Cracks when mixing opaque and clear

[Enchanted Cobwebs]

I wind some opaque, then clear then more opaque in a random fashion to get a lovely combo swirly mix. I have only batch annealed so far and I'm using vermiculite yet the only beads that have cracked are these combo ones. Some that didnt crack in the vermiculite cracked in the kiln some survived in the kiln. These are not large beads, no more than 12mm - 16mm and I dont have problems with beads that are entirely opaque or clear wound in the same way. I will start garage annealing (just as soon as I have batched the backlog) and that may overcome the issue but I just wondered if anyone else has experienced this or am I just trying to do something that is known not to work?

[sarah t]

where are they cracking ?

it could be a couple of things ...incompatability or thermal ...but the type of cracking is sometimes an indication of which one it is

they should work fine lots of styles use opaque and clear glass combinations  so my best guess would be different cooling temperatures ...you might need to reheat all the class to an even temperature before begining the cooling process 

[Enchanted Cobwebs]

cracking in half, hole to hole along the mandrel. I generally blast them with heat as my shaping isnt great. I find if I get it all really fluid I can spin them into shape, I continue to turn till the glow subsides moving down the flame then under it. I try to aim for just cool enough so it doesnt fizz in the vermiculite and scare me when I smell burning!

[sarah t]

down the mandrel is usually thermal ...but i am  not sure what you are doing wrong so it might be the cooling process so i would hope that garaging than annealing would do the trick

[Enchanted Cobwebs]

Thank you, I will try garaging next weekend but the first batch will be these troublesome ones just to see if it makes a difference. Glad its not because I should shape up the opaque before applying the clear as I get a much better random swirly effect by just blobbing it on!
Thanks
Helen

[AnnaSpanna]

I have found that some CIM opaques just simply don't like clear - especially non-CIM clear

[Enchanted Cobwebs]

one of the opaque colours is CIM, I thought I'd get that lovely cats eye effect that I see in the rod but didnt get that either!
I think I will stick to the effetre and not mess around till I'm ready to garage

Thanks all
Hx

[Kalorlo]

What colours are you using? You could try making the core clear and covering that in a thin layer of the first opaque rather than making the whole core opaque and see if that helps at all. It can make colours that don't like being encased behave better.

It does sound odd, though.

(Pretty much none of the CiM colours keep the cat's eye look - apart from African Gray which is a limited edition and we haven't had over here yet!)

[stuwaudby]

You probably have a combination of a couple of issues here:

I use alot of the CIM semi transparants. They are much stiffer (more viscous) than effetre at the same heat. This is either because the glass has a higher melting temperature or because it is radiating heat faster. Using CIM over efftre can be difficult because the CIM is stiffer and can drag or pull the underlying effetre at intermediate heat. Whilst the COE of the glasses are the same the CIM will be harder then the effetre as it cools.

Mixed transparant / opaque beads are much more prone to cracking because of the different radiant properties of the two types of glass. (i.e. one will radiate heat faster and cool faster). Garaging will slow the cooling and allow both types of glass to stay at more even termperature.

I bought a bead cube because I was loosing about 1 in 10 beads, I rarely loose one now.

[james M knox]

Enchanted Cobwebs,

Is the vemiculite cold or do you warm it before you use it?

I have a small electric hotplate for my vemiculite that I have set for 40 deg C, this warms the vermic and gets rid of any chill from sitting in my shedio and also removes any condensation that may form from the cold nights. 

Till I started heating the Vermic, I always had a few beads crack completly in half, now it is vary rare and usually only the first bead cos I didnt wait long enough. 

JMK

[stuwaudby]

Before my beadcube I used a Tesco's value slow cooker filled with vermiculite (and still do).

http://direct.tesco.com/q/R.100-2168.aspx

It got very hot when used on the low setting but I always had dry hot vermiculite. I didnt find any difference to bead survivability between using it hot or cold.

The element went after about a year, but for £12 who cares?