Help needed! How to best fix sandblasted copper?

First, you need to find out how much material you need to remove to get to a flat surface. How large is it?

Take a micrometer, measure it, and then work on a patch with some 600 grit SIC sandpaper (probably wet) to see how much you need to remove to get to a uniform finish. Now, work out if this is acceptible. If not, it is possible to build up copper on the surface with electoplating, though this probably needs to get to a flat surface first.

On the plus side, they've probably strengthened the horn a bit by work-hardening the surface.

Blasting copper, even with soda was a bad idea. The impact of the media can bend the metal.

In the future, remove the paint by mechanical or chemical means, and remove the oxide with chemistry. you'll save yourself a lot of headaches.

For the oxides, you can use Sodium Bisulfate (aka Sparex Safety Pickle, aka Granular pool acid), it's cheap and does a good job, but can be hard on your clothes. Or you can use what I personally recommend, Citranox. It's made by the Alcanox people. It's citric acid based, and you can just about bathe in the stuff. It's about $30 for a gal of concentrate, that'll make 50gals of solution.

Now, back to the problem at hand.

Bead blasting will only frost the pits. Shot peening will only pound the snots out of it.

The problem with polishing is that it is essentially an abrasive process. You have to grind away every pit and scratch until the surface is shiny.

Before the blasting this wasn't so bad, but now you have pitting in every nook and cranny. They are hard to reach with a buffing wheel without grinding off the detail that made them nooks and crannies in the first place.

If polished is what you want, I only see one possibility. Burnishing.

If you had a really big tumbler you could work it with fine media. Barring that, hand burnishing is the way to go.

Rig up wooden forms to keep the horn from collapsing under the pressure and use a polished steel or agate burnisher to rub the surface back to something approaching a polish.

Paul K. Dickman Paul K. Dickman Studios The Repair, Restoration and Reproduction of Antique Metalwork

Tragically, I remembered my 6th grade shiny penny experiments too late. I should indeed have just bathed the horn in pickle. Actually, the blasting company showed me a sample of chemical oxide removal on the horn, and indicated they were going to do this for me instead of blasting. It looked great and got my approval, but they gave it to another employee to finish the job and he just blasted away with no regard for the surface or value of the item. The horn is ruined primarily because of a communications problem at the blasting company.

The horn is fairly large (32" long, 8" diameter at the mouth, 4" diameter at the throat, purely conical), and i've only seen small hand-held burnishing tools. Can I accomplish the burnishing with just a shiny piece of drill rod? I can support the horn down the middle by placing it over a piece of pipe, that's not a problem.

Would the burnishing be in lieu of polishing, or just the step prior to buffing? A buffed a similar horn (one without blasting damage) with tripoli and then white "rouge", and got the results I am looking for, but I honestly don't know what to expect from burnishing.

-Adam

On a conical section a round rod would work fine. You'll something like a rectangular bar with the end slightly rounded and polished to allow you to dodge into seams and corners. A hardened tool is best, but a soft steel tool will work on copper. Watch that no grit gets imbedded in it though. For flat areas, you would want something about the size and shape of a teaspoon's bowl mounted to a handle big enough to place a hand on each side and put your back in to it.

Conceivably it could be the final polish, (in ancient times they made mirrors out of burnished bronze) but I wouldn't push my luck.

Start slow and light until you get a feel for it. Then go heavy enough to remove the scars and a light once over to work out the burnishing marks.

Some oil doesn't hurt.

Paul K. Dickman

One other possibility:

If you can find an instrument maker, tinsmith, or silversmith with a blowhorn stake and planishing skills, he can probably planish the surface smooth, or at least get it to the state where it can be polished without losing a lot of material. Is this JUST the horn (or the horn is detachable) or is it an assembly? Either planishing or burnishing might require annealing the metal first, since it was probably surface-hardened by the blasting. That's a problem if you've got other parts attached to it that might not take heat.

Regards,

Bob

Whatever method you decide is best for smoothing the surface could be alternated with plating, so that you end up filling the pits and removing the peaks.

I would find a local chrome plating company in your area and consult with them. They can chemically clean the remaining areas of the piece and bath dip in a electro-plating copper solution to re-build up the surface that was blasted. I would do this before attempting any other method of polishing (physical or chemical).

Best of luck

Jim Vrzal

Cyl> I took a rare 1930s copper fog horn to a media blasting company for

Find someone to do a copper plate for you...

Electro-plate...

It will look better than new...

It only costs a little electricity to deposit copper. They refine the ingots that way and each is hundreds of pounds.

Do they come out shiny? Probably not. There's a chemistry that will tend to produce a smooth surface. I don't know what it is. Chemicals, temperature, concentration, agitation, time, and current density per square inch or foot. Current heats the solution. Cooling may be required.

I sure hope you have replacement value on your homeowner's insurance, or a good lawyer.

Good luck!

Yours,

Doug Goncz ( ftp://users.aol.com/DGoncz/ )

Read about my physics project at NVCC:

Electroclean. It will remove _no_ metal. I have used it on copper and brass as well as silver and gold plate many times with excellent results.

Ted

An english wheel could fix that, light pressure, it would put the shine back on the surface without changing the shape. Go to the yahoo group metalshapers, and ask the question, I think there is someone on that group that restores horns from memory, it would need a special english wheel to get down its throat.

regards,

John