Fine grained low plasticity cone 10 melting silty clay. Ravenscrag Slip is a blend, its principal ingredient is mined from a large deposit near Ravenscrag, Saskatchewan, Canada. It was originally developed as an almost-complete base cone 10 glaze (it is feasible to use 'as is' on either green or bisque fired ware, to achieve a light colored silky matte surface). The material can be viewed as a 'plastic feldspar' that provides good melting and excellent slurry properties in a light colored material. In educational and beginning pottery settings it is an ideal starting-point for material-blending style glaze development and experimentation with colors, opacifiers, variegators and matting agents.
Ravenscrag Slip is a silty clay and has good application properties (when used pure and not applied too thickly). However, when glazes employ a high percentage of Ravenscrag Slip they will crack on drying (and crawl on firing), it is important to roast a portion of the powder (e.g. the GR6-A recipe). For use as a base coat or in multi-layer applications consider adding CMC gum also.
Glazes are most often prepared using the traditional method of simply adding water until the desired consistency is achieved (rather than targeting specific gravities and conditioning with flocculants). No flocculant chemical additions are needed, it applies well across a range of viscosities (although more viscous slurries can create overly-thick applications).
Left: Raw Ravenscrag Slip powder. Right: Roasted at 1080F. This is a silty clay, thus it has a relatively low drying shrinkage. Where a glaze is applied thickly or the percentage of Ravenscrag is high, shrinkage cracks (which produce crawling during firing) can occur. We recommend a mix of roast and raw material in recipes. Roasting the Ravenscrag powder at 1000F greatly reduces the shrinkage. Use a firing rate and hold-time-at-1000F appropriate for the wall thickness and size of your bisque vessels (e.g. 500F/hr and 30 minutes for thin-walled small vessels, slower and longer hold for large ones). If any of the powder within is black, increase the hold time for the next firing. Adjust proportions as needed (more roast if the glaze cracks on drying or more raw if it is drying too powdery or not bonding well).
The roasted material has a weight-loss of about 3% on firing (vs. 9% for the raw powder). This difference can be ignored in most cases. But, to be more precise, use 3% less of the roasted powder (multiply the amount by 0.97). For example, suppose you need 1000 grams of a 50:50 raw:cacline Alberta Slip mix for a glaze recipe. Use 500 raw and 500*.97=485 roast.
Since Ravenscrag Slip is a siltly material it does not work well for burnishing.
Ravenscrag Slip is the opposite of Alberta or Albany Slip: The latter is a low-melting, plastic, and dark burning; this is higher-melting, non-plastic, and light burning. It melts to a silky-matte surface at cone 10. Ravenscrag.com demonstrates the addition of fluxes to get betting melting if needed.
Adding about 20% Ferro 3134 frit will produce a base transparent glaze for cone 6. You can add colorants (i.e. stains, metal oxides), variegators (i.e. 5% rutile), and opacifiers (i.e. 10% Zircopax) to create almost any effect.
Since it is basically a clay material a significant amount of gas is generated during decomposition during melting. If not fired high enough and not fluxed adequately very thick layers could tend to form unbroken bubbles in the glaze layer.
For more detailed information go to http://ravenscrag.com
Drying Shrinkage: 3.5-4.5%
Ravenscrag Slip really shines in its ability to produce a good floating blue glaze at cone 6, this is GR6-M. The speed of cooling in the kiln affects the appearance (faster cool gives better blue). The left M340 mug was fired using our drop-and-soak PLC6DS firing schedule, the other, also M340, with the slow-cool C6DHSC schedule.
|Plainsman Clays Ltd.
702 Wood Street, Medicine Hat, Alberta T1A 1E9
Phone: 403-527-8535 FAX:403-527-7508