How thin can a large hole bead be?

yellowbird · #1 2010-11-30, 7:40pm

chrissij · #2 2010-11-30, 7:50pm

You mean tall, or wide? I know that it has to be at least the same diameter of the mandrel all the way round. Just like you're not supposed to press beads thinner than the width of the mandrel on either side.

yellowbird · #3 2010-11-30, 7:57pm

right I mean wide .
because I have heard that the walls should not be thinner than the mandrel.
With a big hole that seems impossible .
I was ask to make beads that would slide over viking knit.

houptdavid · #4 2010-11-30, 7:58pm

How large of a mandrel?
The idea of a bead being x sixe of the mandrel doesn't hold true with the big hole mandrel. Think about it... a bead on a 1/2" big hole mandrel would have to be 1/2" thick

houptdavid · #5 2010-11-30, 8:01pm

Over VK would most likely be a dipped 1/4" or 5/16" depending on the ends
5mm should be fine for width

yellowbird · #6 2010-11-30, 8:08pm

thanks for the help
but why for the larger is it okay? and how do I figure out what will be too thin?
Not just for this project but in general.

houptdavid · #7 2010-11-30, 8:23pm

think of it this way

__________
__________
1/16" Mandrel wrapped with 1/8" glass
__________

But when you get big

______________________
______________________ 5mm

Like 1/2"
______________________
_______________________

That theory doesn't hold true
PPP to small they will crack

yellowbird · #8 2010-11-30, 8:30pm

I'm going to think about it while I study the inside of my eyelids

thanks again!

Moth · #9 2010-11-30, 9:22pm

I think the width of the glass having to be equal to the mandrel mainly applies to when you are squishing a tab or flat shaped bead. You don't want the area you squished to be thinner than the mandrel because it will shock too easily. You have these two solid wings of glass holding deep heat, then two shallow areas of glass cooling faster and your heat differential is too great causing thermal shock.

You have more play with the larger mandrels if you keep the bead profile round (like for a pandora bead).

Mainly you just have to have enough glass around the mandrel to maintain heat. If you can maintain heat, you can go pretty darn thin. It's just most people don't want to risk it. The mandrel width= glass thickness is a rule of thumb. Kinda like the only use 5% of 96 coe frits on 104 coe base beads. Or some people say 20% as long as you don't encase it. Or some people just put on as much as they want, encase the crap out of it and dare you to prove them wrong. LOL

Make it. If it lives you did it right.

If you were using a 5mm mandrel (for round numberness sake) then your bead would have to be 15mm in outer diameter to follow that rule. Too big for a pandora bracelet, in the comparative sense to what you can buy direct from pandora. There's are significantly smaller (usually).

~~Mary

yellowbird · #10 2010-12-01, 9:10am

well that's an eye opener!

Thanks Mary for taking the time to explain that so well.

Moth · #11 2010-12-01, 10:56pm

You're welcome. Just remember...the thinner you go, the harder it is to keep your heat balanced. Lotsa stress. You need good heat control for sure if you are working with large holed, long thin barrels. Tiny beads are way easier.

It's important to keep the mandrel hot too...not just the bead. Occasionally heat the mandrel to glowing for an inch or so to each side of the bead holes. Mandrels wick heat from your bead and this helps slow that down and acts as a bit of a heat sink as well so you can travel some heat through the mandrel into the bead. Another thing to remember, even with beads that aren't thin, is to make sure you bring the whole bead to a uniform heat after it is in its final shape, before you put it in the kiln.

Bring the whole thing to a deep, low glow. Not soupy dripping hot, just a good low deep glow. This equalizes the heat throughout the bead so that before it goes into the kiln all the areas of it were the same temperature which relaxes internal stress. Then, heat it slowly in the far reaches of the flame until the glow subsides and you can put it into your kiln.

A cheat that I use sometimes on thermally tricky beads is to turn my bench lights off so I'm working in relative darkness. You can read the heat a lot easier when the lights are dimmer.

~~Mary

Moth · #12 2010-12-01, 10:57pm

Think of the glass rings people are making. Those are like 3/4" diameter mandrels but the rings are thin.