All Glossary

Medium Temperature

These are stoneware glazes that fire in the range of 1200C (2200F). They often contain boron to assist with melting.

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Details

In functional ceramics, this term generally refers to glazes and bodies that mature from cone 4 to 7.

Bodies

Mixtures of natural light or white burning minerals and clays almost always produce bodies that do not mature or vitrify until cone 10. This is because whiter burning clays are more chemically pure, thus contain little or no feldspar. To produce a body that will vitrify at cone 6 generous feldspar additions are thus needed (PV clay is a notable exception). Bodies made using kaolin and ball clay can require up to 40% feldspar to mature, this is a challenge because the feldspar addition comes at a big cost to the plasticity (a further plasticity hit is that silica is also almost always needed, reducing clay content to 50% or even less). Darker burning clays contain iron oxide and other minerals, it is common for them to mature at or even below cone 6 on their own (although some red fireclays do exist).

Glazes

At these medium temperatures, it is difficult to compound glazes that will melt well without the need for powerful melters like zinc and boron. Thus, a medium temperature glaze contains mostly the same kinds of ingredients as a high temperature one, but additionally, it typically needs a power flux (boron is by far more popular and less troublesome for potters, whereas industry uses zinc/boron for fast firing. Typically, frits are employed to supply the B₂O₃ or ZnO (historically in pottery Gerstley Borate and Colemanite were common sources of B₂O₃, but industry uses frits). Boron has a low thermal expansion and thus is an ideal additive since it reduces the tendency of glazes to craze. Since there are no practical insoluble sources of pure boron, glaze chemistry is normally needed to determine how to best incorporate boron-sourcing materials into recipes that use troublesome natural sources.

Related Information

Plainsman M2 being delivered at the plant site in 2015

M2 is a dark red burning, middle temperature, moderate plasticity, low contaminant stoneware clay. It makes a great base for brown firing low and medium temperature clay bodies, less than 50% is needed for good body color. Plainsman Clays has been surface-mining it in Montana and importing it to Alberta since 1980.

Iron red glazes

I worked on this G3948A cone 6 glaze effect for 40 years! Now I have it. Here are the secrets:
-A fluid-melt glaze having the right chemistry for the iron to crystallize.
-The right amount of iron oxide in the recipe (too much and it over-crystallizes, too little and the effect is compromised.
-A dark burning clay body or engobe underneath, in this case, Plainsman Coffee Clay (we also use L3954F engobe on lighter clays).
-The right firing schedule (initially we used the C6IRED schedule but later we found the C6DHSC worked well also).
-An electronic controller and kiln packing density for a repeatable firing curve.
-Careful control of slurry thixotropy and rheology to enable consistently thick enough application that the melt runs (thus a catcher glaze is needed).
This cost about $20/kg to make in 2024 (enough to make more than two 500ml jars of brushing glaze).

The Magic of Rutile Glazes!

Rutile is a high iron and titanium mineral mined in Sudbury, Ontario. When kilns cool this glaze "parties and dances", creating amazing rivulets and crystals of blue and navy. Both of these glazes are made from materials mined by Plainsman Clays (in Montana, Alberta, Saskatchewan). Both produce a wide range of effects with different thickness, bodies and firing schedules. This effect is achieved by firing to cone 6 (2200F) and slow cooling down to 1400F.

G2926B cone 6 transparent liner glaze: Proven, reliable, durable

While colorful and layered glazes on the outsides of pieces get lots of praise and glory transparent or white glazes providing the functional surface on the insides of pieces often get little attention from potters. Really, what good is an attractive piece if the food surface is crazing, blistering, leaching or cutlery marking? Or if it converts the piece into a time bomb? This cone 6 liner glaze, G2926B, is an example of how I found a glaze, recognized its potential and then adjusted the recipe to resist crazing on our clay bodies, fire durable and leach resistant act as a base to host colorants, opacifiers and variegators. I get the best fired results using the C6DHSC firing schedule and very good performance as a dipping glaze when the slurry is thixotropic. One of the reasons this recipe is so widely used is that it is well-documented having a code number that Google indexes. Drinking from a mug having a quality and fitted functional surface and a nice crisp line dividing the outside and inside glazes instills pride in me as the maker. What is the outside glaze? It is the G2934Y matte base recipe plus 8% Mason 6027 stain. The clay is MNP which I make myself.

G2934 cone 6 DIY matte glaze: Reliable, durable, adjustable, stainable

These pieces were made from Plainsman Polar Ice and fired to cone 6 using variations on the PLC6DS and C6DHSC schedules. The dipping glaze is G2934Y, a recipe variant of G2934 having a finer micro-surface texture (it has the same chemistry but the MgO is sourced from a frit and talc instead of dolomite). These mugs display varying degrees of matteness depending on the cooling rate of their firings and the percentage of glossy G2926B base we blend in. As an MgO matte, this glaze is can have a surface very pleasant to the touch. It fires durable, can be quite matte without cutlery marking and it has very good slurry and application properties (as a dipping glaze). It has a very low thermal expansion (won’t craze). It works really well with stains (except purples). It melts even better than the glossy!

Mason stains in the G2926B base glaze at cone 6

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 overfired (it is rated to cone 8), adding 1-2% Zircopax normally fixes this issue.

Mason stains in the G2934 matte base glaze at cone 6

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.

Two cone 6 matte glazes opacified with 10% Zircopax

The clay is a buff stoneware. The upper two samples are G2934, an MgO matte. The one on the right has 10% zircon added to opacify.
The bottom two are G1214Z1 (transparent version and opacified-with-zircon version).

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