|Monthly Tech-Tip |
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I gathered these downstream from the Oldman River dam in southern Alberta. My ultimate goal is to paint on underglaze designs, possibly a clear glaze over that, then fire them to cone 6. The Oldman River exits the Rocky mountains nearby, so these billions of rocks are not far from their original source. Upper right: The rocks as I gathered them. Lower left: After firing to cone 04 (about 1900F). Lower right: After I poured water on them! Seven disintegrated in front of my eyes. With sound effects, even popping! Generating high heat and steam in the process. That means they are all limestone or dolostone. The firing drove off the carbonate. But the calcium sulphate wants it back and welcomes it with a fanfare. #5 and #10 cracked badly. The unfractured ones appear to be iron-bearing silt and sand stones, they should withstand considerably higher temperatures.