aluminum oxide Q?

[Rosanna Gusler]

was going through the new granite city tool catalogue and saw aluminum oxide polishing powder. one third the price of cerium. anybody here ever use it on glass? rosanna

[David Paterson]

I tried a very fine aluminum oxide powder several years ago. It wasn't very finely graded, although the stated grit size was very small (1200 I think). Just one larger particle will leave lots of scratches.

Also , one theory as to why cerium works as a polishing agent is that it reacts chemically with the surface of the glass, causing the glass to flow. I believe there is some reality to this, from years of looking at a piece as the polishing progresses.

I now use exclusively diamond powder and cerium. Diamond powder is very finely graded, and a little bit lasts a long time.

Diamond powder will embed itself in many materials, simply by spreading a little bit over a wet surface and using a flat piece of glass to rub it in. I usually use a damaged paperweight. My favourite is a particular type of plastic that I glue to a magnetic backing. Once embedded in plastic you can do light grinding on that plastic for quite a while, and only need to add a carrot or two to the surface to refresh the wheel.

60, 80 or 100 grit diamond embedded in plastic is quite aggressive at removing deep scratches from a rough grind, yet it leaves a smooth finely ground finish. I presently use 60 or 80 grit. In fact, it is so aggressive, I usually use it as the first grinding step if I am only doing a bit of stock removal to even out the bottom of a piece. Diamond on plastic will never chip an edge badly.

Next, 300 grit diamond will remove the finish from the last step. My final step uses 600 grit diamond on plastic, followed by cerium on a piece of carpet glued to a magnetic backing.

It is actually quite cheap and easy to build your own wheels. Diamond powder seems expensive, but it is very finely graded, and cuts very aggressively. On a moderate size surface, such as the bottom of a vase, spending one or two minutes per step is about the norm, with the cerium possibly taking longer.

[Scott Dunahee]

What kind of plastic, David?

I just bought a full set of diamond pads for the 18" lap I'm building and I'd like to know.

BSD

[David Paterson]

Just curious, what did the 18" diamond pads cost?


The plastic I use is available at many lumber yards and building supplies.

It comes in 2' x 4' sheets.

It is used as a light diffuser for fluorescent lighting.

There are different styles in terms of the pattern that is pressed into the plastic to break up the light. The one I use has an eggshell finish on one side, and hundreds of little irregular pyramids on the other side.

The eggshell side is the top in this application. Because it slightly irregular, it prevents hydroplaning. Also, this type of plastic is not only quite hard, it has a slippery feel to the surface.

It is brittle to cut. I use a bandsaw.

[Scott Dunahee]

I bought them from HIS glassworks. I have a grit lap that I run at 80 and everything after is diamond to cerium. I bought a 140 grit hard plate and the purple, brown, red and felt wheels.

It was around $800 for the set, which is fine as I know from experience they last a long time, especially since I'll be doing stock removal with SiC grit.

I'm always open to saving money later on though.

BSD

[Pete VanderLaan]

I am of the belief that diamonds will never give you the quality surface achieved with silicon carbide, pumice and cerium. I find most polished stuff I see to be substandard but it is also the glass. A good glass for polishing should be high in Potassium and should contain some barium . Commercial glass bodies do not put the money into the batch materials to ever give you good refractive indices. You could polish bottle glass all day and it will remain low lustre.

Diamonds are fast for a while but always throw long scratches that have to be polished thru. Once the sweet spot is worn away on a pad, it slows way down. I am currently using a 30 inch diamond out of necessity and I really long for my old system. I will admit it gives you a clean studio though and that's important. As soon as I can get back to 60 grit SIC, a 27.5 mu aluminum oxide and Canneto Lipari pumice followed by superox PMD cerium, I will be happy again. My diamond pad came from China, has been as good as I have seen them and cost a bit over 300 dollars each. Josh and I bought a bunch together.

The glass does not flow when polishing with cerium. That is utter crap. There is a theory that the earth is only 10,000 years old too. It impedes children from learning.

How's that for a curmudgeonly response?

[Scott Novota]

Strong with the curmudgeon you are.



Gnat.
.

[David Paterson]

"The glass does not flow when polishing with cerium. That is utter crap."


Actually, this issue goes back a long way. Not everybody would agree with you.


From an article by Robert Sabia & Harrie J. Stevens of Corning Incorporated:

"Investigation of the nature of chemomechanical polishing (CMP) for glass requires an in-depth knowledge of the influences of abrasive physical properties and slurry chemistry. In the present work, eight cerium oxide abrasives were used to polish fused silica on a polyurethane pad.

The CMP mechanism is concluded to be a chemical reaction at the particle/workpiece interface promoted primarily by mechanical interactions such as abrasion, with both cerium oxide and rare-earth oxyfluoride constituents promoting the reaction."

