Since I'm impatient, I've been running around the internet trying to find out when I can 'force cool' my little experiment by cracking the kiln. However, during the search, I came across this site:
http://www.glass-fusing-made-easy.com/open-fusing-kiln.html
Which has this paragraph in it:
QuoteWhenever you would like to re-fire your glass a second time, whether to slump it or add more to the object, remember that it's now doubly as dense and can only be heated at about 450° per hour. The rule of thumb here is to heat up half as fast as the first firing and cool down half as slow.
I've never heard this before, and it seems bizarre. If you need to fire it three times, are you supposed to do it at a quarter the original speed?? ??? If you keep firing it, will your large platter eventually end up a super-dense, matchbox size weapon of mass destruction with the density of a black hole? ;D
I don't know the science behind it, and having repeatedly refired only small pieces where you can ignore almost any firing schedule and get away with it, I rest in blissful ignorance to why it is so. I'm sure Dawn knows a lot about it, though ;) I don't think it will dense to a black hole, but if you are planning to do something bigger than 5x5cm I would take any advise seriously. Too bad to lose something you have worked on for a long time!
??? weird. and definitely not true!
This website is full of useful info http://www.warmglass.com/basic.htm
and if you are using Bullseye, their pages have firing schedules etc and they tell you how many heating / cooling cycles their glass has been tested for
Thanks Julie & Julia :) I'm playing with float for the time being.....and if I stay up another couple of hours, I might be able to sneak a peek and find out how the experiment went, lol!
It is true that the more times you fire, the slower you should go - as Julie says, with smaller pieces it doesn't matter so much (I mean, if you can chuck 'em in a microwave kiln in 30 minutes, then I am sure you can fire quite fast! ;D ;D)
But when I am doing, say a 25cm dish, first firing I'd consider 266 C per hour, but a second firing, I'd go for 166... and with my larger dishes, especially one that went in for a 3rd or 4th firing, I annealed for an hour.... overkill maybe, but I would rather err on the safe side.
I know Bullseye recommend slowing down too for subsequent firings too.
I can remember Dennis Brady saying on here that beyond four firings, the glass becomes less stable - not sure about it getting denser, perhaps they just mean it's thicker as its now one 6mm thick piece instead of two 3mm???
When I am in a real hurry, I will open the kiln lid wide at around 100 degrees, and shut it again quite quickly - just to allow heat to escape a bit quicker, otherwise they hold their heat too well! I'll do that a few times until its cool enough to remain open.
That's a top opener though Sabine, is this in your Caldera? I don't think I'd do it in a front opener because of it cooling unevenly across the piece then.
Is it cool enough yet???
Thanks so much Dawn, that kind of makes sense! The Caldera has a top door thingy, too, but... Having just gone outside in a t-shirt and undies, I can confirm two things:
1) kiln is still too hot to mess with, and
2) baby, it's cold outside!!!
;D ;D ;D
I wonder if they are referring to casting with nuggets or frit? Where you start by filling your mould with pieces of glass? If so then it would certainly be denser when it has fired. I suppose that there are other air gaps and voids in most fused pieces too prior to firing - though maybe not accounting for it being doubly dense.
It must be dark matter making up the missing density. Ask Prof Cox - he'll know all about it. ;)
I am not sure they know themselve what they're referring to....
I was just browsing round the site, trying to find out what schedule they actually suggest.... can't find it, maybe its me - but they only say go slower second time (slower than what??) - oh hang on, found another page where they suggest heating up 6mm glass at a rate of 250 to 450 degrees fahrenheit.... then they say heat the second firing, go slower, only 450 per hour.... hmm, isn't that the top end of what they suggested for a standard thickness fuse?
Oh, and they suggest that slumping occurs between 1300 and 1400 degrees fahrenheit... that's 700 to 760 centigrade - I know they might be generalising, but slump Bullseye at even the very low end of that range and you'll have a pool of glass in the bottom of your mold. I don't even slump float glass anywhere near as hot as the top end of their suggestion, so not sure what glass they're playing with....
I think you're right Sal, they can only be talking about dark matter, because I not sure they're talking about normal fusing glass at all.
