medium temperature oxidation range (or middle fire) that most potters work in using electric kilns. Orton cone 6 fires to about 2200F (or 1200C). Potters who fire ware cone 5 or 7 may also consider themselves to be working at "cone 6" (using the term to designate the range or type of ware). This range represents the best investigated and experimented glazes among potters and hobbyists, there are millions of images on line of ware being made and commercial products that support it.
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Cone 6 is the lowest practical range for feldspar-fluxed white plastic bodies. Vitreous porcelains can be made with up to 30% feldspar (the remainder being silica, 20-25% is needed to prevent crazing, and kaolin/ball clay). White stonewares need about 20-25% feldspar. Typically a bentonite addition is needed to augment plasticity. Buff and brown burning bodies offer more flexibility since they often employ clays that already contain natural quartz and fluxing minerals (so less feldspar powder is needed).
Clays made using feldspar can be made to vitrify to zero-porosity density at cone 6 (including porcelains and stonewares). Since vitreous ware can be made at cone 6, many do not bother with the extra time, trouble and expense of firing to higher temperatures. Companies making prepared glazes serve the middle temperature range with a wide array of products.
This glaze, G2926B, is our main glossy base recipe. Stains are a much better choice for coloring it than raw metal oxides. Other than the great colors they produce here, there are a number of things worth noticing. Stains are potent colorants, the percentages needed are normally much less than metal oxides. Staining a transparent glaze produces a transparent color, it is more intense where the glaze layer is thicker, this is often desirable in highlighting contours and designs. If you add an opacifier, like zircopax, the color will be less intense, producing a pastel shade the more you add. The chrome-tin maroon 6006 does not develop well in this base (alternatives are G2916F or G1214M). The 6020 manganese alumina pink is also not developing here (it is a body stain). Caution is required with inclusion stains (like #6021), the micro-bubbling here is not likely because it is over fired (it is rated to cone 8), adding 1-2% zircopax normally fixes this issue.
Stains can work surprisingly well in matte base glazes like G2934. But they perform differently in a matte host glaze. The glass is less transparent and so varying thicknesses do not produce as much variation in tint. Notice how low many of the stain percentages are here, yet most of the colors are still bright. A good reason to minimize stain concentration is to avoid leaching. We tested 6600, 6350, 6300, 6021 and 6404 overnight in lemon juice, they passed without any visible changes. It is known that MgO mattes, like this one, are less prone to acid attack that CaO mattes. A down-side to the MgO-matte-mechanism is that chrome-tin stains do not work (e.g. 6006), high CaO content is needed in the host glaze to develop the color. The inclusion stains 6021 and 6027 work very well in this base. As do the 6450 yellow and 6364 blue. And the 6600 produces an incredible gunmetal black. The 6385 is an error, it should be purple (that being said, do not use it, it is ugly in this base). The degree-of-matteness can be tuned by blending in some G2926B glossy base.
Medium Temperature Glaze
These are stoneware glazes that fire in the range of 1200C (2200F). They often contain boron to assist with melting.
Low Temperature Glaze
In ceramics, glazes are loosely classified as low, medium and high temperature. Low temperature is in the cone 06-2 range (about 1800F-2000F).
Also called "middle temperature" by potters, cone 5 (~2160F/1180C) refers to the low end of the temperature range at which most hobby and pottery stonewares and porcelains are fired.
G2934Y - Cone 6 Magnesia Matte Low LOI Version
The same chemistry as the widely used G2934 but the MgO is sourced from a frit and talc instead of dolomite. It has a finer surface, less cutlery marking and staining.
G2934 - Matte Glaze Base for Cone 6
A base MgO matte glaze recipe fires to a hard utilitarian surface and has very good working properties. Blend in the glossy if it is too matte.
G2926B - Cone 6 Whiteware/Porcelain transparent glaze
A base transparent glaze recipe created by Tony Hansen for Plainsman Clays, it fires high gloss and ultra clear with low melt mobility.
Electric Hobby Kilns: What You Need to Know
Electric hobby kilns are certainly not up to the quality and capability of small industrial electric kilns, being aware of the limitations and keeping them in good repair is very important.
Glaze Recipes: Formulate and Make Your Own Instead
The only way you will ever get the glaze you really need is to formulate your own. The longer you stay on the glaze recipe treadmill the more time you waste.
|By Tony Hansen|
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