Blasted Sandblasting?

[Terrie Corbett]

Hi all,

(Brock and Avery this is a good news/badnews situation!).

The good news is that we got a new sandblaster -- the bad news is that I may have a larger learning curve than I realized!

(1) I fused 2 layers of glass 13" x 19" (2) then I slumped it on a double curve bullseye mold 15 3/4" x 15 3/4" by 2" (I also had kiln furniture with fusing paper on either side of the mold just in case it didn't slump in the middle as much as it seemed like it would) and (3) I elevated the mold so that it would heat more evenly.

It slumped wonderfully, everything was fine, it was free-standing-- it looked ok and then (4) I decided I needed to sandblast the back of it -- which I did. Bad move
I didn't like it so much. It was ok, but the clear areas had looked better than the "frosty" areas now looked. So I put it back on the mold and tried to polish it again. This time a large "swell" came up in the lower slumped area. (Discounting my ignorance) was the sandblasting the culprit? What little "get up and go" this piece had got up and went!

Thanks for any assistance.

Terrie

[Steve Immerman]

Terrie,

I have not had this problem, but I wonder if somehow, in placing the piece back in the mold, it was positioned in such a way that a tiny bit of air was trapped under the deep curve. I don't know if the sandblasting played a role in this.

If your mold is like mine, there are no holes in the mold for air to escape. When heated, the trapped air expanded and gave you that raised area. You might want to try drilling a few small holes in the mold in the bottom of the curve and try again - unless the area is now so distorted that it won't be able to slump back down and maintain your design.

I wonder if others have had this experience, and if in general when re-slumping into a mold one should re-evaluate the location of air holes and consider drilling more since the spacial relationships have changed?

Steve

[Barbara Muth]

I Have found when firing sandblasted surface down that it is difficult to get a clean non-frost surface anyway. My approach in this situation would have been to slump it flat, then blast, then use diamond pads to bring it up to 400 grit, then slump anew. Maybe I do too many steps, but it is the only way I have found to avoid frost veils on the bottom. (unless I use my trademark "flip'n'fire" technique)

B

[Terrie Corbett]

Barbara and Steve,

Thanks for your input. I have "many miles to go" when it comes to problem solving, but I'm in this for the long haul! I will drill some holes in the mold as you suggest, Steve, that may be the key. Barbara I want to try the cold-working next time also! I am not opposed to additional steps! I'm appreciate your responses.
Have a good afternoon!

Terrie

[Cliff Swanson]

Just lending my $0.02 in support of what Steve already said.... I frequently sandblast and refire pieces either because I want to really clean the surface, or to produce a matte finish. I have not had any bubble trouble that i can directly attribute to blasting. I have also reslumped pieces on occasion when i'm not happy about the symetry or positioning of the final piece relative to the mold, but all of my molds have holes in them, so i assume that I have not had bubbles because of the escape route for any trapped air.

There is a lot in the archives related to preventing large, destructive bubbles from forming in work. They are due to air becoming trapped and then growing in size beneath the glass during heating above the softening point. Check out the archives by searching for "bubble squeeze" or "shelf bubbles". The bottom line is that heating between about 1000 or 1100 and up to around 1250 should be slow...maybe 60 dph. This enables the glass to sag down onto the shelf, squeezing the air out as it does so.

Cliff

[Terrie Corbett]

Thanks, Cliff,

Will do.

Have a good evening.

Terrie