What's the lowest temperature you can anneal at?

[Fiona]

Hi All,

I've been making cocktail stirrers and batch annealed them on Friday by suspending them each end in a chicken wire cage as I was annealing beads on mandrels as well.  Because they had no support along the length of the rod when I took them out the next day they had all bent slightly in the middle.  I've since carefully put some back in on the floor of my kiln with the curve at the top so that they would hopefully slump back straight which has helped a bit.  The thing is I want to make quite a few more as I hope to sell them but I don't want to use my kiln for only a few at a time so was wondering if putting them in at a lower temperature would stop them bending or would it be better if I made several shelves out of my chicken wire and laid them on those?

Help please. 

[flame n fuse]

what programme did you use?

[Magpie]

You need to get the glass up to a temperature so that the molecules are mobile but the glass isn't molten (stress relief point), so they can move around in the stressed areas to make sure there are no stresses in the bead. Then the slow cooling ensures that the whole bead or cocktail stirrer cools at the same rate, the thin bits as well as the thick bits, so you don't introduce more stresses (the stresses happen when different parts cool at different rates so that the contraction of the glass as it cools pulls in different directions; cooling slowly and in a controlled manner stops this happening). You need to cool below the strain point, the lower temperature, below which if there are any stresses left they'll be permanent. It's the slow cooling which destresses the glass as much as the top temperature.

However the stress relief point is different for different glasses, so we tend to use 520°C for effetre as the bods who make it recommend that, but if you're finding distortion of your glass then the chances are your kiln is working hot or your glass is awfully soft. Trial and error with a clear glass rod and some polarising filters may be best, though awfully long winded. Supporting the rods would be good, though if the temp is too high you may get kiss marks.

There's a good, though short, wikipedia article on annealing here.

[Fiona]

Hi, the programme I used was taking it up to 510°C over 3 hours then holding it there for 1 hour then ramping it down 50°C per hour until it was at 378°C at which point I turned the kiln off and left it overnight to carry on cooling down before opening it.

I used 006 clear Effetre and frit to make them.  Because of kiss marks and beads sticking to each other I always try to anneal them so none are touching each other but the stirrers at each end do touch the wire shelf although this didn't affect them so I am assuming that laying them on a wire shelf would be ok.  However because the top of the stirrer is not flat it would still mean that they wouldn't lie completely flat on the wire shelf so I'm still concerned that the rod might bend slightly.

I also want to make Christmas decorations such as icicles so am concerned that they would bend too.

[SilverGems89]

You shouldn't get sticking/kiss marks annealing at 510, that suggests to me your kiln is getting too hot! What kiln do you have?

[Magpie]

That's a reasonable program, it wouldn't affect 'normal' beads, though I wouldn't expect the kiss marks on normal beads either using that programme (I'm assuming you're batch annealing so the kiss marks don't come from putting the bead in too hot). So it sounds like your kiln is running hot.
Hopefully someone will be along soon who knows the ins and outs of checking kiln temperature, there's ways of doing it with cone-y things rather than with a thermometer. If you find out how hot it is then you can adjust your temperature accordingly.

Hopefully someone else will come along with more experience of sculptural work with soft glass to say whether 510°C is too hot or not.

[Fiona]

I'm not having problems with sticking/kiss marks and hopefully never will.  The problem is with the rods bending.  They go in straight and come out bent.  I'm guessing it was because they were suspended in the air with only the ends holding them up and no support in the middle but when I put some of them in a second time on the floor of my kiln with the bent bit up at the top the ones that didn't roll over did seem to straighten out a bit although not perfect so I'm worried now that for future ones I make even laying them on a shelf it will still happen as the top of the stirrer is not flat so they will being lying on a slight incline.

The kiln I have is a Prometheus Pro-7.

I do occasionally anneal as I go along if I think I''m going to be on the torch for long enough to warrant it and maybe making bigger beads and I've never had a problem.  I batch anneal more often than not purely because until recently I haven't had much time at the torch this year so don't feel I can justify putting a kiln on for the basic small beads I've been making so wait until I have quite a few to put in.

I do think it is a sculptural thing because apart from the bending of the rods everything was fine but now I'm paranoid about anything sculptural I might try distorting in the kiln.  I would like to make Christmas decorations such as icicles which I would want to put in the kiln as I went along so they didn't crack but now I'm worried that just putting them in any old how will end up with me taking out the next day bent decorations instead of straight ones.

[julieHB]

Very weird!! Soft glass should not slump at normal annealing temperature, even when gravity is at help, like in your case. To me too it sounds like your kiln is running hot.

You could try to leave them in a bed of vermiculite to support them along the length.

[Fiona]

That's not a bad idea Julie.  I might have to get some and see although I was hoping not to have to buy anything.  Perhaps if I let the top of the stirrer hang freely and just have the rod resting as flat as possible on the wire rack it might be ok.

I hope it's not the kiln running too hot.  I only bought it earlier this year and have only used it a few times.  I do have it on now trying to straighten some of them and have noticed that it's on the soaking programme at the moment which is set at 510°C but the kiln temperature currently says 517°C.  I was told that kilns fluctuate 10-20°C and this is ok but perhaps I should set the programme to 500°C instead.

[Glyn Burton]

You could get some pyrometric cones to check that your kilns pyrometer is accurate I know you have to buy them but its better than wasting glass and time in the long run.
Glyn

[Fiona]

Where do you get those from?

[Steampunkglass]

Where do you get those from?

I think pottery supply places, as I know ceramics people use them. You might find some parts of the kiln are staying hotter than others 

[Fiona]

I not sure it is though as I annealed a dozen of these and they were suspended on wire on two rows across the whole width of the kiln. Since then I've laid some on the floor of the kiln with the bend at the top and although not perfectly straight it does look like they straightened up a bit. I've never had any problems with beads in the kiln so assumed gravity was playing a role in it as they are long rods instead of small compact beads.

[julieHB]

Slumping, or draping of glass happen d/t heat and gravitation. I have only heard of coe104 glass slumping at temperatures at around 600-620 degC, which is way above the schedule you are using. That is a bit worrying, and I would check the kiln temp. if I were you.

I have met lampworkers who anneal at temperatures as low as 470 deg C - at this temperature you need to hold longer, though.

I have put soft glass xmas icicle decorations straight on the kiln shelf with no slumping whatsoever, using my normal annealing schedule.

[Glyn Burton]

Pyrometric cones are available from potters suppliers. They are designed to bend over at a given temperature and are very accurate they show not just the temperature but the amount of work the heat has done. They sometimes are quoted in Fahrenheit which is a bit of a pain but it is easy to convert. I used 12 across the bed of my big kiln which showed a marked variation in temp. But as Michael Cardew said "it doesn't matter how bad the kiln is as long as you are used to the bugger"