marble polishing query

[bsorcs]

Gents,

I must polish 2 old white marble dresser tops, 12-13 sqft total. Conditions range from sub-gloss to coarser-than-honed on the pieces; they also are from 0.003"-0.013" out of flat. Preliminary sanding will have to be done pre-polish. In looking at several polishing products [MB, Samich/Klindex, Tenax] I note that all contain oxalic acid and/or oxalate salts and, variably, carnuba, resin, and shellac. These components surprised me for the following reasons. The low-pH oxalates because of the seemingly universal dictum to avoid acidic agents when working with calcite-based stone; the ?filling/shining? agents because of the potential for eventual yellowing.

In perusing a sculpture forum I found recommendations for applying oxalic acid, or a combination of acid and [tin] oxide, to a smoothed [~1200 grit] marble surface. A generalized statement was that the acid 'melted and fused' the surface and the TiO2 abrasion provides gloss. From a geochemical perspective, this might, in essence, be correct, as the abraded surface would comprise highs and lows. The highs would tend to be pointy and would have higher surface area-to-volume ratios than the rest of the surface. The highs, then, would be more readily dissolved than the flats or lows. The low pH of the acid also could be a requisite for use of really small grit...<0.05 micrometers.

Given the above, I'll likely sequentially hand sand [wet] the pieces, ending with 3000 grit. If 3000 does not get me sufficient gloss [don't know what 'sufficient' is] the question then is how to finish...finer grits, commercial compounds, or oxalic acid and very fine ploycrystalline alumina. I recognize that this hand job will take a bit of time.

Your thoughts?

[dustinbraudway]

How do you know it is out of flat that much?

I think you are way over thinking it. Just bring it back up thru the normal grits and finish off with an acid. Yes acids affect polished calcium but it also is what gives it the pop. We could go into crazy intense conversations about acids and ph and what not. Or you could just be done with it. Not trying to be an ass.

Are you familiar with polishing with wet pads or dry sandpaper? What is your background? Just trying to figure out your skill level

[bsorcs]

Out of flat: I put a couple of precision straight edges on 'em and used the automotive gauges.
Over analysis: Probably so; first time around the polishing track. Want it done as well as is possible as the pieces are part of my better half's great grandmother's bedroom suite [which I also have to touch up/repair].
Background: Hydrogeologist/geochemist; moderately skilled woodworker with large old machines; pretty good cook; decent piano player; make really good cocktails.

[Dan R.]

Refinishing older heirloom pieces may dictate that some of the original "charm" be left intact, in the form of original shape (out of flat).

Highly doubtful that you will be able to hand sand this to a "flatter" surface. May I suggest precision grinding, similar to what we do on surface plates?

Never used tin oxide on marble, only on granite with lead wheels, real old school. The yellowing you fear, is a non issue, especially on great-grandmother's heirloom pieces. An acid will pop the shine easily, especially for a novice. 3000 grit is overkill. A suitable finish can easily be had with 1200 and some chemical.

.003-.013 out of flat?? I suggest you refer to the skill listed and have a few cocktails to lighten up....and possibly gain some needed perspective.

[BrentP]

You're overthinking this. 0.003? In terms of flatness over several feet I'd recommend thinking in terms of sixteenths. I can guarantee that no passersby will notice the lack of flatness, though I do understand the quest for perfection. You can spend days on this project or you can get it done in a couple of hours and have plenty of time for cocktails during or after.