Twice as dense??

[sparrow]

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:

Whenever 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? 

[julieHB]

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!

[flame n fuse]

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

[sparrow]

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!

[Zeldazog]

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!  )

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???

[sparrow]

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!!!

[Redhotsal]

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. 

[Zeldazog]

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.

[sparrow]

Well......  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 

[Chameleon]

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. 

[Flyingcheesetoastie]

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..... 

[Zeldazog]

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!

[Flyingcheesetoastie]

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!)

[sparrow]

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!

[sparrow]

I'm not peeking......I'm not peeking......I'm not peeking......argh!