new be ; help with cracked bead

[Susan]

Hi me again .I made some beads yesterday .I was happy with them .not too well made but very pretty !!! I got a bit too happy and made a large one .Very very pretty but when I looked this morning this large bead has very small cracks a bit like crazed glass just in the top .I don't want to loos this one as I really like it .I haven't put any thing in the kiln yet as I want to put a lot in together .
Should I not put this large bead in the kiln and just use it or keep it on the side to look at !!!!
I don't know why this bead cracked maybe because I had two phone calls while I was making it .I did keep it in the flam while talking but I know I didn't look after it that well .Poor bead it really wanted to be a nice grow up bead one day but I didn't give it a good start in life!!!!

Susan

[♥♥Tan♥♥]

Hiya Susan, you need to do a bit of research on annealing, that will tell you more about keeping your bead at the right temp. I wouldn't put that large bead in the kiln its about to go pop at any minute by the sound of things

[Susan]

Poor poor bead. Looks like it will only have a very short life !!!I did put them in a blanket to cool down over night but maybe that wasnt as slow as it should be .I dont want to have my kiln on when Im working as I dont make very many at a time yet Im soooo slow .

[jobead]

I cool in a fibre blanket and batch anneal, I rarely get cracking with this method.I would say your problem is with the phone answering !! A waft out of the flame while the bead is hot is the most likely culprit.

Focus on the bead everything and everyone else can wait (that's my motto, though people don't necessarily co-operate with me  )


Jo

[Trudi]

My friends and family know better when I'm at the torch - I ignore the phone. They text me and I'll reply between beads 

[stuwaudby]

Afraid the bead is a gonner. It's annoying but I always get the odd cracked bead when I check in the morning.

It is possible to slowly warm the bead up and eventually remelt it, but difficult and potentially hazardous because the bead might split with a bang. I do not recommend it.

The crack is caused by the stress caused by the temperature gradient through the bead. As the bead cools heat is lost from the surface and the centre stays hot. Glass is a very good insulator, this means that the heat transfer from the centre of the bead to the surface is slow. Glass also contracts as it cools. This means that when a bead is cooled too quickly it's outer skin contracts faster than the inner core. The result is that the outer skin squeezes the inside of the bead causing stress in the skin. If a crack appears then the stress at the point of the crack is massively amplified and the crack spreads very quickly.


What needs to happen is the centre of the bead cools at a similar rate to the surface so the centre is able to contract at the same speed as the surface.

The amount of heat lost from the surface of the bead is proportional to its temperature. This means that a hotter bead looses surface heat faster and causes more stress. As a bead starts to loose it's glow when it cools the glass molecules stop flowing and begin to bind tightly together and stresses build up. It is very important that this stage of cooling happens slowly. The best way to do this is place the bead straight in a hot kiln (garage) and then let the kiln slowly cool down over a few hours. The easiest alternative is to waft the finished bead at the back of the flame to allow it cool more evenly and then place it in an insulating material. These beads will still contain glass which has cooled at different rates causing stress.

Batch annealing slowly raises the temperature of the Glass to the point where the molecules are able to move against each other, but not hot enough to melt and deform. The temperature is then slowly reduced to evenly cool.

Larger beads are much more susceptible to cracking because the centre holds more heat as it cools causing a larger temperature gradient between the centre and surface.

I find some mixed colour beads are much more susceptible than others. Bullseye clear pink/white is one of the worst. This is because the pigment in the different colours of glass can radiate heat at different rates. Some pigments also have an effect on the molecular structure of the glass meaning that they can be more brittle. If I check my rejects can, most of the split beads have an opaque base colour.

The plus side of all this is that split beads can be melted down in the kiln and make great cufflinks:


(Some of these were split with a chisel.)

[Susan]

Thank you all for your help and thank you Stuart for explaining all that and your idea about the cufflinks I would never of thought of that.


Susan

[jobead]

Cufflinks, what a great idea, off to raid my rejects jar 

[Susan]

OK some some times its just a bad day for beads!!!I put my newly made beads in a fibre blanket very quickly from the torch so they wouldn't cool down and this morning I had lots of very " flat beads " .

No cracks .

Today we try again!!!!!

I am going to need all that glass that I ordered yesterday from Martin but now I have lots of good ideas of what to do with my odd beads  (what do you do with cracked beads post )

Susan

[stuwaudby]

Susan, if the glass is anywhere near molten when it goes in the blanket it is likely to crack. Becasue the temperature is so high the heat loss is rapid, even in the blanket.

Cool them slowly at the back and above your flame. Make sure all the glow, and more has gone before moving to the blanket.

[cwest]

What temperature does the kiln have to be to melt the broken bead into a cab?

Carolyn

[stuwaudby]

We use a bullseye fuse cycle which goes upto just over 800 DegC. Don't leave the temperature up there too long or the glass will de-vitrify. There are plenty of fuse cycles available on the net.