grinding and other abrasions effect between 2 fused surfaces - WarmGlass.com

grinding and other abrasions effect between 2 fused surfaces

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sbanthony
Posts: 6
Joined: Wed Feb 16, 2005 4:12 pm

grinding and other abrasions effect between 2 fused surfaces

Postby sbanthony » Thu Sep 24, 2015 10:34 am

Hi all,

I have an uneven surface on a piece of glass that I want to flatten before fusing it onto another piece. What is the coarsest grit level I can get away with and not compromise the fuse? The surface that is "ground away" will be down and not show on the final surface, so the question is more about what might compromise the result of the fuse, which will be somewhere between tack and full fuse. The glass being adjusted is transparent turquoise, the background is opaque, if that matters. I understand that grinding edges and surfaces gives scummy surface fusing results, but does it actually compromise the fusing process?

I will most likely be using diamond hand pads to accomplish this, possibly pumice, almost certainly not an actual grinder.

Thanks for any tips!
--SB

Morganica
Posts: 1079
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Location: Portland, OR
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Re: grinding and other abrasions effect between 2 fused surfaces

Postby Morganica » Fri Sep 25, 2015 12:23 am

That's a tough question to answer because it's going to depend on the heat the glass receives after it's been coldworked, and for how long. Obviously, if you nuke the heck out of the glass long enough, it'll turn into a puddle and you'll get a smooth, flat surface, and if you barely kiss the surface with a little heat, you'll pretty much retain the entire abraded texture. The question is, what combination of abrasion and heat will give you what you want?

As a rule of thumb, the less heat you apply with the schedule, the more hand polishing you'll need to do to the glass to refine and smooth it to a final finish. If you increase the heat, the surface will melt more, so you can stop at a coarser grit level and let the kiln do the work of softening the surface and letting it flow smooth.

You also need to understand that the side of the glass on the shelf is the best-insulated and therefore the coolest of the entire piece. The underside also has the weight of the glass resting above it to hold it down, so that surface is going to be the least-changed in the kiln. The face-up surface will soften and flow more than the underside.

If I am planning a second full, flat fuse to the glass, I don't need to coldwork much past 100/120--the heat of the schedule will smooth over the surface. I'll usually go to 220 anyway, because the finer grit doesn't catch as much swarf. (When you're coldworking, especially with machines, it's a good idea to keep a bucket of soapy water handy---as you finish a grit, dump the glass piece directly into the bucket to keep the swarf (grinding residue) from lodging in crevices and potentially causing devit. Keep the pieces wet until you can rinse them out and scrub them well with a brush and more soapy water.)

If I'm firing to a firepolish or tack-fuse (higher than a slump, lower than a full-fuse), I'll usually coldwork to 220 or 400-grit, depending on the surface quality I want. If I am slumping and firepolishing in the same firing, I'll coldwork to 400- or even 600-grit, which gives me a really nice polished surface. Backing off to 220 gives me a matte finish.
Cynthia Morgan
Marketeer, Webbist, Glassist
http://www.morganica.com/bloggery
http://www.cynthiamorgan.com

"I wrote, therefore I was." (me)

sbanthony
Posts: 6
Joined: Wed Feb 16, 2005 4:12 pm

Re: grinding and other abrasions effect between 2 fused surfaces

Postby sbanthony » Mon Sep 28, 2015 4:50 pm

Thanks Cynthia!

It doesn't quite address my exact problem, but there was so much info that I was able to formulate a better informed plan.

I have a question about the swarf. WHY does it have to be kept wet until scrubbed? (an early lesson learned when I switched from stained glass to fusing, LOL) How come it doesn't work to re-wet it later? What's the mechanism that causes that to be ineffective?

Thanks again,
--SB

Jerrwel
Posts: 129
Joined: Thu Feb 10, 2005 6:25 pm
Location: Charlotte, NC

Re: grinding and other abrasions effect between 2 fused surfaces

Postby Jerrwel » Tue Sep 29, 2015 12:41 am

sbanthony wrote:I understand that grinding edges and surfaces gives scummy surface fusing results--SB


Is this assumption true? If not, you may be going after the wrong answer and Cynthia's input is probably what you need. The issue of 'scummy' surface is probably related to residue from grinding or not grinding adequately and not the result of grinding per se.

Have you thought of firing the glass to be ground separately to obtain the surface you desire before firing the two pieces together?
Jerry

Morganica
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Joined: Mon May 19, 2003 6:19 pm
Location: Portland, OR
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Re: grinding and other abrasions effect between 2 fused surfaces

Postby Morganica » Tue Sep 29, 2015 11:38 pm

Mostly you just keep the ground surface wet until you can scrub off the swarf completely. If it dries into the pits and crevices of the rough surface, it's a lot harder to get out, even if you rehydrate. Grinding crud left on the surface is what leaves the cloudy, scummy appearance--it can provide "nucleation" sites for the glass to start crystallizing, i.e., devitrification.
Cynthia Morgan
Marketeer, Webbist, Glassist
http://www.morganica.com/bloggery
http://www.cynthiamorgan.com

"I wrote, therefore I was." (me)


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