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Fusing and Stained Glass Tips and FAQ's => Slumping => Topic started by: Lorac on July 24, 2014, 12:16:14 PM

Title: slumping break and mould pinch - RESCUE REMEDY anyone?
Post by: Lorac on July 24, 2014, 12:16:14 PM
A disappointing first fuse in a 4 section Candle Bridge - glass has pinched mould so I am unable to release from mould and edges have drooped unevenly causing [I think!] the glass to break. [closed sharp crack]
   I realise now that I probably made the piece slightly too wide...should I measure to absolute edge or just to the edge of curve?  I am guessing the crack is because the edges are ‘pinching’ the mould? And I am not able to remove it!  One book suggests I turn upside down on new thinfire paper and slowly  to 620 degrees and allow it to soak for 30 minutes. It suggests it may ‘drop’ onto the shelf but with a bridge mould it wont be the same as a bowl mould will it! Any suggestions how I can rescue mould? If I can get glass off in one piece I would keep it for my Christmas table but I fear it may be a right off!

2.   The edges of the piece were completely straight before slumping so the ‘drooping’ suggests that it may have been too hot at 735 following the schedule from Mould supplier, Warm Glass. The Brad Walker book says that Slumping at a higher temperature than 704 is ‘almost never necessary’! For me as a novice I understand that lower and longer may well be the answer but I have no idea how much to adjust the temperature and/or time by.   My smaller Candle Bridge slump had wavy edges too which meant it was a bit rocky. I am not in a hurry so happy to give it as long as it needs to avoid this!

Any suggestions about trying to release mould AND/OR measuring for candle bridge AND/OR suggested process temperature to avoid sagging??
It is such a relief to have found Frithappens! Early fusing days can be very lonely and frustrating with no one to ask!!
Thanks to anyone who reads and ponders this!
A steep learning curve indeed but the best one I have ever climbed!

Title: Re: slumping break and mould pinch - RESCUE REMEDY anyone?
Post by: marklaird on July 27, 2014, 02:11:31 AM
I can't advise on question one as luckily it's not a situation I've had.

Question two. The slumping temperature really depends on your kiln. I've got an sc2 and I ramp up at 222 to 520, hold for 25 mins, then ramp at 333 up to 650 and hold for 20 minutes. Then it is down as fast as possible to 516, hold for 50 minutes, and then anneal down at 60 per hour to 371, then off.

I've only used a couple of smaller plate moulds so far but been really happy with the results.

If you have any other slumping moulds is it worthwhile trying with a simpler design first to get the hang of the kiln.

Anyway, hope this helps.


Title: Re: slumping break and mould pinch - RESCUE REMEDY anyone?
Post by: chas on July 27, 2014, 03:28:40 AM
Hi Lorac - and the cheery news is that someone from Warm Glass may well come along and give you the full advice you need, but in the meantime a couple of observations:

assuming it's this mould

it seems to combine two elements designed to drive you barmy:  gently sloping 'shoulders' coupled with four deep square-sided depressions. You'd be inclined to think they're incompatible, particularly as on their own pdf fact sheet they quote "616 for shallow slumping" "677 for deep" both contrary to the front page info "735 for 35".

That said, neither Bullseye or Warm Glass are slouches and they do show an example - so someone made one, once, and the answer will be out there somewhere.

As far as yours is concerned, the 'reverse slump' upside down suggested will almost certainly ruin the cast, so, sadly you'll have to write that one off - but it does seem to be the best way to preserve your mould.

As to why it pinched in the first place, you don't say if it's along the long sides or short but if the former I'm surprised: it looks as if it should release ok (radiused edge, no undercuts) if the latter - blimey, that glass did spread!

Assuming you get to use it again, try taking the measurement square across the mould for width. Because of the radius it will be 'short', but the fired result may be close enough to the edge to be safe and acceptable. If too short, then make the next one a bit wider. Unless you're a lucky slumper, you have to be prepared to experiment. Don't use the full length either, keep it short by a mil or so each end.
And (unless someone who's used this mould or WG does come along) try a lower top temperature - it seems your glass is running rather than slumping.


