Monday, April 02, 2012

Continuing Sterling Clay Experiments - The Bubble Saga

Over the weekend, I ran two more loads of sterling clay, with mixed results. I had the working theory after my failures last week that moisture was the root of my bubble problem. The latest experiments say that cannot be the issue, leaving me without a working theory for what is causing the problem.

Flock earrings
©2012 Vickie Hallmark
sterling silver
Flock earrings, reverse
©2012 Vickie Hallmark
sterling silver
After drying these Flock earring components to death, they fired in my test run perfectly. I went back to the full 1500°F with no signs of overheating. No signs of bubbles, front or back.

Treetop pendant 2
©2012 Vickie Hallmark
sterling silver, before patina
Then I committed the more intricate work waiting on the bench -- two pendants and a sculptural pair of earrings.  The front is pristine.

Treetop pendant 2, reverse
©2012 Vickie Hallmark
sterling silver, before patina
The back of this pendant has major pox. No fiber blanket was used, but I did prevent trapping carbon in the openings during shrinkage by encasing the pieces in stainless steel screen.

After drying for several days, trapped moisture seems unlikely to be the problem. The isolation of the bubbles to the reverse of the piece still stumps me. One hint I see is that textured backs are giving more trouble, although I did get bubbles on one corner of a smooth back earring. The texture was made with tear-aways, lubricated with olive oil. Of course, I used olive oil on the front textures, too, with no problem, so olive oil shouldn't be the culprit.

I'm still thinking. Any ideas?

11 comments:

  1. I can't remember now, but when this happened with COPPRclay for me I believe it was which ever side was facing down blistered... I think... it may be the reverse! I can only speak from base metal clays, but when this happens with base metal, one time everything is fine the next it's blistering, it's indicative of kiln temperature spikes. Some of this can be resolved with a ceramic firing container because they will hold and stabilize the chamber temperature. There is a major caveat to this, you will have to lower the target temperature because they retain heat more than steel containers. The also have to be watched more closely because the ceramic is porous and the carbon can ignite (this is not a common problem, sometimes lower quality carbon is more prone to "burning up") From my experience it seems this temperature regulation problem is an issue with front loading and muffle type kilns, and because sterling sinters and blisters in such a narrow window, even a slight spike in one spot can wreck your piece. This isn't the case with all SC kilns, but this was a problem with mine when firing COPPRclay.

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    1. Interesting, Wanaree! Perhaps it's just a kiln god issue. I will think on what you said. Interestingly, the pieces that were nearer to the door (and should thus be cooler) were the most blistered.

      It's a frustrating puzzle. I hate it when perfect work goes into the kiln and unusable junk comes out. I'm experimenting with rescuing pieces by sanding off the backs.

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    2. I think they are totally recoverable! I hope you can find a resolution, it's one thing to have it happen to copper another to have it happen in sterling!

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  2. As I was reading your post, I was thinking along the same lines as I see Wanaree noted: temperature spikes!

    It may be kiln gods, but painful as it is to suggest more trials with very detailed note-taking, I'd suspect there's at least a bit of a pattern to even their reactions.

    Closer to the door? In a little cage? Could one or both of those be the specific change that provides just enough extra oxygen to spark a tiny, brief carbon-fire that just happens to spike the temperatures at what the kiln gods find to be a particularly susceptible moment?

    Combine that with the unpredictable (a little bit of "something" in the carbon that burns a bit hotter), the subtle (slight variation in distance from vessel edge or other pieces; firing in a square versus round container, ceramic versus steel, etc.), and the quirky (some clay formulas are just more "sensitive" than others). Add a few other minor details (which side faces up or down: in my limited experience with this until I learned to fire bronzes low enough, I'd guess the side facing down is more susceptible).

    I might bet it is not one simple variable, but a multiplicative effect whose occurrence can be at least be reduced by lowering the kiln's temperature setting just a bit and playing with as many of the other options as you can. Please continue to report your findings!

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  3. Hi Vickie,
    What type of lubricant are you using with the clay?

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    1. Janet, I always use olive oil.

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  4. I have had one piece blister in the past year. Since then (3 months ago) I have been firing with a slower ramp in the atmosphere stage; 6 firings at 500F per hour and now 6 more at 800F per hour (then carbon firing the usual way).
    This is based on the theory that I may not have been burning the binder out at the full ramp and that slowing it down may do so. You may want to try it.
    Also, I was able to sand and burnish the blisters away on the ring shank even though it was very badly blistered. It's strong and fully sintered and looks fine.

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    1. Thanks, Celie! I listened in on your CraftCast class the other night, and this was the one thing that got me most excited. I have a note to myself right here in front of the computer to slow down the ramp on the open shelf firing.

      Since I have convinced myself that moisture is not the issue, the next obvious thought is incomplete burnout. That makes some sense with it always be the back side, too. That side is more protected during the burnout, and perhaps gets less oxygen or heats up slower. What is really strange to me is that the tear away texture bubbles more than the smooth texture.

      Yes, I did grind the entire back side of this pendant down successfully. The bubbles are not open voids, but apparently only areas where the density is lower. It doesn't affect the final finish, but just adds work and might ruin an area that couldn't be smoothed.

      Thanks for your comment!

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  5. I know thus its an older post, but I sejm to be having the same problem with my Copprclay. Always the back side, worse with texture. It's not moisture since I let one piece dry out for days before firing. I'm thinking it's the binder, might do two burn outs, one for each side. Any luck figuring this mystery out?

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  6. Shayna, yes I have successfully solved the bubble issue for sterling clay by changing two steps in the process: drying and burnout.

    I no longer use a small hot plate. I have gone back to either a dehydrator or air drying. I speculate that the clay may dry outside inwards and trap moisture that later causes the bubbles.

    And I also slowed down the ramp on the burnout stage to make sure that any water remaining has time to escape before the binder actually starts to burn. I now ramp at 800º/hr to 1000º and then hold for 1 hour.

    I don't use Copprclay, so I can't say how this might apply in that case.

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  7. I am a PMC sterling novice, and have been having issues with bubbling since starting. I was following the manufacturers instructions (such a novice), but want to try your method. Is the burnout stage the first firing? When you do the second firing do you do full ramp? Thanks for your help!

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