Well...... ;D ;D in true experimentation style, I got...a crack, which developed into....two pieces of glass by the time I'd washed it :)
I don't know whether it was because I did exactly the same schedule as before (without slowing down for dark matter glass), whether I was over-confident venting, or too nosey too early...only repetition and elimination will tell, I guess ;D
sorry to hear it broke, you also need to bear in mind any inclusions you might have when you think about temps for cracking the kiln as depending on what they are they may conduct heat (or cold) more than glass which can cause cracks ;) and like Dawn I would also slow my ramp speed for the 2nd firing (or 3rd or whatever)
Dawn, we were also told at uni that beyond 4 firings you need to be very careful with your schedules as the glass is more prone to deviting and other issues, interestingly ive never had problems with beads that have been in and out of the kiln and torch 4 or 5 times.
Is it still float glass you are using Sabine? If so then I would be much more conservative with firing it, ramps, annealing and no peeking! Float is a much more brittle glass to fuse with as it's simply not designed for it, hence why you always should make sure to fuse pieces from the same sheet. I'm even finding with the water-jet cutting that pieces are cracking post-cut because it's simply not designed to be put under stress of any kind, including being bombarded with grit at 50,000 psi!
Oh and hi guys, BTW! I may be lurking on here a bit more often..... ;D
I never knew that Rachel, I do tend to fire REALLY conservatively on float if its more than one fuse and slump - but it makes perfect sense!
Good to see you back BTW!
Also, the tin side can effect things too. I can't remember what the answer is for that, whether to fire so that the tin is on the inside or the outside to burn off or whether it will encourage de-vit etc... I don't tend to fuse it and it doesnt' effect my enamels so I don't even bother to test anymore!
But I do remember making quite a few float castings at college and everytime you fire it, it stiffens up a little more and I think I got to 4 firings before it was just a big crystalised heap that literally chewed itself up when i tried to slump it!
I try to fire as low and long as possible for float and if I can do it in one firing, all the better!
(Waves at Dawn!)
Ooooh, interesting! Yes, it's still float at the moment, I'm trying to keep costs down, but maybe it's a false economy if it's more likely to go crackers, lol! I shall do a bit of maths :) Thanks Rachel!
I'm not peeking......I'm not peeking......I'm not peeking......argh! ::) ::) ::) ::)
Enjoying reading this - Don't peek ;D ;D ;D ;D ;D
Remember Sabine that patience is a virtue and everything comes to those that wait-and also remember that if you live in Bolsterstone its always freezing, those are my cheery thoughts for the day.
Still........
.......not.......
...........peeking.........
..........................
.......must..................
..............................resist.................... :-\
Lol, Glyn, you're right. We have the heating on, and I'm under a blanket with a hot water bottle......
Oooh I so feel for you - I am bad at waiting too..... but I do wait, mostly by firing the kiln overnight such that it will be cold by the morning, but definately too hot to go even thinking about opening it before bed that way I don't notice the waiting soo much - but it does mean I bring my tea out to the studio in my dressing gown and slippers all too often ;D (probably not a good sight first thing in the morning! :o ) It's on its way down as I type, but is at 400C and cooling at 100C/hr.
I've had to adjust my speeds both up and down due to the winter weather and my studio is inside a big concrete block of a building. I had my first cracked piece in the kiln in years the other week and it was purely due to going up to quickly. I mainly use float too Sabine so I'm not judging the economics of it. I use 4mm for dishes and clocks, 10mm for hares and other water jet cut bits. The thicker pieces I ramp at about 250c/hr to 500 then full to whatever paint firing I'm doing. Cooling its more like 45c/hr from annealing to 50c and even then don't peek till its at room temp.
Right.....experiment two didn't crack....but it also didn't do what I wanted. Re-started the kiln with attempt no. 3 at 7am this morning, adjusting a bit here and there :) Very, very slowly, I'm beginning to understand the point behind different ramps, temps and holds, and feeling confident to tweak a titchy bit - thanks for all the help!
Quote from: Flyingcheesetoastie on November 14, 2011, 11:56:45 PM
I've had to adjust my speeds both up and down due to the winter weather and my studio is inside a big concrete block of a building.
A timely reminder for me to check my schedules, and no peeking early... it's the first winter for Kiln Bill and the studio got so ridiculously cold at times last year....
Sooo....would I be right in thinking that winter schedules work in summer (but would simply take longer), but summer schedules might crack in winter? Or is that oversimplifying it? I'm getting my big kiln soon (hopefully tomorrow), so I guess I'll start out on winter schedules :) It'll be stood against the outside wall of the garage studio (only place it fits!)
