LJBridges Posted August 6, 2011 Report Share Posted August 6, 2011 I tried to raku fire for the first time today and the results were awful. I used the following glaze formula Seth Blue Luster 2000 gerstley borate 500 nepheline syenite 31 tin oxide 250 copper carbonate 125 cobalt carbonate So my question is was it the glaze? or Did the kiln get to hot? Link to comment Share on other sites More sharing options...
Chris Campbell Posted August 6, 2011 Report Share Posted August 6, 2011 Most luster recipes I have seen have 80 GB and 20 Neph syenite so that part looks ok. I don't think the colorants would do that but I could be wrong. The glaze looks awfully thick ... Was it applied heavily or did it bubble up? Was this the first time you made your own glaze? Do you know how hot the kiln got? Link to comment Share on other sites More sharing options...
LJBridges Posted August 6, 2011 Author Report Share Posted August 6, 2011 Most luster recipes I have seen have 80 GB and 20 Neph syenite so that part looks ok. I don't think the colorants would do that but I could be wrong. The glaze looks awfully thick ... Was it applied heavily or did it bubble up? Was this the first time you made your own glaze? Do you know how hot the kiln got? The glaze bubbled up which makes me think that it just got to hot, I only dipped the pots once on each side and there was no bubbling or flaking prior to firing. Unfortunately the pyrometer is broken so I'm not sure what the temperature got to. I started making my own glazes this summer and haven't had an issue with any of the others, but they were all cone 6, not raku. Link to comment Share on other sites More sharing options...
Chris Campbell Posted August 6, 2011 Report Share Posted August 6, 2011 I have never had a glaze go that badly ... Sorry can't be more help. Some parts look over fired and others look under. I certainly would not use that batch again until one of our glaze people gives you a better answer. Link to comment Share on other sites More sharing options...
Guest JBaymore Posted August 6, 2011 Report Share Posted August 6, 2011 The photo is a huge file and out of focus a lot.... but from what I can see it looks like the glaze was still melting when you pulled the piece. Never reached temperature. It also looks like the kiln was running in heavy reduction during the climb. The glaze is still outgassing heavily and has not fined out. That reduction situation is a common problem with raku kilns that are being pushed for attempting a fast climb......... too much total fuel input for the volume of the kiln, and the flame cannot mix and burn within the chamber. The "cheaper" the burners used, the more the mixing is an issue since cheap burners can't typically entrain a large percentage of primary air. So the mixture within the chamber is fuel rich since the seconary hasn't been able to mix evenly into the flow. best, .....................john Link to comment Share on other sites More sharing options...
LJBridges Posted August 7, 2011 Author Report Share Posted August 7, 2011 The photo is a huge file and out of focus a lot.... but from what I can see it looks like the glaze was still melting when you pulled the piece. Never reached temperature. It also looks like the kiln was running in heavy reduction during the climb. The glaze is still outgassing heavily and has not fined out. That reduction situation is a common problem with raku kilns that are being pushed for attempting a fast climb......... too much total fuel input for the volume of the kiln, and the flame cannot mix and burn within the chamber. The "cheaper" the burners used, the more the mixing is an issue since cheap burners can't typically entrain a large percentage of primary air. So the mixture within the chamber is fuel rich since the seconary hasn't been able to mix evenly into the flow. best, .....................john The kiln is electric, is it possible that the temperature was reached to quickly? I've attached another picture of the pieces. -Lindsey Link to comment Share on other sites More sharing options...
Chris Campbell Posted August 7, 2011 Report Share Posted August 7, 2011 You also did not get any black which makes me think it did not get hot enough. Try re firing it with witness cones so you can see what temp you are reaching. Link to comment Share on other sites More sharing options...
Guest JBaymore Posted August 7, 2011 Report Share Posted August 7, 2011 If it was an electric kiln then you were way, WAY below the appropriate temperature. Remember the is a likely thermal lag between what the electric kiln's pyrometer's thermocouple might indicate (it is very close to the elements) and the higher mass and further away pieces on the shelves. The faster teh attempted climb, the greater this thermal lag will be. Also the more massive (weight and thickness of walls) the piece, the greater the thermal lag. How are you judging the "done-ness" of the pieces to decide to pull them? One of the more typical ways is to insert a metal rod (I use and old stainless steel glass blowing punty) and hold it near the pieces in question. The rod will be "cold" and will create a clear reflection in the fusing surface of the molten glaze. You can see how much shine there is on the glaze and also how many bubbles are still forming on the surface. This technique works best for glazes that are fully molten at the peak, but with practice you can learn to gauge even matte surfaced glazes based on reflectivity. best, ...........john Link to comment Share on other sites More sharing options...
Marcia Selsor Posted August 8, 2011 Report Share Posted August 8, 2011 I am on the road and can check the recipe when I get back to my studio. raku glazes do bubble up before the flux and melt to a smooth shimmering surface. I called it the "pancake-ready-to-flip" stage. however, 10% copper carb is a lot of copper. I will check my Seth's blue luster to see if that is correct. glazes don't usually peel off like that. What clay were you using? Was the surface clean before you glazed? marcia Link to comment Share on other sites More sharing options...
Idaho Potter Posted August 8, 2011 Report Share Posted August 8, 2011 I've not used Seth Blue Luster, so can't speak to the correctness of the recipe. However, this bubbled result usually means the flux was just starting to do its business and the pots were pulled too early. I use a propane fired raku kiln and check the state of the pieces by sight looking down through the lid's hole. When the sheen is all over the pots I stop the gas feed and pull the pots. The only other time this type of bubbling happened was a small neck on a bottle that caused the glaze to bubble up and fill the neck opening. If you are going to refire, I'd use a heavy rasp to knock off most of the bubbles and give the pot another thin coat of glaze--same or even another. If you can't "sight the glaze", definitely use self-standing cones for 06-05 so you'll know you've reached temperature and can continue firing until temp is correct. Good luck! Link to comment Share on other sites More sharing options...
Marcia Selsor Posted August 9, 2011 Report Share Posted August 9, 2011 I tried to raku fire for the first time today and the results were awful. I used the following glaze formula Seth Blue Luster 2000 gerstley borate 500 nepheline syenite 31 tin oxide 250 copper carbonate 125 cobalt carbonate So my question is was it the glaze? or Did the kiln get to hot? My Seth's Luster recipe differs: Gerstley Borate 2000 Bone Ash 500 Copper Carb 250 Cobalt Carb 12.5 Tin Oxide 31.25 You are over a decibel point on the cobalt carb. AND I have bone ash down rather than Neph Syenite. Mix up my version (from who knows where?) and see if it works any better. Marcia Link to comment Share on other sites More sharing options...
LJBridges Posted August 15, 2011 Author Report Share Posted August 15, 2011 I tried it again yesterday and the result was much better. I didn't let the pieces get to temp. before pulling them. Thanks for all of the help! Link to comment Share on other sites More sharing options...
LJBridges Posted August 15, 2011 Author Report Share Posted August 15, 2011 My Seth's Luster recipe differs: Gerstley Borate 2000 Bone Ash 500 Copper Carb 250 Cobalt Carb 12.5 Tin Oxide 31.25 You are over a decibel point on the cobalt carb. AND I have bone ash down rather than Neph Syenite. Mix up my version (from who knows where?) and see if it works any better. Marcia I will try this version next time, thanks for the help! Lindsey Link to comment Share on other sites More sharing options...
Recommended Posts
Archived
This topic is now archived and is closed to further replies.