Thermal Crack frustration

simvet02 · #1 2010-03-17, 4:44pm

I lost five beads from yesterdays batch. I'm so frustrated. I do need to ask for some advice. Normally I leave the beads in the kiln until the next morning. Sometimes, if I'm up late enough I will bring them up but only if they are near 100F. I bring them up in a blanket and put them immediately in a glass of warm water as close to the temp. as possible.

I had always heard that if they were below 200 it would be ok to do this. Is that not true? Is this why I'm getting the thermal crack, horizontal, exactly in half? Sometimes the cracking doesn't show up until they are drying on a towel or the next day but I always have a few that crack.

I do make sure that they have a nice red glow when they go into the kiln which is garaging at roughly 965F.

The ones that broke were almost all Effetre-all Effetre so I know it's not incompatibility.

Help, and thanks.

pam · #2 2010-03-17, 4:52pm

Can you tell us how fast you are bringing the kiln down and the size of your beads?

simvet02 · #3 2010-03-17, 5:10pm

Well the size of the beads vary but they are on the larger size like 25mmx45mm, that 25mm has been pressed to that width with a crunch press. One was 13mm x 50mm.

As for how fast it comes down, I'm ashamed to say I'm not sure. I got one of Mike's kilns at the Glass Hive, and it was preprogramed I will have to go find the instructions. I believe it soaks at 950 for and hour and a half and then down to 700 over 3 hours. Then it goes off and drops naturally.

The whole thing usually takes about 7 hours at which time it's between 150 and 250 degrees and I unplug it and leave them.

Like I said there have been times where it was close to 100 and the brought them up.

LyndaJ · #4 2010-03-17, 5:58pm

Are you sure that you're reheating to a good glow before they go in the kiln? Larger crunched beads take a little extra reheating to prevent cracks.

squid · #5 2010-03-17, 6:06pm

The air temp in your kiln is not necessarily the temp of the inside of the beads.

wendbill · #6 2010-03-17, 6:08pm

Make sure you heat both sides of the bead as well. That was one of the tips I picked up from someone else here, when there was a similar issue. It made me stop and think that often I used to only reheat one side.

pam · #7 2010-03-17, 6:16pm

Well, the 950 to 700 over three hours sounds fine, but I personally would never turn my kiln off at 700. Thermal cracking can occur below the 700 degree mark. I kiln mine down to 400, then the kiln goes off. You might try that and see if it helps. Also, what everyone else says is right, making sure you get the heat back in the bead after pressing is important.

The only other thing is if your kiln isn't registering the correct temperature. Sometimes kilns can be as much as 100 degrees off.

ewdb · #8 2010-03-17, 7:32pm

you might try skipping the immediate water bath as well. i take beads out at just under 100F but let them finish cooling completely at room temperature before soaking for cleaning... (i'm assuming that's why you're putting them in water?)

simvet02 · #9 2010-03-17, 8:20pm

Another thing that really has me baffled is trying to do reduction or striking, when your done reducing a bead just the way you want it and need to get the red glow you ruin the reduction or the strike, I did it just today when I had the colors just the way I wanted them and needed to reheat the bead before putting in the kiln, darned if I didn't ruin the strike.
Triton is one of those reduction beads that needs to be really cool, like raku before reducing, so then you get it just right and are suppose to get a nice glow??? Hope I'm making sense, if I have to go back to a neutral flame to warm it up I ruin the reduction. I've lost more of those than I care to count for that very reason.

As for the turning the kiln off at 700, I'll go down and get the exact instruction in the morning, it may not turn off until 300 or 400.

From now on I'm going to leave them until the next morning, I have to eliminate one thing at a time and that's the easiest to eliminate as a problem.

Thanks for all your help, I really do appreciate it.

CO_Phantom · #10 2010-03-18, 10:01am

I have a kiln from Mike at the Glass Hive, too, and I've never had a problem with the program that Mike preprogrammed in there for me. I'm not 100% sure of the firing schedule, but I leave the kiln on until the temp says 100 and then I flip it off and leave them in there until they are actually room temp. In Phoenix, it was easier for me to take them out at 100 because room temp out in the garage was dang close to 100, so it wasn't a problem. Here in Colorado where it's closer to 35...I don't wanna try it.

-Amy

GLASSFREEK · #11 2010-03-18, 12:56pm

sounds like your letting them cool to much befor going in the kiln.
The program thats in your kiln is on the safe side for a brick kiln, if you where useing a frax built kiln then yes you would want them to cool a little slower to a lower temp couse frax will not hold heat like a brick kiln.

Mike

Ravenesque · #12 2011-09-26, 3:40pm

Sorry to boost an old thread, I searched and this is the only one discussing it.
Can anybody tell me the schedule that is programmed in a Glass Hive Kiln? I'm using a friend's while she's at a seminar, she left the instructions but it does not say and I'm not able to reach her until Thursday :/

I would set up a new one but I'd hate to make her mad if she's got one set on #2, and if it's a 7 hour cycle that's too long for what I can do now.

PerfectDeb · #13 2011-09-26, 6:22pm

just ring Mike, he'll help you

Ravenesque · #14 2011-09-27, 12:17am

*edit, dah! I figured it out, I'm stoopid. I didn't see the times on the instructions until a bit ago omg. I just saw a sea of letters I didn't understand.

I didn't want to bother him during dinner time, I don't know it yet but I'm figuring it lol. My kiln doesn't have a nice handy controller, I'm not used to this luxury.

I'm house & critter sitting for her so I'm just going to let it ramp up and just do it and see

If not she'll just have a lot of clean beads, I'll do that instead.
Super nice kiln though, I'm going to have to have one!