Yeah I'd say so Sabine - to be honest, I don't usually adjust for winter, but Bill arrived when it was spring, so I may need to slow stuff down.... once I have done that, I will probably leave him be, I tend to fire conservatively anyway - only thing lost is a few hours production time, but it will still be done when I get there as I don't do silly hours at my studio (unlike Rachel!)
Thanks, that would work for me too, I think - it'll be a long time before I have the confidence to mess too much, so if I get something that works in December, I think I'll stick with it ;D
Silly hours? Dunno what you mean, I've only had a few times where I've been there at 1am! But yeah I am in and firing at least one kiln every day mostly. The best idea I had and can recommend is to have a wall themometer above your kiln, not so close that the heat coming off it will effect it but so that you have the actual room temperature to eye when you programme. Also it's good to note the temperature inside the kiln if you've not fired it recently and it's cold, if it's reading below 10, then remember that it's got to heat that cold air, the bricks, the thermocouple, any props etc before the glass, so go easy ont eh poor wee thing!
To be honest, I probably change something with every single firing I put on, but that comes from practice and confidence/arrogance that you know what will happen! I probably should start writing some of the generalisations down, but it varies from kiln to kiln anyway!
Dawn, I have to comment on your new avatar. DH is a Clangers fan.
Well......it didn't crack, and it did pretty much what I wanted it to. Now I need to design it properly with colours and such things, lol!
Following the conversation about winter and summer firings above, most kilns are programmed to heat up by so many degrees per hour, to a particular temp, so I don't see why the outside temp matters too much on the heat up - if it starts colder, it'll just take longer. It might matter on the cool down, if you have a cooling segment which is AFAP, if your kiln isn't well insulated.
Not that I know anything about fusing, but if the glass starts out colder and the kiln heats quicker as the air will heat faster than the glass there will be a greater difference in temperature between the inside of the glass and the outside, possibly leading to a greater possibility of cracking, depending on how thick the glass is that you're playing with.
Back to the original suggestion. Scientifically if the same amount of glass is twice as dense, surely it would be half the size, so unless you've got a magically multiplying kiln by the time you've fired it 3 or 4 times it would be hard to find unless it was massive in the first place.
Tried to answer Flame N fuses points last night but Iphone died, saving a novel of explanation!
Basically FnF is right, kinda, obviously if you are heating really slowly from room temp up to slump/fuse/whatever then the temperature of the outside of the kiln/ inside of kiln won't make a huge deal of difference. For example, when I was at college we were taught to go up at like 75c/hour to 500c then either AFAP or slower to out top temp depeding on flow rate etc. So basically that's 5 hours of gently heating the glass/kiln etc before you even get started. I'm not saying this isn't best practice, but it's certainly not viable for me in my studio when I need to get kilns reloaded every day and as long as the annealing is sound then that's the main thing.
So when I fire my kiln, with say 3/4mm thick glass for a paint or slump firing (my bread and butter stuff) I want to go AFAP up to those temperatures because I know on the whole the glass isn't going to crack doing that and it's the cooling down where I want to be investing the time and electricity to get right. BUT that breakage that occured recently happened in just such a firing wiithout any other factors apart from the kiln hadn't been fired for a few days and was very chilly at 8c. So for me it was a reminder to go a little slower to allow the glass (insulator on the whole), the batts/props (insulators), kiln bricks (insulators) to heat up gradually too, because it was that sudden shock of heat hitting that glass when it was on a batt of 8c that probably cracked it.
Hope that helps to explain my vague reasoning behind programmes, chilly kilns & my perchant for kiln whispering! I'm by no means an expert, but I'm well practised in what I do know!
I think that's the main difference for us Rachel, I still fire quite conservatively (although not quite *that* conservatively) - because more than once a day isn't currently viable for me with various things - so a quicker firing time isn't a requirement - and I for one, do consider you an expert!
Oh I don't quite reload the SAME day, even that's a bit too kamikaze for me but most of my stuff is usually done in 18-24hours. Mind you when I worked at the stained glass studio, I've seen painted pieces go through 6 firings in a day with their gas kiln!
There are some things I don't have much experience of though like fusing multiple sheets etc. I would still need to look up firings and consult manufacturers handouts like the bullseye technotes. But having a good knowledge of your kiln helps with everything.