From:
http://www.informaworld.com/smpp/content~content=a777372782~db=all
http://cat.inist.fr/?aModele=afficheN&cpsidt=882108


AND:

" In experiments conducted by Dr. E. D. Tillyer, of the American Optical Company, a lens surface polished with rouge was tested and found to contain no trace of iron, but after etching away the surface with hydrofluoric acid, a positive test for iron was obtained, indicating that glass had actually been deposited on top of minute particles of rouge. With further etching away of the lens surface, the actual marks of grinding were revealed, which certainly could not have been there if the polishing action were an abrading one."

From: http://www.best-telescope-guide.com/48-theory-of-polishing.html


AND:

http://cat.inist.fr/?aModele=afficheN&cpsidt=2344605

http://www.bovagems.com/eclectic/HTML/19980101_9801GEMPOL.html




.

[Pete VanderLaan]

unless you use a decent glass, you will never get a decent polish. Putting lipstick on a pig does not really fool anyone.

While it is certainly true that managing the PH of your slurry medium as well as the precise temperature of the liquid to insure that the maximum amount of cerium can be kept in suspension, I would wager that you don't do that .

I actually did. What really matters however is the quality of the cerium that you use as well as the quality of the pad upon which you polish. That on top of not using a glass that is really not engineered for polishing will give you predictable mediocre results. I know of at least fifty different cerium compounds that range from economy to premium. It's probably a waste of time to use premium cerium grades on soda lime glasses with no meaningful amounts of potassium or barium or lead in them regardless of whether the glass might flow and somehow become pretty.

[David Patchen]

I've found cerium really does make the glass flow if you coat your glass with a paste of the stuff, bring it up in a box to 990 - 1050 (depending on size/thickness), punty it and put it in the glory hole. Flows like crazy.

How's that for a smartass answer?

Is it possible that the cerium *does* very finely abrade the glass and that abbraded glass powder finds it's way into the micro-scrapes and pits on the surface you're polishing, thus trapping some of the stuff 'under the glass'? Dunno.

[Tim Bassett]

I have no idea how cerium works but have been using a slurry of cerium on a woollen carpet wheel with great effects...the only problem was that the cerium always got itself onto the body of the piece I was polishing and as this area is sandblasted we had a lot of trouble removing the spots left on the sandblast surface.We polish on a pneumatic drum using various grades of silica carbide belts starting at 60 grit then to 120..240..320..400..600..cork and then cerium. The 400 prob in not necessary but this is the time we double check that it is all good.
Yesterday we recieved some cerium belts from HIS, WOW are these things good...no mess so no spotting problems and as fast as anything the best polish we have ever had...a bit pricey but they remove the problems we have had with the slurry leaving a mess on the body of the piece. I really recommend these belts for anyone that needs a high high polish and don't want the mess.
Just my 02c

[David Patchen]

Why not blast *after* polishing and don't risk getting the cerium into the surface? I always sandblast after the bottoms are finished.

[Edward Skeels]

Originally posted by David Patchen

Is it possible that the cerium *does* very finely abrade the glass and that abbraded glass powder finds it's way into the micro-scrapes and pits on the surface you're polishing, thus trapping some of the stuff 'under the glass'? Dunno.

No David, it doesn't work that way. Guessing how something works got civilization the flat earth theory.

Cerium does change the surface chemically.

[Brian Blanthorn]

I think U will find nothing better than cerium 4 final polish on a good surface

I can only say what I see

And with cerium + felt I can get the appearance of "melting" of the glass if I get it hot enough

I actualy try not 2 as its close 2 cracking but I can get qite a pronounced smearing look

Brian

[David Paterson]

"I can get qite a pronounced smearing look"


I have seen this smearing look, which is what led to my original comment.

[Edward Skeels]

re; smearing

If you repolish with cerium (felt wheel) at 90 degrees to the smear can you re-smear in a new direction?

[Pete VanderLaan]

Tammy Fae Baker can.

[Charles Friedman]

I heard that when Tammy Fae died, they had to surgically remove her make-up: they found Jimmey Hoffa.

[Paul Stevenson]

That's a neat tip David. How do you secure your disk on the wheel? Magnetic sheeting as in magnetic vehicle signage?

[Brian Blanthorn]

Originally posted by Edward Skeels
re; smearing

If you repolish with cerium (felt wheel) at 90 degrees to the smear can you re-smear in a new direction?

Definately yes

I can remove a deep spot / scratch and can smear the scratch as tho melted

I do try n not smear as it can B realy pronunced

Certanly looks flowing / melted

I realy try n get everthing done on Rosie Lap

Brian

[David Paterson]

"That's a neat tip David. How do you secure your disk on the wheel? Magnetic sheeting as in magnetic vehicle signage?"


Yes. You can buy the magnetic material from a sign making supply, or from some art supply stores.

I attach the plastic to the sheeting using a spray adhesive on both materials. With the right spray adhesive, you can separate them later, so that you can add a new piece of plastic.

The wheels are basically throw aways, in that they are very cheap and easy to make. You only need a few dollars worth of diamond powder on a new piece of plastic to start grinding.

Also, if the wheel picks up some grit in one spot and is putting a scratch on your piece, you can often remove this by running water over the wheel and scraping the offending part of the wheel with a knife blade.