Bead cracked in half 5 weeks later

[Kimster]

Last night I was just stringing up some beads for a friend, and one fell apart in my hand. I made them a few weeks ago - this one was opaque fritty. I know this can happen, but I'm a bit nervous now about other beads I've made, especially since a local shop wants to stock some  It's the first bead I've had crack in a year, apart from my very first ones. Is there any way I can tell, particularly with opaque ones, that they're 'sound'? Would welcome any advice!

[Redhotsal]

It was probably cracked already but just holding together. I make really large lentils and occasionally one may have a very fine crack running down it. It's really hard to see with opaque beads. My only advice is to look really closely at each bead - I do a bead inspection after I've cleaned and dried everything as part of the cleaning regime - just to be on the safe side. I've had one instance where I was photographing a bead for sale and it wasn't until I was messing with the photo that I saw the crack - it was apparent on the macro photo but almost impossible to see with the naked eye. Don't worry unduly though - if you haven't had any cracks in a year you're doing something right! 

[Hamilton Taylor]

Was it definitely stress cracking (across the bead), or incompatibility cracking (fragmentation/crazing of frit edges)? You can usually tell what caused the breakage by close examination.
Both of these issues can be ameliorated somewhat by ensuring your annealing schedule is adequate - unless using really fancy glass, you need to anneal between 490C and 520C (pick a number, most any will do...), and go for overkill - make the schedule hold there for 30mins.
Then, ramp down slowly (50C/hr) to about 400C (again, playing safe).
You can go a bit quicker to room temp from there.

Cracking due to excess stress should be completely cured by the above process. Incompatibility issues will not be cured, but may be made tolerable. If you still have the problem, consider using different glasses in mutual contact.

If you want to re-anneal other beads you are concerned about, put them in a cold kiln and ramp to your annealing temp at a lowish rate (250 - 300C/hr), then anneal as above.

If you want to try a potentially destructive stress test on opaque beads, put them in the freezer for 30 mins, then drop them into hand-hot water. If they survive that, they'll survive pretty much anything...

Hope some of this helps...

Sean

[Hamilton Taylor]

Oops, Sal got in first.

[Redhotsal]

Yours is more technical though Sean! I was just thinking that usually when I get these sort of cracks it's because the bead has cracked even BEFORE I've got it to the kiln - if I've let the bead get too cold and then given it a quick lick of the flame (which is what causes the crack). I then place the bead in the kiln blissfully unaware that I've cracked it! 

[Shirley]

Crumbs, Sean, you're an impatient bunch North of the border! 30 minutes hold is overkill? An hour as a matter of course over here in the East.

Some good advice here from Sean and Sal. I can't add anything else other than to say been there, done that, and it's usually a thermal shock caused by a mistake on my part rather than incompatibility.

[Hamilton Taylor]

Yep - no amount of annealing is going to solve that one...

[Hamilton Taylor]

Shirley - fair point, it does depend on size - lots of huge lentils going around these days - my stuff tends to be quite small.
Indeed, the longer the better, really...

[Ian Pearson]

The annealing schedules for beads always amazes me. The length of time one holds at a certain temperature always seems far longer than any piece of boro but that maybe because I work clear and can see stress in glass using starin veiwer. Set my oven to min time and temp to remove stress using strain viewer to double check. No need to over anneal. At Flame Off we will have a couple of strainviewers to demo but I appreciate this doesnt work with colour - shame.

Ian

[Kimster]

Thanks so much for all the advice. It was definitely stress cracking. Sean - thanks for the freezer advice! I put all 300+ in the freezer/hot water this weekend, no more casualties so I assume they're OK. Seems I'm overdoing the holding temp too - 90 mins as a matter of course!

[Hamilton Taylor]

There is no such thing as 'overdoing' the annealing time - honestly, the longer the better! There is a table of annealing times against max thickness floating around somewhere, I will try to find it if people want it?
Annealing is the easy bit, just as important is the cooling phase - nice and slow to the strain  point (~450 for soft), then a bit quicker if you want, but if there's time, you can go at the same rate to room temp.

I suppose it's worth mentioning other things that can make the glass structurally unsound - anything like bubbles, inclusions (like silver in SIS), sharp frit edges (like on 'sugar frosted' beads), fume layers between layers of glass, hot/cold glass interfaces (where glass hasn't fused properly - like bad dots), 'captured' bead release - any of these have the potential to form the propagation point of a crack. There's probably others...

Sean