Polishing out scratches by hand?

[Jeannie Clark]

I have a black and white slumped bowl that had a few small scratches on the inside curved surface (not sure how). I tried to use a 3M extra fine hand pad to remove them but made the matter worse as the area that is spoiled is larger than the original scratches. I have some cerium oxide, and wonder if I use that on a cloth it will restore the glossy surface - or whether to try fire polishing in the original mould? The dish has actually been purchased by a customer subject to the surface being restored.

[Bob]

Hi Jeannie,

I think the fire polish solution will work better. I have never had much success removing scratches with hand polishing.

Cheers,

Bob

[Jeannie Clark]

Thank you Bob - I'll do that .

[Franzeska]

I'm a confused newcomer. From previous posts on another topic, I came away thinking that it is not advisable to fire polish in a mold. If it is okay firepolish in the mold in the instance at hand here, why not just combine steps to firepolish and slump in one firing? I'm sure I'm missing something, so please be kind when you correct my thinking.

Thanks.

Franzeska

[Bob]

Hi Franzeska,

Good question. Under normal circumstances you do not fire polish rough edges during slumping. Jeannie is looking for a way to repair (or salvage) a piece with a few surface scratches. To achieve this by hand polishing is very challenging because you not only have to remove the scratch but you have to bring the surface to a high polish consistent with the glassy lustre of the rest of the piece. This requires using a succession of finer grits and ultimately cerium oxide. From my experience the area around the scratch gets marked and the final effect is a larger area with a slightly matte finish that is as annoying as the initial scratch. If you had a tool that minimized the area being ground in the repair it might be possible.

Resorting to a fire polish will repair the scratch but will "over slump" the piece. The underside of the glass will pick up texture from the mould and will be rough compared to the original finish of the back. This is probably why people don't recommend doing a full fire polish when slumping. If fire polish is the only remedy then there is a solution to the highly textured back of the bowl. First refire to remove the scratch by fire polishing Next sandblast the back with a fine abrasive (say 220 grit) to remove the rough texture. Put the piece back in the mould and refire to about 1150F or 620C . This will bring up the lustre of the back slightly ... lovely soft matte finish. Instead of refiring after sandblasting you could apply a sealing agent such as Liquid Lustre.

Another risk that Jeannie will have with fire polishing to remove the crack is that if air is trapped between the glass and the mould she could get large bubble. The solution is to ensure that the vent holes in the mould are open and are at the last place where air would be trapped between the glass and the mould.

As an aside... if you finish the edges of a piece prior to slumping then the slight amount of fire polish in a normal slump will clean up any slight haziness in the polished edge. In other words you don't have to bring the piece to a perfect polish. It saves time and effort.

Really hope this clarifies why I answered Jeannie as I did.

Cheers,

Bob

[Morganica]

To add to what Bob says...I usually firepolish during the slump to gloss up coldworked edges. It requires that the area to firepolish be coldworked to a relatively fine, smooth finish (I usually go to 400 or 600 grit, depending on the gloss I want). Then I fire at the high end of the slump range and hold for an extra couple of minutes. It would be the same for anything coldworked to that level, including surface defects, i.e., you get rid of the defect, get it as close as possible to the rest of the surface, then let the kiln take it the rest of the way.

The first few times you slump+firepolish, you keep an eye on the kiln during process temps, and stop the process as soon as the slump is finished and the edges show gloss. And you check every time you use a new kiln, or a new mold that's pretty deep.

That's different than a typical firepolish to correct surface defects such as scratches, or a firepolish that melts clear powder into the surface to hide devit. With those, you're looking for the glass surface to soften enough that it spreads and covers the defect. The amount of heatwork necessary for that level of softening is more than needed for most slumps, and the glass can start sliding down into the mold and collapsing in on itself (in addition to taking on too much mold texture). So...you must firepolish first, slump next. If the piece is already slumped, you probably need to fire to flatten AND firepolish, then slump again.

[Franzeska]

Thank you so much for going into depth on this topic. I understand so much better now.

Franzeska