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Here are more micrographs of sandblasted BE glass comparing SiC with Al2O3:


Clear, unblasted, firepolished


Clear, blasted with Silicon Carbide, firepolished


Black, blasted with 120 grit Aluminum Oxide, firepolished


For more photos, go to: http://www.clearwaterglass.com/SandblasterTest.htm

Steve

this is SO cool. brian is right when he says 24/7 research and development. rosanna

Wow!... that makes it really clear... doesn't it?

To summarize, the silicon carbide may be faster and last longer, but for fusing, the duller, cheaper abrasive gives better results (and more shocks).

Thanks for posting these photos Steve.

Tony

Tony Smith wrote:Wow!... that makes it really clear... doesn't it?

To summarize, the silicon carbide may be faster and last longer, but for fusing, the duller, cheaper abrasive gives better results (and more shocks).

Thanks for posting these photos Steve.

Tony

The AO test (#3 ?can't remember) was on black and the SI test #1 was on clear. I wouldn't expect the blasting to be as photographic on black as on clear. Are image #1 and image #7 equivalent maybe? Those were the only two tests I saw that compared AO to SI on clear glass after firepolishing. The AO one states it was firepolished to 1450F....thats firepolishing? Seems like fusing to me. Was the SI one at the top firepolished to 1450F too? I'm tired and pissed at my job and not thinking very clearly so maybe I should revisit it tomorrow.

Interesting pictures Steve and good investigating too. Thanks to Nikki and Tony too.

Looks like the silicon carbide grit is blasting deeper holes in the glass that seal over with firepolishing trapping air. I've seen the same problem if I blast with aluminum oxide and then coat with an overspray like borax. Unless fired really hard and hot the surface is always cloudy from trapped bubbles.

I looked at some 60 grit silicon carbide with a high power eye loupe and the grit looks very sharp and angular and some pieces even look like long splinter like crystals. I couldn't see much in my 120 grit aluminum oxide sample, too fine.

Ron

Ron Coleman wrote: Looks like the silicon carbide grit is blasting deeper holes in the glass that seal over with firepolishing trapping air.
Ron

Ron,

That theory sure seems to explain what we're seeing.

Steve

Ron Coleman wrote:Interesting pictures Steve and good investigating too. Thanks to Nikki and Tony too.

Looks like the silicon carbide grit is blasting deeper holes in the glass that seal over with firepolishing trapping air. Ron

Hi Ron and Steve,
Nikki also showed me those photos - but in my experience, fire-polishing doesn't soften the glass surface enough to let it and trap ubbles - they usually just come out with smoother edges around the perimeter.........

I understand how an overspray could trap bubbles, but not fire-polishing....

an inquiring mind,
judith

Judith,

Keep in mind that to the naked eye, the bubbles on the firepolished surface are nearly impossible to see. Steve had seen some very, very tiny bubbles or pits on the surface of some glasses that he was working with, and the problem became more apparent when he tried Brock's irid to irid technique using SiC.

The purpose of the microscopic examination was to try to understand Steve's experience seeing the haze after using SiC and not with Al2O3. From the micrographs, it's clear that the haze is actually microscopic bubbles that are trapped under the surface. Whether these bubbles can be seen by the naked eye wasn't really the issue.

Tony

Ron: That's an interesting observation with the 60 grit silicon carbide. I looked very carefully for a distinguishing fracture pattern between the samples blasted with aluminum oxide and silicon carbide at the finer grit, but could not see differences. I suspect differences are there, though, and that silicone carbide may cut deeper or finer fissures that more easily trap bubbles. I'd be interested in examining and photographing a 60 grit sample. Are there comparable Aluminum oxide grit sizes available for comparison?
db: Images were taken in reflected light , with only a small amount of transmitted light used for clear samples, hence the dark background with clear samples. Many more were taken if you have an interested in seeing additional images.
Judy: It might be interesting to look at blasted samples with degrees of firepolishing. The pattern of healing over may show us something more about the formation of bubbles. The whole project has been fun and interesting. The glass samples were really hard to keep clean; fingerprints and dust on samples was like looking at sludge spread out on a field. Steve and Tony: May I keep the samples for a little while longer? I'd like to try a different scope.
Nikki

Having my blast chamber already filled with SiC 120 as I like to carve I would be interested if there is any difference related to the pressure of the blast. I could blast up some samples of black, clear and clear on black that were blasted at different pressures. I was wondering if you blasted SiC at a lower pressure (I currently use 40) if the lower pressure might not give you a similar finish that the AlO3 does?

Also, what if you firepolish before you fuse? I do just few enough pieces of irid on irid that I would do the extra step if it means I don't have to reload my blaster. That's a test I can do but I would be interested if someone has already done it.

phil

Nikki,

I appreciate the time that you have put into this project. The samples are yours as long as you want them. There may be more information to get out of them.

