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If we demand clay bodies with great working properties, why do we not demand the same of our glazes?


If your glaze cracks on drying like this, it will crawl on firing. Calcine some of the Ravenscrag.


Calcine it like this.

You May Need to Calcine Some of Your Ravenscrag Slip

Ravenscrag slip is not a plastic clay, it is silty clay. It still shrinks, but much less. As such, higher than normal percentages can be tolerated in glazes and slips before excessive shrinkage causes drying cracks. Even pure Ravenscrag can be successfully used if not applied too thick and water content is not too high. Notwithstanding this, sometimes cracking can occur where the layer is thicker and the proportion is high (thicker application is sometimes needed for good coverage, such as with mattes). It makes sense to optimize these recipes by calcining some of the material. For example, you might try a 75:25 raw:calcine mix to supply the Ravenscrag called for by the recipe. Remember that Ravenscrag has an LOI of 10%, so you need to use 10% less of the calcine powder (just multiply the amount by 0.9). For example, suppose you need to supply 1000 grams of material to a glaze. Use 750 raw and 250*.9=225 calcine. Then watch how the slurry performs and adjust it again later if needed.

To calcine a ceramic powder simply fire it high enough to destroy the plasticity but not so high as to sinter it (producing particle bonding that creates grit that could affect glaze surface quality). Fire the uncompacted powder in bisque vessels to 1000F (Cone 022 or red heat). For large or heavy-walled vessels fire slower (e.g. 200F per hour) but for smaller ones (especially thin-walled ones holding 500-1000g) you can fire much faster (e.g. 500F/hour). Hold for the time necessary for the heat to penetrate (start with 30 minutes). If any black powder remains in the center extend the soak time next firing.

Ravenscrag Slip is a product of Plainsman Clays and near Ravenscrag, Saskatchewan, Canada.