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GR6-A - Ravenscrag Cone 6 Clear Glossy Base

Modified: 2020-12-15 14:34:50

This Plainsman Cone 6 Ravenscrag Slip base is just the pure material with 20% added frit to make it melt to a glossy natural clear.

Notes

This is the base cone 6 Ravenscrag recipe, it fires as a transparent glossy. It has an addition of the most common North American borate frit, enough to produce a good glass at cone 6. Most other glazes based on Ravenscrag use this as a starting point (although they might substitute the frit for another).

The character of this is not the same as transparent glazes made from materials like feldspar, kaolin, silica, wollastonite and frit. This is a natural clay material having some iron content and particles you can feel (up to 42 mesh). So it forms a less sterile glaze, one having some variegation, fine speckle and slight blush or amber coloration. It is most glossy at a complete cone 6 (for a lower temperature you may need to increase the frit a little).

It is best to at least hold the temperature at the end of the firing to minimize firing defects. Even better, use the C6DHSC firing schedule.

As a transparent, this glaze it is best suited for use on light-burning stonewares (e.g. Plainsman M340). On whiter porcelains and stonewares it may not fire to a sufficiently transparent glass (consider using G2926B or G3806C instead). On darker clay bodies this recipe may fire too cloudy. Is has a low enough thermal expansion to fit most bodies, but if it does craze consider substituting Ferro Frit 3195.

As noted, this is a base recipe, well suited to additions of opacifiers, variegators and colors. Add zircopax to get a white (about 10%). The white produced will be somewhat variegated (non toilet-bowl-like). For variation-with-thickness, to highlight edges of contours, use a lower percentage of zircopax. For even better variegation, add some titanium dioxide (1-2% to a mix already containing 6-8 zircopax). Add colorants and variegators (with or without opacifier). If colorant additions affect melt fluidity, add or reduce the frit percentage to compensate.

If it cracks during drying, increase the roast clay at the expense of raw clay.

Glazes having a high percentage of Ravenscrag Slip are most often prepared using the traditional method of simply adding water until the preferred viscosity is achieved (the material has inherent properties that produce functional slurries for dipping). Control of drying shrinkage and slurry character is achieved by varying the proportion roast and raw powder in the recipe. For us, a weight ratio of 80 water to 100 powder (2000 water for 2.5kg of powder) produces a 1.5-specific-gravity slurry that, although fairly runny, gives the right thickness on 1-2 second dip on 1850F bisque-ware (there is some dripping, but coverage is even and it is quick drying). This recipe actually does not respond to flocculant additions that gel traditional mineral-blend glazes to a thixotropic state.

Related Information

Roasting Ravenscrag Slip instead of calcining

This is the Ravenscrag Slip I used to calcine at it 1850F (about 10lbs in a bisque vessel). I am now roasting it to 1000F instead, this produces a smoother powder, less gritty. To make sure the heat penetrates for this size vessel I hold it for 2 hours at 1000F. If your container is smaller you could do less time, if any black material remains inside, then do a longer soak on the next firing. It is not actually calcining, since not all crystal water is expelled, so we call it "roasting". Why do this? Ravenscrag Slip is a clay, it shrinks. If the percentage is high enough the glaze can crack on drying (especially when applied thickly). The roast does not shrink. The idea is to tune a mix of raw and roast Ravenscrag to achieve a compromise between dry hardness and low shrinkage. Technically, Ravenscrag losses 3% of its weight on roasting so I should use 3% less. But I often swap them gram-for-gram.

Emulating a speckled reduction fired stoneware in oxidation

The body is Plainsman M340S. Cone 6. Left to right: G1214Z calcium matte base glaze with 6% titanium dioxide added. GR6-A Ravenscrag base with 10% zircopax (zircon). G2926B glossy transparent base with 10% zircon (this one produces the white "Kohler Toilet Bowl" appearance we are seeking to better). G2934Y silky magnesia matte base with 10% zircon.

Ravenscrag Slip at cone 5R and 10R

Cone 5 GR6-A glaze at cone 5R on Plainsman M340 (left) and pure Ravenscrag Slip at cone 10R on H550 (right).

Ravenscrag Slip transparent and Alberta Slip blue glazes by Tony Hansen

The mug is the buff stoneware Plainsman M340. Firing is cone 6. On the inside is the GR6-A Ravenscrag transparent base glaze. The outside glaze is GA6-C Alberta Slip rutile blue on the outside. The transparent, although slightly amber in color compared to a frit-based transparent, does look better on buff burning stoneware bodies this.

Cone 5R mug with GR6-A Ravenscrag glaze

Ravenscrag GR6-A glaze with Frit 3134 and Fusion F-12

As this cone 6 melt flow test demonstrates, the Fusion F-12 is giving very similar performance to the Ferro. The GR6-A recipe is just 80% Ravenscrag Slip and 20% frit (10% zircopax has also been added to opacify). The degree-of-melt on the glazed tiles in also very similar, however notice that some of the whiteness has been lost on the second F-12 tile (the first is frit 3134). We are attributing this to the better melting of F-12, that is amplifying the color of iron present in the Ravenscrag Slip. For the third tile we reduced the frit to 15%, that has whitened it somewhat. Likely an increase in the zircon and would whiten it more.

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By Tony Hansen

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