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.
All of these mugs are made from glazes whose major component is either Alberta Slip or Ravenscrag Slip. They are all fired at cone 10R.
Ravenscrag Slip is a silty clay and has good application properties when used pure. However, because it shrinks on drying and can crawl (e.g. inside sharp corners), especially in recipes having high percentages, it is best to use a blend of roast and raw, especially for multi-layer application. Visit http://ravenscrag.com for information on how to roast (or calcine) it. 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 over 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 hold time. 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%
+100: 0.0-0.1% 100-150: 0.5-1.5 150-200: 2.5-3.5 200-325: 5.5-8.0
Ravenscrag GR10-C silky matte over L3954N black engobe on H550. Fired at cone 10R. GR10-C is just 90% Ravenscrag Slip and 10% talc.
Plainsman H450 (buff stoneware) mugs fired at cone 10R with pure Alberta Slip on the outsides, G1947U transparent (left) and pure Ravenscrag Slip liner glaze. By Tony Hansen.
|Plainsman Clays Ltd.|
702 Wood Street, Medicine Hat, Alberta T1A 1E9
Phone: 403-527-8535 FAX:403-527-7508