Using dropout mould

[Martman]

Hi all

I have used a square dropout mould dropping 60 mm, the glass was a single piece of bullseye 6mm clear (freshly cut).  During the firing I noticed that bubbles were appearing in the middle of each of the four edges at the edge of the slope. 

Can anyone tell me what caused  the large air bubbles to form on the 4 sides, on one side around 15mm high, also on the under side it looks like the glass has separated ie the rough side separated from the smooth top.

The glass was placed smooth side up, should I have fused it first?

The process temp was 660C manually increasing to 680C

MartMan

[Zeldazog]

Any chance of a photo Martman?  I can't really picture what you mean - both of where the bubbles have formed, and what you mean by the glass seperating?

When giving your firing details, it helps to give the ramp up speed too, as this is often crucial in glass firing, as it can be just as important as the process temperature itself.

[Warm Glass UK]

When we do drop outs (or any type of slumping), we always fire the blank first to fuse it and then slump it in a separate firing as the temperatures are different - I suspect that this is your problem.

[MeadMoon]

I don't understand the replies.  If it's a single piece of glass, then what is there to fuse?

[Pat from Canvey]

I don't understand the replies.  If it's a single piece of glass, then what is there to fuse?

I think what they are saying is that they full fuse 2 pieces of flat glass first before any further work is done. That way, as the glass slumps it thins, but not enough to cause problems. Anyone else with a more technical answer?

[MeadMoon]

But Martman says:

<snip>the glass was a single piece of bullseye 6mm clear (freshly cut). <snip>

I totally understand that if there is more than one piece of glass, then they need to be fused together first before slumping, but this is a single 6mm piece if I'm reading it correctly.

Is there any chance that the bubbles form because the glass is catching on the mould rather than slipping?

[Zeldazog]

What is confusing about asking for a photograph and the full firing schedule, Meadmoon?

To be fair to Warm Glass' reply, whilst Martman *did* say he'd used a single piece of 6mm Bullseye, A) as far as I know, Bullsye per se doesn't come in 6mm thickness (it's Tekta) and B) he did ask himself if he should have fused it first.

[Martman]

Thanks for all the replies and sorry for the delay in getting back to you all.

It was a single piece of 6mm tekta it was not fused as I did not see the point.

Canvey has made a very good point did the bubbles form because the glass got cough on the mould.

Well the I did place the glass on the mould rough side down and the glass is the same size and as square as when it was first cut.  I had expected all of the glass to be drawn down leaving the edges of square drawn in.  

So what caused the glass to catch on the mould? Was the process temp 660-680 not high enough?
The mould Is 8257 from warm glass

http://www.warm-glass.co.uk/mould-8257-square-drop-out-od-25-x-25cm-p-1298.html?page=3

MartMan

[MeadMoon]

What is confusing about asking for a photograph and the full firing schedule, Meadmoon?

To be fair to Warm Glass' reply, whilst Martman *did* say he'd used a single piece of 6mm Bullseye, A) as far as I know, Bullsye per se doesn't come in 6mm thickness (it's Tekta) and B) he did ask himself if he should have fused it first.

Hi Dawn,  There's nothing confusing about the request for a photo & schedule - sorry if my reply implied that there was.