Title: Re: slumping break and mould pinch - RESCUE REMEDY anyone?
Post by: Lorac on July 27, 2014, 07:52:15 AM
Hi Chas
Thanks so much for your helpful response! I did contact Warm Glass but their response was
"If you feel that the recommended firing is too high then reduce the temperature by 20C, if it is then too low, increase it by 10C etc until you get the perfect slump. Slumping isn’t easy and as a novice you are challenging yourself by using the most complex moulds."  But like you I felt that the glass was running. reducing by 20 degrees to 715 still seems high..what is your view on that?
I bought my Hobbyfuser 3 from Warm Glass and have followed the schedules they give for the many moulds I have bought. I did ask what they thought of Brad Walkers 'not above 704 but they didnt answer that one! They said....."The recommended slump for the moulds are based on tests carried out by the manufacturer in a medium sized kiln (50cm x 100cm) as all kilns fire different, there is a requirement to test to achieve the best results."  I had hoped that their expertise would be able to help by offering recommended schedule adjustments for the kilns they sell. I also remember them saying that they use Hobbyfuser kilns so naively assumed they would know about the moulds they sell.Troubleshooting states " Small kilns such as Skutt Firebox may require hold time at process temperature reducing." but I dont know how Hobbyfuser 3 compares to that. As my Hobbyfuser is a top loader I assumed the ". The chart shown below is also based on the assumption that you are using a top fired kiln, e.g. Nabertherm GF75, GF240 etc. " included the hobbyfuser. Do you know which kiln it is closest to?

Their Elegant Candle holder took 4 attempts to get it right but obviously firing flat then tacking for embellishments and then 3 slump firings is not recommended! Presumable the above suggestion about reducing by 20 then trying again with 10 degress less means 2 separate firings?
How many attempts do you think is ok with one piece of glass? I hoped that the mould schedules given would be suitable for the Hobbyfuser 3!  The other moulds have required higher temperatures if anything so it is confusing to find that this one seems to need lower?!
I accept I am a 'novice' but dont want to stay one so am trying to understand the science behind it all!
Am SO grateful for your advice though Chas and appreciate I may be expecting too much and asking far too many questions!
I am going to try and attach a picture or two! when I have transferred to other lap top. Technology is a challenge for me!

Title: Re: slumping break and mould pinch - RESCUE REMEDY anyone?
Post by: Nicknack on July 27, 2014, 08:45:57 AM
You have to remember that every kiln has its own idiosyncracies!  My SC2 is a case in point. If I use the Bullseye programme ( top temp 804c for 10 mins), I get a semi puddle, but with trials ( using glass I'm not too worried about) I've discovered that 800c for 4 mins is perfect.  I fired up my second hand Fusion 10 yesterday, and after the seller told me that it's fine with the recommended settings, I used those, and everything turned out nicely. :D


Title: Re: slumping break and mould pinch - RESCUE REMEDY anyone?
Post by: Zeldazog on July 27, 2014, 10:01:03 AM
I agree with Nick, even if you're using the same glass, the same design and the same kiln, things will still be different.

I full fuse in a Hobbyfuser at 797, which is good way under Bullseye recommended firing temperatures. 

Warm Glass simply cannot test every variant of mould, in every kiln, and even if they did, it is not that straight forward - ALL kilns are different, even the the same make and model (think ovens, I know more than one or two bakers who pretty much have to start again to get the perfect cake when they have a new cooker).

In addition, just to complicate things, the glass can can affect things too!  Some glass "softens" at an earlier temperature than others.  To see this in action, try this

Get standard black and standard white - cut a large and small square (2cm and 1cm will be enough) of each - place a small black on top of a large white, and small white on top of a large black.  Take it to high take fuse, almost full fuse.

You might have to tweak temperatures a little, but at some point, you will find that the white sinks into the black completely, but the black sits atop of the white.  This is because black softens before white in the standard range (there is a stiff black too).

Lots of things to consider when fusing and slumping - I have to say, I feel that 735 seems rather high for a slump, as in a shallow mould, that would certainly distort (experience tells me this!) - and I personally would have tried lower in the first place.  You can always pop a piece back in if it's under-fired.  Over fire and you've almost certainly lost that piece. 

I've had an issue with the very small deep square slumper, the trinket dish one - it has to go really high as it's small and square, but I find it goes lop-sided if I am not careful (my trusty Hobbyfuser probably has a slight hot spot) so I often slump it in stages now.

Fusing is easy.  Getting it spot on, every time, is not so easy.  I've been fusing glass since 2006/2007 and I still get throwaways....

Title: Re: slumping break and mould pinch - RESCUE REMEDY anyone?
Post by: chas on July 28, 2014, 02:49:05 AM
I think Nick and dawn have pretty well covered the points already... you'll be getting the impression that slumping is not an exact science - to begin with. It becomes an exact science when you have a schedule noted down that works for that particular mould and that particular glass - which you discovered by accident or experiment. It's a sort of kissing frogs thing...

