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Last spring I got it in my head to make a concrete bowl with broken bottle glass aggregate. I had a bunch of blue and green glass bottles on-hand, and broke these up by submerging them in a big galvanized washtub and bashing them with a fence-post driver. I had a book with a broken-glass concrete recipe, and I mixed up a small batch and pressed it between two stainless-steel mixing bowls. I set the stacked bowls in a corner of the porch, covered them with a soaking-wet towel, covered that with a plastic garbage bag to hold in the moisture, and weighted everything down with a pair of cobblestones.  Here’s what the cast form looked like when I knocked it out of the mold a week later…

At this point, the instructions in the book I was working from advised the reader to “sand or grind the surface to expose the glass.”

I had no idea what I was in for.

I tried every kind of sandpaper in the toolbox, to nil effect: scrubbing a quarter-sized area for half an hour wouldn’t even begin to expose the glass aggregate beneath.  I bought some flap-wheel abrasive disks at the big orange store and mounted them on my heirloom Sears Craftsman auto-body grinder, and didn’t fare much better.  I tried sanding wet and sanding dry.   I even experimented with acid-etching.  The more aggressive carbide and diamond polishing media available from the local hardware outlets was all intended for use on floors.  It was rigid and flat and wouldn’t work on a curved surface without grinding facets all over it.

I found this set of eight “soft” polishing pads on Amazon for $50, and took a chance. The business side of each 4″ diameter pad consists of a polymer honeycomb that looks sort of like the bottom of a sneaker. The elastomer, whatever it may be, has industrial diamond grit embedded inside. In use, the matrix slowly wears away, exposing fresh grit. The back of each pad is covered with “loop” Velcro and marked with silver numbers indicating the pad’s grit size. The Velcro is also color-coded in case the numerals wear off, which hasn’t happened to mine yet. But it’s a nice detail, anyway.

The set also includes a medium-hard rubber pad holderwith black “hook” Velcro on one side and a 5/8-11 threaded brass insert. That’s a standard grinder arbor thread and it fit my old Model 315 perfectly.

I turned the bowl upside down and fit it over an old bar stool so I could work on it standing up. My first experience was not good. My grinder only has two settings. I flipped the switch to “HI,” plugged it in, turned it on, and started polishing. The pad quickly overheated, despite my diligent efforts to keep the bowl wet while I was working, and the cement binding the Velcro to the pad holder melted and failed.

Frustrated, I complained to the seller, and was pleasantly surprised by their response, which was to A) send me a free replacement holder, B) tell me how to fix the one I already had using rubber cement, and C) explain why it failed in the first place. The new one showed up in the mail three days later, but I still actually haven’t used it because the rubber cement fix worked great and held up fine once I slowed the grinder speed down. Which I did by switching to “LO” and using a universal motor speed controller from Harbor Freight.

With my equipment problems resolved, I set to with the grinder. At the lower speed, I was able to get away without using water. I spent about 10 hours working at the largest (50) grit size, just grinding away the so-called “cream” to expose the glass, and then worked down to smaller and smaller grits, spending about two hours each on 100, 200, 400, and 800 grits. There was a really striking improvement in the step from 400 to 800, and for a day or two I fully intended to polish the whole bowl all the way out to 6000 grit. But then I lost patience and just applied a polymer sealant (specifically Arrow-Magnolia International’s Glo-crete) inside and out. This provided a nice, shiny, “wet-look” gloss.

It was still a heckuva lot more work than I counted on, but I dunno how I would’ve done it at all without this set of pads. And it looks like even the 50 grit size still has quite a bit of life left in it, as you can see from the detail photo above, which was taken after the bowl was complete.

No part of the set bore any kind of manufacturer marketing, so I don’t know anything about where they come from besides the product link on Amazon and the seller website at GranitePolishingPads.com. They are marketed as “for granite counter tops,” and honestly I have no idea how well they perform in that application. But for curved surfaces, at the proper grinding speed, they have my unequivocal endorsement.

Sean Michael Ragan

I am descended from 5,000 generations of tool-using primates. Also, I went to college and stuff. I write for MAKE, serve as Technical Editor for MAKE magazine, and develop original DIY content for Make: Projects.


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Comments

  1. Brian Bloniarz says:

    Congrats, that looks fantastic! How were you able to polish the insides of the bowl, I wasn’t quite sure.

    That HF Router Speed Controller deserves a review of it’s own! Cheap and versatile. I’ve got one and am using it to control the heating element of my homebrew coffee roaster.

    1. Sean Ragan says:

      Thank you! But I didn’t, actually, polish the inside of the bowl, since it was going to end up as a planter filled with dirt, anyway. I sealed it to keep water out, but the inside surface was otherwise as it came from the mold.

  2. George says:

    Looks like the pads I used to make a concrete/crushed glass kitchen work top. To take the “cream” off first I used diamond grinding discs like these: http://www.amazon.co.uk/dp/B000LFVXAU/ref=asc_df_B000LFVXAU7301433?smid=A3P5ROKL5A1OLE&tag=hydra0b-21&linkCode=asn&creative=22206&creativeASIN=B000LFVXAU&hvpos=none&hvexid=&hvnetw=g&hvrand=8751173791554157120&hvpone=&hvptwo=&hvqmt=

    Pretty aggressive – in fact very unpleasantly dusty – but worked great to get down to the glass/concrete matrix before polishing.

  3. PeteB says:

    You really want a “polisher” rather than a “grinder”, and a wet polisher is best. The speed is much lower than your typical angle grinder.
    For the initial grinding with coarse grit you should do it after only about 4 days or less of setting time, The cream is a lot softer then and it’s a lot quicker to get the glass exposed.
    Using water is great since it cools the pads and also extends their life, but makes a giant mess. I used a contraption that hooked to my wet/dry shop vac to suck up the water when I made my first countertop. It sucked up the water but was a hassle with the wire, water feed hose, and shop vac hose all attached to the grinder. Water also keeps the nasty dust out the air.

    When I used glass aggregate, I ended up putting a bunch of bottles and a fist sized rock in a cement mixer. After the glass was broken enough I pulled the rock out and ran the mixer with just the glass for a while. This takes the nasty edges off the glass, but makes an infernal racket. A little water made it quieter and kept the dust down.

    Where did you find your broken-glass concrete recipe?

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