Rectangular moulds

[Stephen23240]

Hello, I’m sorry if this has been answered before, I’ve looked but not been able to find an answer! I am new to glass fusing and am having fun experimenting. I’ve had success slumping with circular moulds but I tried a rectangular mould for the first time and had two abject failures! The mould is 8″ x 5″ x 1″ with a flat rim (3/4″) all the way round. Two faults have occurred: the first being that the sides of the rim have been pulled in so that the rim of the finished article is not rectangular but are curved. The second fault is that the glass has not filled the corners of the mould leaving a flat base that is only half the size it should be.

I have identified two areas that could be at fault: the firing schedule and the shape of the blank glass. Changing the schedule (reduced the upper temperature) did not succeed. Would a different shape of blank have a positive effect?  I would very much value your expert advice out there in forumland!

many thanks, Steve

[Webetilin]

I am very new at this but I have slumped in a similar mold several times in my class.  The instructor told me that longer soak times are required to get slumps to fill corners well.  I’ve not had the curved sides issue. If the temperature of the class doesn’t equalize properly, it will slump unevenly.  Soaks at various temps are supposed to fix that.  I’ve been experimenting at home with ramping up in stages with short soaks to allow the glass time for the heat to even out.  Maybe someone with proper expertise could elaborate on this.

Flee at webetilin’ Studios

http://www.facebook.com/webetilin

http://www.webetilinstudios.com

[Stephen23240]

Flee, thanks for your reply – both ideas sound feasible to me. I’ll adjust my schedules accordingly and give it another go!

Steve

[wordana]

You don’t provide a firing schedule, so this is just a guess, but I would start with lowering the top temp to avoid the “dog boning” as it is called, and increase the hold time to give the glass more time to settle into the mold.

Also, you should be fusing with at least two layers of glass to help avoid the sides pulling in. You can adjust the shape of the glass blank (making the sides convex to correct for the glass pulling in at the sides), but with the mold you describe, that should not be necessary.One adjustment could be to make the glass a little larger (1/4″) than the outside rim of the mold, which might help keep it from pulling in as much.

Dana W.

Jester’s Baubles Fused Glass Designs

http://www.jestersbaubles.com

[Stephen23240]

Dana,

many thanks for your reply. I’ve now tried this mould three times, reducing the upper temperature and increasing the hold time (1st time – 733°C & 15 mins; 2nd time – 680°C & 15 mins; 3rd time – 700°C & 20 mins). The third one was the most successful but there was still a certain amount of “dog boning” (describes it perfectly!).

I think the next one I’ll use slightly oversize glass and two layers. What do you advise with regard to upper temperature and hold time? I endeavour to change a single variable each time but fear it would take me forever to reach a sensible conclusion!

[wordana]

You will have better success with two layers of glass. Because of surface tension properties, glass wants to be 6 mm thick (which is two layers if you are using standard 3 mm glass).[Think about how water beads up when you put a teaspoon of it on a smooth surface.] Any glass stacked that is thinner than 6 mm will contract and any glass stacked so that it is thicker will spread (unless dammed). Using a single layer of glass is asking for dog-boning, unless you are firing at very low temps.

I typically do my slumps at 1225 deg f (663 deg c). I also incorporate a hold at 1100 on the way up. Quite often, by the time I actually hit 1225 (593) I don’t have to hold very long to finalize the slump.

That’s my experience, at any rate .

Dana W.

 

Jester’s Baubles Fused Glass Designs

http://www.jestersbaubles.com

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