As for "How many attempts do you think is ok with one piece of glass?" the enemy is devit: the clarity may suffer, but you can 'work' the glass through the kiln as often as you like and it remains the shape you want. I'm pretty sure that if you took, say, three attempts to get the required slump with an experimental (and sacrificial) piece, then went back in with schedule 3 and a fresh piece, that one would be successful in one hit.

The point has been made that kilns and glass differ. By way of illustration and without wanting to confuse, the 'basic slump' top temp for our Nabertherm F110 is 673deg for 10 minutes but the one eventually used for a mould with two cavities of our own design was 835 for 35mins. You can imagine the experimentation that went into reaching that conclusion (with help from this forum) and the joy with which is was finally noted and used for 40 repeat firings without a single fail! That was with float, btw. Apparently Bullseye glass 'works' at 15/20deg less than float.

Good luck!


Title: Re: slumping break and mould pinch - RESCUE REMEDY anyone?
Post by: Lorac on July 28, 2014, 03:28:38 AM
Dear Nick, Dawn, Mark and Chas
Thank you all so much - it is fantastic to have so much experience to tap into! I can tell that I have a long way to crawl out of the novice pool but am just so glad there are superheroes out there who are willing to lend a hand!
I think I am just going to have to bite the bullet and accept that firing a plain 6mm texta piece to get the mould schedule right is the best way to go. I am still very nervous about peeking in the kiln and knowing what to do but Milton Keynes wasn't built in a day eh!
I too had problems with the extra small square slumper [why oh why did I start with that mould!] but have got to grips with it now I think.  Knowing whether to increase temperature AND/OR time is where I struggle!
I can appreciate now that Warm Glass cant test all the moulds and am very grateful for the information they have given me.  I did 2 terms of fusing glass weekly class but it didn't really cover any kiln information - understandably! All my previous experience was stained glass so it was like starting again. But I love it!
think this should link to photobucket........
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will keep fusing!
cheers all for helping

Title: Re: slumping break and mould pinch - RESCUE REMEDY anyone?
Post by: MeadMoon on July 29, 2014, 03:42:16 AM
That looks like an annealing crack so maybe you didn't cool down slowly enough.

The glass is lovely; did you do your own design?  I use an SC2 so my firing schedules would be different anyway, but I took the advice of someone on a different forum and tried slumping at a much lower temperature for longer (630 for 25 minutes down from 680 for 5) and a hemi-spherical bowl turned out ok, but using almost the same schedule in a soap dish mould resulted in an incomplete slump so I still have more experimenting to do.  The only problem that I had at 680 for all the moulds that I've used so far was slight devit with the deeper ones.

Have you tried the upsidedown firing to try and release the mould yet?  Let us know how you get on if you do.

Title: Re: slumping break and mould pinch - RESCUE REMEDY anyone?
Post by: Lorac on August 01, 2014, 03:32:16 AM
Thanks yes, own design with xmas in mind {in July?!!}
Annealing crack? Ah havent come across that one yet. I assumed it was that glass, having 'run' down side a bit - pinched the mould as it cooled?
Which stage of the schedule [following Warm Glass schedule] do you think I should change when/if I try it again? Be really glad of your advice as it is a lot of glass to ruin!  I know I need to make it narrower [thanks to advice from Chas] and lower the temperature from the Warm Glass schedule so it doesnt 'run'.
many thanks
[haven't been brave enough to try the release method yet as I am not sure what I do when and if the mould is released?!! turn kiln off and allow to cool to room temp with lid shut????? I am guessing i have to decide really quickly??? ]

Title: Re: slumping break and mould pinch - RESCUE REMEDY anyone?
Post by: Zeldazog on August 01, 2014, 04:31:15 AM
When Elaine refers to annealing crack, she means it happened during annealing (cooling down), not heating up.

You may have something when you question whether it's because of the glass pinching the mould, in combination with cooling too fast.  As already been said I think, you've picked a rather complex mould shape and this is why.

There's a couple of loose, general rules about slumping, mould materials and inside or outside.

When using ceramic moulds, you generally slump into/inside the form.  When you use stainless steel, you generally slump over/outside.

This is because during cooling, glass contracts sooner than the ceramic, so, if it was outside, it could 'pinch' and crack.   However, when using stainless steel, the metal will contract sooner than the glass, so if you were to slump inside the shape, it could 'squeeze' I have successfully slumped inside stainless bowls, but they were a continuous radius on the curve and so the glass was slowly lifted up.

