kiln too hot??

[mindy23]

Hello all,

I have a weird kiln question to ask..

I got a new firebrick kiln, a paragon bluebird and Quinton kindly supplied it with an annealing schedule.  It garages at 500 and ramps up to 510 for 1 hour before slowly ramping down.  I used to anneal in my old kiln at 520 but I tried programming in that programme with this kiln and my beads left lots of little annoying "kiss" marks on each other.  Heres the question, is it possible that the thermocouple from kiln to kiln varies slightly and this kiln, although showing at 510 is hotter???  That sounds really stupid. 
I remember reading somewhere a while back that each kiln is different and there was a way of establishing how yours was.  I think it involved using a glass whos melting point you know and watching to see at what point it melted and noting what temp the kiln thought it was at amnd comparing that with the known melting point for that glass.  However, I might have just made that up but I do have a faint memory..

Anyone know anything about this?

[glassworks]

i may be hotter - but should not be significantly so - they are all calibrated etc before they ship em...

there are cones that you can buy which when fired give you the exact temprature - they bend over as they get hot and are quite precise.. don't know who stocks them though?..

the xl REALLY holds its heat well - far FAR better than an unlined kiln (one with fibre insulation).. we have not had a problem with kiss marks though - is it possible that you are sticking them in when they are still a little soft and that the heat in the kiln is then keeping them soft enough to mark each other?.. if they are soft enough to mark then they are DEFINITELY over the annealing temp anyway, and you can easily lower the temp of the kiln - but i must admit it is unlikely that the temp of the kiln is significantly out.. to get kiss marks the glass has to be soft, and that happens at about 550-570 degrees?..

i'll fire a question over to the factory for you if you like, and hear what they say?...

[Zeldazog]

the only thing I would say, is that don't thermocouples get inaccurate with age?  So is it possible, that although your old kiln said it was firing at 520, it wasn't really, it was firing at 500??

I dont' know much about bead annealing and the glass you use, so I could be talking gobbledegook!

[Mary]

Maybe your thermocouple has got dislodged? If it's not sticking into the chamber far enough you might get a false reading.

[Redhotsal]

I shouldn't have thought a temp of 500 - 550 deg would be hot enough for the glass to stick. Sounds like the glass is touching as soon as you put the beads in the kiln. You're going in at about 800 degs or thereabouts, so the sticking will occur as the beads touch. Once a bead has been in the kiln for, say ten minutes it will have actually cooled down to the garaging temp - be it 500 or 520 which should be fine - i.e. not hot enough for the glass to stick.

It might be that the new kiln doesn't lose heat as rapidly as the old one ('cos of the fire bricks) and so it takes longer for the beads to cool to the garaging temp. If so, there is more opportunity for the beads to stick together??

[mindy23]

Thanks for these replies.  I had been working on an extra large kalera last week using cim pecan and it cracked the next day.  So, I thought I would try the kiln at the old slightly higher temp , so I set it to 520 and made 2 sample beads, plain boring moretti beads.  I put them at opposite sides of the kiln rack and gave it 5 minutes and then put them together.  I left them like that for a minute and then took them out and let them cool in the fibre blanket and when cooled, they had definately left a mark on each other.  I know its very odd because 500 is 500 is 500, right??? 
I'll check the thermocouple, never thought of checking it wasnt damaged or something.
I think I'll devise a test so that I can check that thermocouple is actually reading the temperature correctly. I have the slumping point temperatures for moretti so I'll experiment and see when it starts to sag, if in fact it does!!
I cant work out what else I could've done.  WHen I first got the kiln I got a few marks but I assumed I'd put them in too early but I NEVER let them touch till they've been  in a while.  I move them along when the next beads ready to go in so by that time they definately arent glowing. 

Off to experiment now, thanks for the suggestions.

[Vicki]

Aw, what a pain Mindy, having to do tests when you could be making groovy beads

[Dora]

Mindy,

Each kiln reads the temperatures differently - 'calibrated' or not or whatever - please remember that the temp shown on your kiln is only an approximate reading and not the actual reading. It largely depends on where the thermocouple has been placed in the kiln. If this thermocouple is placed too far into the wall of the kiln and is partially 'hidden', it will read differently than if it is placed sticking farther out into the kiln. Co-incidentally I was talking to Craig from Arrow Springs about kilns & temp settings when I was at the Bead & Button show a few wks ago.

Different colour glass from the same COE family will absorb heat differently and you will note that some glass rods will have different melting points (eg black will absorb heat faster than clear glass). Theoretically we should be annealing the different colours at their annealing temperatures. Generally I use an extremely slow ramp at the beginning but start with a fairly high temperature. 520°C is above annealing temp, so this is my general thumb rule for annealing & soaking my beads (COE104 glass).

Your beads sticking to the ground is probably due to the kiln hotter than 500°C. I had that problem with one of my kilns (Fusebox kilns from EK Miller) which I then placed an aluminum lattice (bought from DIY store) which is screwed onto the bottom of the kiln and then placed my mandrels on this lattice.

I can't agree with Quinton about the XL holding its heat well - I have the XL kiln and once the door is opened, the kiln loses its heat so fast that I have to make sure my mandrel is held in such a position that there is no unnecessary time lost when I open the kiln door to place the bead in. The temp drops from 510° to 470°C if I hold the door open for about 30s. So, when that happens, the kiln will reheat to a much higher temp than the original set temp 510°C (for soaking) and when that happens, there is the risk of the beads being molten enough  to get stuck to each other or the floor of the kiln.

You should anneal at 520°C or at least 510°C for 104 COE glass and let soak for at least 30min before ramping the kiln to strain point. 500°C is a little too low for annealing.                                   
 

[mindy23]

Thanks for that info Dora!
THe kiln actually garages at 500 but ramps up to 510 for one hour when I'm done so its not actually annealing at 500, sorry I must have not made that clear.  I'm really grateful for your reply because what you say about the kiln dropping in temperature fast I find too, I'm really surprised by how much it can drop and I wondered if it was during the kilns attempts to get back to the correct temperature when there was an overshoot.  I'm sure I remember reading one of your posts before on this very subject about each kilns temp being slightly different, I think it was in reference to cim annealing temps and how it was best to test and see with your own kiln as they are all a little different.
I'm going to be cleairng my workshop up tommorrow and running some experiments.  I suspect the kiln is a bit hotter than it thinks it is or at least, is having patches of beign hotter.  I am making a larger kiln rack too so there wont be any need for the beads to huddle up !
The blueburd really heats up the workshop too, sometimes I feel like I am also being annealed, my sc2 was so small and kept itself to itself! I do love this kiln though, there's just so much space!
Thanks again everyone for your help