View Single Post
  #77  
Old 2005-09-27, 5:27am
Moth Moth is offline
Mary Lockwood
 
Join Date: Jun 21, 2005
Location: Boonies
Posts: 5,831
Default

Quote:
Originally Posted by Sharon Abood
Thanks for the tut! It is beautiful. I tried it last night and like Katja said I can't make a round bead but I tried it and it looked pretty good going into the kiln but came out with two cracks in it. I have had other beads that crack when I try to take them off the mandrel. What am I doing wrong? I am using Morretti glass and I am annealing at 925. I just set the kiln and let it cool down on it's own overnight. I would appreciate any suggestions you can give me.
Thanks,
Sharon
sabood@insight.rr.com
Hi Sharon, I'm glad you are having luck with the tutorial.

Now let's see if we can fix your cracking problem.

First, up your annealing temp to about 968 and let your beads soak at that temp for 20-30 minutes. Rule of thumb is 15 minutes for every 1/4" thickness of glass. Then, if you have a digital controller, program it to ramp your temp down to 830 over a period of about 2 hours. (I'm very conservative about this...Probably lots of people do it faster, but I make lots of encased beads that are chubby.) 830 is the 'strain' point. The slower you cool down from the temperature at which the glass is still moving 968 to the temperature at which the glass locks into its structure (830), the more stress will be relieved, and/or the less stress your glass will aquire during the process. (If you don't have a digital controller, you will have to babysit your pyrometer)

Any stress introduced into your glass before you hit the 830 point, is permanent stress. This means it will remain stress in the glass even after the bead is cool. That is why I am so conservative about it.

Any stress introduced into your glass below the 830 point is temporary stress, which means once the bead has reached a uniform temperature throughtout, aka...totally cooled down, that stress is gone. That is why I will ramp down from 830 to room temperature at a faster rate. Usually takes about 5 hours for my kiln to cool down enough to take the beads out.

Now, if you go TOO fast from 830 to room temp, you can thermal shock your beads and they will crack. Just like putting boiling water into a glass if it has been in the freezer. POP.

So, in a nutshell, you are annealing at too low a temperature, (actually you aren't annealing at all), and you are ramping down too fast.

Another thing to keep in mind is at what point to put the bead into the kiln. Your beads could very well be cracking before you ever get them into the kiln (but I doubt it in this case). Once you have your bead in its final shape, and uniformly heated, you should waft it in and out of the flame to let the glow die down. Your goal is to bring the temp of the bead down as close to your kiln temp as you can. (Which is a guess actually). You are trying to get the inside of the bead and the outside of the bead at the same temp. Once the glow is practically gone, this is when to put the bead quickly into the kiln. I use lauscha to encase, so I put them in still glowing (but not brightly). You will find what works for what type beads you are making.

Hope that helps you out!
~~Mary

PS- It is impossible to OVER-anneal, so in my opinion, you can't go too slow.
__________________

To view links or images in signatures your post count must be 5 or greater. You currently have 0 posts.


To view links or images in signatures your post count must be 5 or greater. You currently have 0 posts.

To view links or images in signatures your post count must be 5 or greater. You currently have 0 posts.


Last edited by Moth; 2006-09-30 at 9:26pm.
Reply With Quote