Please explain thermal shock to me

[rosebud101]

My lentil shaped beads, not any other shape, that I press, are starting to crack from hole to hole in the center. After searching the threads, I came to the conclusion that this must be thermal shock. What can I do to prevent thermal shock? I am not encasing. I work on a HH so that makes it more difficult to do.
I'm just hoping that someone can give me a tip or two. Thanks in advance.

Mallory
rosebud101

[Dennis Brady]

When glass is heated, it expands. When it cools, itcontracts. This creates stress in the glass. If part of the glass is too much hotter than other parts of the glass, the glass with crack. That is thermal shock. Thermal shock occurs while the glass is being heated or being cooled from having been heated. If your beads had cracked later after you finished making them, it likely isn't thermal shock, but more likely it's either inadequate annealing or incompatibility. It's very common for inadequately annealed beads, or beads made with incompatible glass, to come apart even a year after being made.

Only COE compatible glass must be used - and all beads must be properly and adequately annealed in a kiln.

[Karen Hardy]

See Mallory. See Mallory make bead.
Pretty bead.
See Mallory admire the pretty bead.
Look, Mallory, look!
See Mallory let the bead get too cold.

CRACK goes the pretty bead.

See the new crack that goes up along the bead from
one side to another side.
Oh no. Now Mallory is sad.
Sad Mallory. Poor pretty bead.

[Judi_B]

Try heating along the mandrel from hole a little before popping your bead in the kiln. If you do admire your beautiful bead (and who can resist when they come out good!) be sure to heat it up evenly again before putting it in the kiln.

[CO_Phantom]

Are you batch annealing or garaging them in a kiln right after making them? What temp is your kiln at? What types of glass are you using in your lentils? Any different color combos there that you aren't using in other beads?

My guess is that because you are pressing them they are cooling too quickly before being put in the kiln to anneal...much like admiring them too much before putting them away. Do what Judi said to combat that.

[rosebud101]

Thanks, the information helps. I will not admire my beads any more before putting them in the kiln. Once in a while, they look pretty!

[CO_Phantom]

Just not for too long.

[Emily]

Try giving them a warming to even out the heat base before kilning them. If they're big, warm them in the flame, but not enough so that the surface gets runny -- you don't want to ruin your shape or your decoration. Hold the bead out of the flame a little bit so that the surface can cool a little while the heat penetrates into the bead, then warm again. Do this a few times if the beads are big. You want the bead to have an even glow the whole way through.

[bclogan]

Also, if you're using a press, make sure to put them back into the flame briefly to re-heat. Just not too much or you'll lose the shape.

[Carolyn M]

I had a few beads crack recently, I know why, it was because I was wrapping them in silver wire and didn't want it to burn into globules, so I let them cool too much.

My question is, how long after they are made can thermal cracks appear? I don't want to sell this set only to have more crack for the buyer.

[Otter's Flame]

One other consideraton, are you encasing them? If so what brand of clear are you using...it may be something prone to cracking.

[DesertDreamer]

I nominate the other Karen for a Pulitzer!!!!! ROFL that's great!

And yeah, the brass presses will suck the heat out of a bead faster than you can blink...you need to reheat them to a dull glow before popping 'em in the kiln (nice to work out the chill marks, too, IMO).

[Lisi]

Since you have to work out the chill marks anyway, that should be enough re-heating to prevent problems. Do it in this sequence - evenly heat all over and get rid of chill marks, evenly heat to nice even glow all over, place in kiln on rack right away. The rack is important, because going in glowing like that, if you place them on the floor of the kiln you will have "kiss marks".

So how do you get this even glow all over when it's a pressed shape?? Round beads are easy to get the glow even all over, because they are round! With the flattened shapes you have to spin your mandrel fast and really watch what you're doing to get the even glow all over.

Kinda like this, and I wish I had a video to show you how I do it - Spin spin, then back and forth between holes, flip to the other side do the same, spin spin, get outta the flame, then spin your chair, face kiln, and pop that sucker in there pronto. Goofy explanation I know, but this works for me. No cracked beads.

[navarre]

or to put it another way - run from the sauna and dive into the ice pool.

now "That" is thermal shock.

couldnt help myself