Should I fire face up or down?

[Kev]

I have another design in mind..well actually  a few, that require some of the elements to be cut with a ring saw and fitted in the body of another piece of glass. Obviously the fit will not be a perfect fit but will be close.

Question:

If I fit it together and there are still some small gaps or voids between the pieces, and I fire that as the face of the piece face down, will the cap drop down to fill in the voids or will the bottom layer spread to fill in the voids, and do I run a higher risk of bubbles?

What would be the effect reversed with the fitted glass on top of a base layer?

I was thinking by firing it face down  that the lines usually stay truer to the original shape, but that was with pretty straight cuts and no gaps or voids. Using the saw can sometimes leave a gray edge, and because the edges are so wavy, there is no way to use a fine grinder to regrind the edge after the saw.

I know I could do 2 complete layers and add the elements on top, but would prefer to incorporate them into the actual layers without adding relief to the piece.

Kev

[crazyone]

Not a clue,maybe try two small test pieces one face up one face down.

[Kev]

Well that's always a possibility..but if the knowledge is already out there, it will save me glass, money and time...LOL

[Kev]

I suspect I'll fire it face up, but I like the sense of depth from having a clear cap over the color.

[Judy K]

I would go face up with enough heat to contour fuse but not full fuse. This should seal the top layer and hopefully it will spread enough to fill the gaps. Maybe 1400F. Fill any gaps still showing with tiny chuncks or frit of the same glass. Then full fuse it a secong time to get it flat.

Or decide you want the same color lines that the base glass would provide. Like mosaic grout.

Try lightly sand blasting the edge after you use your saw , then scrub like crazy with dish soap.

[Kev]

Are you saying to contour fuse instead of full fusing in order to preserve the original lines if fired face up? I would rather not use frit if possible, and if the original shapes spread a slight bit to fill the gaps, that would still work well.

[Stephen Richard]

Face down to just above tack fuse.  Then flip, clean and fire again.
OOPS edit:   the tack fuse should be at quite a high temperature, so that some glass will move into any spaces left.  So maybe I should have said contour fuse.  Although I don't now what either of those terms mean to you, as they are so imprecise.

[Kev]

OK....not understanding here..why contour fuse as opposed to a full fuse?

If the glass needs to spread  to fill in gaps, is it going to spread enough with contour fusing?

If contour fused with the colored pieces face down and the slight gaps under the clear one piece cap, what is going to fill in the voids...clear glass from the cap or the spreading colored glass under the cap?

[Stephen Richard]

The reason glass has crisper lines on the bottom is that it is less hot. Therefore the glass from the top bends and takes up any gaps.  However the temperature on the bottom at a fuse that is inbetween tack and contour fuse is not high enough for the glass to distort.
Of course, feel free to take the item to a full fuse. That will test how much distortion is induced - not very much, I'm pretty sure.
I am of course assuming that the gaps are only minor due to the inevitable innacuracies of hand cutting. 

[Judy K]

In my experience, during a full fuse you will get the full sheet of glass seeping though the cracks between the pieced glass. If it is straight lines on the bottom those cracks are almost none existent, but a randomly cut piece can have 1/16" gaps. The glass over or under it will seep into this space at a full fuse.

If you want to minimize this seapage try contour fusing with the pieces on top. The top glass will be hotter than the bottom and start to spread a bit, but if you let the bottom get really moving too, it will move into the cracks. So stop at a temp, before this happens, anneal and set the glass.

Now when you full fuse a second time the glass is one piece and less likely to change much other than to flatten out at 1465F.

If you really want to make sure this works put a couple 'chips' of the same color glass on top for a third layer. The extra volume of the third layer will push the top glass into the cracks during your contour fuse, go up to maybe 1400F. You just want to fill those cracks so they don't show. Then set the glass in place and full fuse later for a flat surface.

[Kev]

Cool! Good info!

Thanks to all of you for adding input and info.

[JustBijou]

Sigh, I have never posted before and it says my post has been posted but I can't find it. Here is the photo I was trying to attach to it....at least I hope it attaches this time. This is fired face down to a full fuse, sandblasted then slumped. You can see between some of the pieces that black glass from the back seeped through.

[Becki]

Your pieces have to fit very tightly and precisely to avoid color from the back coming through.

[Stephen Richard]

Black glass is also the most flowy of the glasses.

[Kev]

Woo Hoo...another fuser and sandblaster too!

[Vic Rothman]

fill the gap with powdered frit and fire down

[ct4mom]

Very nice JustBijou