I discovered this technique recently, through pinterest, initially. The technique is best described here: https://thepotterywheel.com/underglaze-techniques/. You can also look up bubble glazing on youtube.
You need: Low, wide plastic bucket, I used the kitchen sink. A plastic cup, a straw, underglaze, detergent.
Method: Mix the underglaze, detergent and water in a 1:1:3 ratio (approximately, the ratios can vary according to how deep you want the bubble colour to be) in a cup. Place your bisque or dry greenware in the bucket and hold the cup above your pottery, blowing bubbles with the straw and directing it into the pottery. If you want to apply the same glaze on the other side, carefully flip the bisqueware over, keeping the bubbles in as undisturbed as possible and blow more bubbles, making sure you cover all of the bisqueware. Most instructions say to use bisqueware, but I tried it on greenware and it worked fine. Dry the piece and apply transparent overglaze once dry on bisqueware, otherwise fire then apply overglaze.
Tips: as the glaze is diluted by detergent and water, a little goes a long way. In one session I glazed this pinch pot, a bud vase and a utensil holder with one tablespoon of underglaze, and had probably half of the amount I used left over. Also, for food safety reasons, please wash the kitchen sink really well after you're finished and don't reuse the plastic bucket for food preparation. Underglazes are made of trace metals, and if ingested can be toxic to your body.
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