Slumping and Fusing Questions

Answers to Common Slumping Problems

Is my glass stressed out?

Zebra fused glass by a student
I have been producing small pieces of slumped float glass in under 2 hours. They have survived successfully but I can see light brown colors around some of the textured areas. Could this be stressed? How do I eliminate this problem?

P.S: this is a very cool site!!!!

Milly’s reply:
Hi David, thanks for your enthusiasm about my site, you can come back!

As for your glass – well, I’m amazed. Amazed that you’re managing to produce slumped pieces in that short time scale, and amazed that it’s not broken even before you open your kiln.

Stressed it will be. Have a look at my Kiln Firing Schedule on this page to see why I’m amazed. Fired glass – especially glass that has been taken to high temperatures and been asked to ‘do’ something (slump or fuse, for example) – needs to go through what’s called an annealing cycle.

‘Annealing’ means holding and cooling down the glass at an appropriately slow rate to allow it to safely make the transition back from a viscous liquid to a solid state. All its molecules are in disarray and need time to realign. There’s no rushing it. You can’t over-anneal something, so it’s always better to be conservative.

You don’t say how big your pieces are, but even bead makers anneal their work, so it’s definitely something you need to research. At 6mm and 10mm it’s reasonably thick, too.

Float glass isn’t all the same, sadly. Find out from the manufacturers of the glass you’re using what their annealing temperature is, and ask them for a couple of generic firing schedules to get you started. Then start experimenting – your kiln will have hot and cold areas, too, so it’s all a bit complex. Bet you wished you’d never asked now?! I hope this points you in the right direction.

Best reusable mold material?

I have been making glass fusing molds from various types of plaster and managing to get 1-4 firings before they start falling to pieces….what is the best thing for me to use to make moulds that is not going to cost me an arm and a leg…and can you tell me why a mould has to be dry before firing and what happens if it is wet? Sharon

Milly’s reply:
Good questions! One of the cheapest and easiest materials for slumping molds is clay. It’s simple to mould into shape, is relatively cheap (you can use bottom of the range clay, nothing fancy!) and you can re-use it again and again.

You have to fire it first – a bisque firing – and then make sure you use a separator to prevent the glass from sticking to the molds. Use a kiln wash for this – dilute with water 4:1 – and paint on in four thin layers, each one dried out before adding the next. Try not to get brush strokes – they will pick up on the glass.

One word of warning; I never put that dirty brown stuff anywhere near my kiln, it will contaminate your lovely CLEAN kiln! If you have a friendly potter/ceramicist, get them to fire the fused glass molds for you. It’s great that you’re making your own moulds – it adds a uniqueness to your work that you won’t find elsewhere.

Glass casting molds, or any molds that go in your kiln can damage the elements, break kiln shelves or cause devitrification if used damp. So it’s good practice to dry them out beforehand. Having said that, I used to dry my glass casting molds out during the early part of the kiln firing schedule with no adverse results, but casting is a bit different to fusing or slumping, as the face of the glass isn’t exposed.

Mould Mix Suddenly Too Wet

I’m trying to create different slumping molds, and recently they have not been working out so well. The mixture seems wrong but it is the same one I have always used. I just wondered if you know if flint can go off after a period of time, or if you know what might be at fault. I use the ratio of 1 pt water 1 lb plaster to 1 lb flint or quartz. Recently they have not been combining well and seem too wet. Any advice you could give would be much appreciated.

Milly’s suggestions: Hi Judith, thanks for your question. Moulds – argh! They can drive you mad… Flint can’t go off, no – so that’s one potential problem out the way.

With regard to the ratios – did you have a look at the recipe on my Mould Making Instructions page? I find volume easier than weight, so I have three large yoghurt cartons and measure 1 flint, 1 grog, 4 plaster and 2.5 – 3 water. A bit unscientific I know, but you can always add a bit more plaster if it looks too wet. It is a moveable feast – sometimes the same recipe you’ve been using forever suddenly need adjusting for no apparent reason.

It looks like you’re using 1:1 ratio, with no strengthening grog? The grog – fired clay broken up into tiny pieces or powder – strengthens the moulds, whereas the flint allows them to resist high temperatures. You can buy grog from pottery suppliers, it may help.

One last thing; I’m assuming that you’re putting the plaster in last? That’s crucial, and should be sprinkled (sieved ideally, but careful of the dust) to create little peaks poking out above the water before you start mixing. If you’re not getting peaks of plaster, there’s not enough. It’s difficult to explain without seeing it, but I hope this is helpful.

Problem slumping bowls

When I slump a 12″ bowl at 740c hold for 4 minutes, the result is fine, the outside has a nice shine. However when I slump a 5″ small bowl using the same kiln firing schedule, it does not slump totally into the bowl. If I move the temp up to 760c they slump fully, but I am left with a frosted effect on the outside. What am I doing wrong? Janet Rogers

Milly’s suggestion: Good question Janet. Slumping is a case of lots of trial and error, as you’ve probably found out! The additional weight of the 12″ glass will make it slump more effectively than the 5″ bowl – that’s why you’re having to increase the top temperature.

You don’t say what type of glass, or what glass slumping mould you’re using, so I’ll guess that the ‘frosted’ look is either devitrification, or the glass is picking up the surface of the mould because of the higher top temperature.

I’d have a go at not increasing the temperature – keep that at 740C – but increase the soak to 10-15mins, depending on your kiln. That will give the glass time to sink down into the mould. Alternatively, you could keep the top temp at 740C and the soak at 4mins, but do the heat work on the way up by slowing down the ascent. As I don’t know your kiln firing schedule, it’s a bit difficult to say any more than that! I hope that helps.


4 thoughts on “Slumping and Fusing Questions”

  1. Hi,
    Newbie here. If a piece hasn’t entirely slumped, has wobbly bottom, can I re-slump it with a longer hold time at the top slump temp previously used? TIA

    • Yes you can re-slump pieces. The longer soak at top temp should do the trick. It’s a long learning process and you seem to be figuring things out and staying on the right track, well done you 🙂

    • Yes. You can flatten it out by firing it again but it’ll never be exactly the same as it was as an unfired sheet – it’ll pick up some kiln shelf texture.


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