Ravenscrag Slip melts by itself to a silky cone 10 glaze (no additions are needed). This recipe adapts it to cone 6 by adding 20% Frit 3134.
When mixed with water to create a thixotropic slurry, this will perform was as a dipping glaze. It will apply evenly to bisque, produce minimal dripping and dry within seconds. It will be adequately durable for handling. If bisque walls are thin, it will be necessary to glaze the inside and outside of pieces as separate operations (with a drying time between).
If you are glaze laying (using this as a dipping glaze and painting a commercial gummed brushing glaze over it) there is a risk of crawling. It may be necessary to add some gum to this, converting it from a dipping glaze to a base-coat dipping glaze.
This glaze is most often prepared using the traditional method of simply adding water until the desired consistency is achieved (do the initial mix with 8 parts water and 10 powder). We find that 1.45 specific gravity works for us using our tap water. No flocculant additions are generally needed and application properties are very good as long as the slurry is not too viscous or too runny (dries quickly on bisque without cracking and, after dipping, there is minimal dripping).
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.
The body is Plainsman M390. These are commonly used base glazes. The top one is an MgO matte, the next is a calcium matte. They behave very differently to these additions. Notice also that thickly applied titanium dioxide is very different. Tin oxide fires whiter than zircon (e.g. Zircopax).
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. G2934Y silky magnesia matte base with 10% zircon.
|Plainsman Clays Ltd.|
702 Wood Street, Medicine Hat, Alberta T1A 1E9
Phone: 403-527-8535 FAX:403-527-7508