The candle bridge mould requires the glass to slump into the 'wells' but over a curve - at the same time.  Add to this that you're effectively doing two processes that require different working temperatures: slumping into small, precisely defined shapes requires the glass to flow more freely - higher temperatures to take on that shape.  Slumping OVER a gentle curve can be done at a much lower temperature to avoid the glass running too far....

You can see why this is a complex mould shape, it requires such a careful balance.  And that's before you add the other variables of what glass you're using, what kiln you're using....

If I am unsure of a mould shape, I always start with lower slumping temperatures.  Unfortunately, it is an expensive hobby to experiment with!

This tip-sheet from Bullseye is one of the most informative of the many that they do - it explains what happens at what temperature, about annealing and devitrification.

Title: Re: slumping break and mould pinch - RESCUE REMEDY anyone?
Post by: Lorac on August 08, 2014, 12:50:30 PM
Yey! Big THANK YOU to all for encouragement.....I took kiln to 620 [watching closely] and as soon as it showed a tiny gap between glass and mould [with mould upside down and glass on piece of thinfire] I change programme to 2 minute hold and allowed rest of schedule to carry on as normal
Not only is the mould fine but the piece is still usable [by me only as crack is still visible!]
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cheers for support everyone
 :) :)

Title: Re: slumping break and mould pinch - RESCUE REMEDY anyone?
Post by: chas on August 08, 2014, 01:15:31 PM
Well done - talk about victory from the jaws of defeat!!

Title: Re: slumping break and mould pinch - RESCUE REMEDY anyone?
Post by: Pat from Canvey on August 09, 2014, 07:51:43 AM
That is very pretty and I see no crack.

Title: Re: slumping break and mould pinch - RESCUE REMEDY anyone?
Post by: ajda on August 09, 2014, 11:20:07 AM
Wow! You must be pleased with that for all sorts of reasons - a lovely piece of work and a great reward for your patience - I'm sure I'd have lost it all: patience, mould and glass! And what of the crack? It's the scars of experience that give us character... and anyway, it makes a great excuse to keep it for yourself!

Title: Re: slumping break and mould pinch - RESCUE REMEDY anyone?
Post by: MeadMoon on August 11, 2014, 05:17:57 AM
Yay!  Well done!  I lose patience easily and wouldn't have managed to do this so successfully.  Both my "small bead" moulds are thus broken after only one use each :)

Title: Re: slumping break and mould pinch - RESCUE REMEDY anyone?
Post by: Warm Glass UK on August 11, 2014, 08:40:54 AM
Hi Carol

We have just seen this and are obviously disappointed that you don't feel that we were as helpful as we could have been as we believe that we are generally good with free lifelong technical information and we spend literally hours on the telephone giving advice to customers and that it's something that we generally do well at - I'm sorry that we didn't meet your expectation on this occasion but I have checked the correspondence and our advice at the time still stands as our best estimate of a schedule. If you would like to speak to someone for more advice, please don't hesitate to call either Simon or Megan on 01934 863344. Many thanks.

Title: Re: slumping break and mould pinch - RESCUE REMEDY anyone?
Post by: Lorac on August 14, 2014, 08:22:10 AM
I would love to know what your feelings are on the slumping advice for the Candle Moulds.
The Bullseye tip sheets you link to suggest Slumping 593 - 704  so us novices are a little nervous of exceeding that by nearly 30 degrees!
Your schedule for the mould suggests 735 which has caused some grief in terms of glass 'running' for me - [you have suggested reducing by 20 degrees and then going up if not successful] I am trying to find out what the difference is in terms of effect on the glass between taking temperature down so it feels 'safer' and holding for longer?? or being brave and giving it the higher temperature for shorter time.
Have you had any feedback from other customers on this? and if you do would you normally add tweakings to your schedules? I know it is not an exact science and I know you are happy to discuss problems on the phone but if experiences could be pooled somehow it would be a great service for your customers!
Has anyone at your end used the 4 candle bridge mould in the Hobbyfuser?
many thanks

Title: Re: slumping break and mould pinch - RESCUE REMEDY anyone?
Post by: Warm Glass UK on September 15, 2014, 10:32:54 AM
That’s a tricky one... my advice at the moment would be to go with the firing schedule with the mould, the hot temperature is needed to create the dips for the candles to sit in on the candle bridge mould. As a slumping rule in general, it's always better to fire slower, lower, longer; however complex moulds like the candle bridge mould and also smaller moulds will need hotter process temperatures to mould the glass. I would follow the firing schedule given as this is tested and should be correct but we'll also run some tests in the studio on the Candle Bridge mould....