It would be interesting to know what shape the bottom of the crater is in the SiC sample compared tot he Al2O3 sample. Any chance we could do a cross section view? Do you have access to that type of cutting and polishing equipment?

Tony

Tony: My sectioning hardware is made of glass bars and razor blades!. But It may be possible to get an idea of the character of the blasting fracture in cross section by simply scoring (on the back side) and breaking off a small strip of the sample. OK with you and Steve if I try that?
Phil: If you'll send the samples, I think it would be really interesting to compare images from different blasting times (as well as the grit size). There are still a lot of variables that should be controlled for, in the best of all possible worlds, such as the degree of contamination of the blasting grit, blasting pressure, distance of the nozzle from the sample, magnification of the image, etc. We could even replicate the experiment on different blasters (I have access to different blasters locally). Judith has a lot of experience and suggested additional possible tests to determine possible causes for the observations. If we keep track of such details our results might be publishable. . This feels like research. I love the scientific method. It's so much easier than art.
Nikki

Nikki,

I'm all for scoring and breaking to expose the cross-section of the bubbles. It's a simple solution. I'll check here at work to see what they have for equipment, but my guess is that they use lapwheels to make their micrography samples.

As far as Phil's point... pressure is just a variable (I used 20 psi... I think Steve did as well) but the impact of the grit on the glass is affected by pressure, nozzle diameter, grit size, mass flow and distance to the glass... just too many variables to be able to make sense of it. You would have confounded results.

Tony

ps: god I love research!!!

Aside from the fusing appearance issues (which affect fusers more than me; a luke-warm glasser), are you-all speculating that blasting with SI weakens the glass in general and would be best avoided altogether?

Nikki O'Neill wrote:Steve and Tony: May I keep the samples for a little while longer? I'd like to try a different scope.
Nikki

Nikki,

You don't have to send mine back at all, and you can cut them or do whatever you want with them.

Steve

db wrote:Aside from the fusing appearance issues (which affect fusers more than me; a luke-warm glasser), are you-all speculating that blasting with SI weakens the glass in general and would be best avoided altogether?

Don, I wouldn't venture so far as to say that. I think that since the depth of the crater is very small, it is insignificant relative to the thickness of the glass, and therefore shouldn't degrade the strength of the glass by any measurable amount. I have no evidence to prove that, but that would be my hypothesis.

Tony

Phil Hoppes wrote:Having my blast chamber already filled with SiC 120 as I like to carve I would be interested if there is any difference related to the pressure of the blast. I could blast up some samples of black, clear and clear on black that were blasted at different pressures. I was wondering if you blasted SiC at a lower pressure (I currently use 40) if the lower pressure might not give you a similar finish that the AlO3 does?

Also, what if you firepolish before you fuse? I do just few enough pieces of irid on irid that I would do the extra step if it means I don't have to reload my blaster. That's a test I can do but I would be interested if someone has already done it.

phil

Phil,

Before we took these microscopic photos, I tried various types of glass, various blast pressures and distances with SiC, and different firepolish temperatures. I could tell just by looking at the glass with the naked eye, or a 5x magnifier if the surfaces were hazy. (This was not with the double irid technique, but on the surface of two layer pre-fused squares.)

I found that:

1- with opal glasses that the haze was harder to see (maybe because the tiny bubbles were obscured by the opal glass?). Also, less noticeable on clear glass.
2- I varied the blasting pressure from 15-40 psi without any significant change.
3- I changed the grit from 120 to 220 with minimal improvement.
4- I tried various firepolishing temps from 1325 to full fuse, but could not get the haze to disappear.
5- I tried various techniques to clean the glass after blasting (including Avery's multistep ritual) without any effect.
6- I switched to Aluminum Oxide and the haze disappeared both on the suface of firepolished pieces, and irid sandwiches.

So, I could not find a combination of techniques with SiC that would eliminate this haze. I could get away with using it on opals, because the effect was hard to see, but I couldn't get what I considered an acceptable finish with transparent colors.

Steve

db: I suppose if the blasted glass were very thin there might be a noticible difference, but we're just looking at a very thin plane (surface). I doubt that structural strength would be an issue for most fusers.
Nikki

What other controls were in place for this? Do we know that pressure, nozzel size, distance of nozzel from glass, etc. were all the same?

Paul,

So far, I've viewed this as an "all or none" phenomenon at the macro level. ie Either visible haze is present or not. The only variable that I found that made a difference was changing abrasive, so these photos were made to answer the question:

"Am I nuts, or is there something different about the surfaces blasted with SiC vs Al2O3?"

So, the finer points of blasting and firing techniques have been ignored.

However, as Phil mentioned, there may be more to it that just the abrasive, and it may be possible to successfully use SiC by finessing the blasting pressure, grit size, firing schedule, and blasting technique. However, I'm not sure you need micrographs to prove this. You can just look at the glass see if it is acceptable.

I wasn't really trying to turn this into a research project - just to get my pieces to come out OK, and to evaluate my sanity.

Steve