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Showing results for tags 'blisters'.
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Brandee Ross posted a topic in Clay and Glaze ChemistryThis is a problem I have had intermittently since I started making pottery at home in 2010, and lately it seems to be happening to more pieces each firing. 1. Are these pinholes or blisters? Sometimes they are sharp on the edges. 2. How can I correct this? Helpful (?) details: I am doing a slow bisque firing to cone 04. I hold for 10-15 minutes at peak depending how tight I've loaded the kiln. Bisque firing profile: 80/hr to 250F, 200/hr to 1000F, 100/hr to 1100F, 180/hr to 1676F, 80/hr to 1945. Glaze firing to cone 6. I do a programmed "slow" firing on Bartlett controller to 2167F and hold for 15-20 minutes depending on load. Witness cones show cone 6 achieved. Stoneware clay, made by a local manufacturer (all-purpose Goldart-based body rated cone 6 to 8. Contains 4.5% fine grog). 10 cubic foot kiln, electric Kiln is vented with a Vent-a-Kiln hood that is only 2 months old, replaced broken downdraft vent. Problem occurred with both vents. Trouble occurs sporadically with all my glazes, which I mix. The green glaze recipe in example photos: Rutile green: Talc 5, Custer Feldspar 22 Whiting 4 Silica 26 EPK Kaolin 17 Ferro Frit 3134 26 ADD Rutile (light) 6%, Copper Carb 4% Happens on all types of work: mugs, bowls, etc. I've read on the forum about correcting pinholes with a slower bisque but I feel my firing is pretty slow already. Do I need to slow it more? Why does it only happen to certain pieces? Sometimes two identical pieces glazed and fired at the same time in the same way result in one unblemished piece and one as shown above. Working from home on my own and really feeling out of my depth... and incredibly frustrated.