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Old 12-15-2008, 06:18 PM   #1
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polished slate project

My house built in 1963 has a nice looking fireplace. It was built without a hearth -- see pic -- which I guess must have been okay at the time. These days it needs a hearth.

I have a slab of light coloured limestone that matches the fireplace stone somewhat and currently this is sitting on a small wooden platform to give me a code-meeting hearth. My project is to build a higher platform to bring the hearth up to just below the fireplace opening. The little stand will be built from steel angle and faced with CBU (non-combustible) on which I will put slate which will be the main focus of this project.

I want the stand and hearth to be removable, as someday I will likely replace the operating fireplace with a gas insert, and at that time the whole hearth can come out to gain back the space.

Near the fireplace I have a polished slate floor, also original from 1963. Now slate doesn't itself take much of a polish, so it's waxed with some sort of acrylic wax, but it looks good and is in good shape. (pic)

My project is to face the hearth stand with slate that will look like the slate on the floor nearby. I have a rough idea how I would do this, but I am interested in any suggestions or comments on how to get a good result. I am unsure how exactly the original floor was done and info about that would be interesting for sure. My plan, roughly, is:

I will use off-the-shelf 12x12 slates. My local big-box mart sells slates by the each, and I can pick through their supplies to find uniform colour and texture tiles. I can even make the rounds of a few stores to put together my stock of slates, and time is on my side if I want to go back at different times.

The substrate will be CBU, bolted to a steel angle-iron frame. The largest area is the front at just over 1 foot by 5 feet. The CBU will be 1/2 inch material (or 5/8ths -- if such a thing exists) supported around the edge and down the centre.

I will cut the slates into pleasant puzzle-pieces with my saw. No problem there.

I will attach them to the CBU with thinset -- probably slightly modified, since it's good and sticky. It probably won't matter what I use here, exactly.

The grout lines in the floor I'm mimicking are around an inch wide and filled with non-sanded white grout of some kind. My project will be on a smaller scale and I'm thinking of 1/2 or slightly wider grout lines. I will want to use a white non-sanded grout to be similar to the floor. I am uncertain what my choices would be here. The grout will be in place during figuring and polishing -- I think. I would want it to have a similar hardness to the slate (pretty soft) to avoid serious problems in the figuring and polishing steps.

After installing and grouting, I need to figure out how to grind the surface flat and polish it. This is the area of greatest uncertainty. I am going to be on a budget, so I don't want to lay in a bunch of expensive tools. I would consider renting items that would be readily rentable.

I was thinking that slate is pretty soft and I could put a large grit carborundum disc in my angle grinder and use this to do the main figuring of the slate surface. Even going through discs quickly, they are cheap to use. I could also use a belt sander -- not sure what would be better/worse than using a grinder. My grinder is a single-speed 10K RPM. I would consider looking for a variable speed or maybe a polisher that could go slowly.

For the polishing part, I am starting to get lost. I have an expendable variable speed random-orbit sander that I would use with water flowing over the surface and could work my way down to small grit. I could use powdered grit and a piece of glass to work by hand. Once the material is smooth to 600 grit, I can splash it with the same acrylic wax as on the floor and I think it will be good to go.

Should I just go get one of these, or similar wet polisher?
One nice thing about having this tool is that I could also touch up the existing slate.

Any other ideas about how to handle the polishing on the cheap would be most welcome. Also any comments on the overall plan, choice of materials, pointers on grout, or my planned process would be welcome.

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Old 12-15-2008, 06:54 PM   #2
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Unsanded grout that wide will shrink and crack...you need to use a sanded. Now, the sand particles can be fairly small. Done well, a sanded grout isn't necessarily rough - it will be if you wash too much cement out of it. I'd be surprised if you got a good polish on slate with those hand tools...I suppose it is possible. The stone dust could be a major problem, though.
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Old 12-15-2008, 07:32 PM   #3
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Here's how i would do it.
I would use an orbital sander with Hermes or similar sandpaper,you'll need to go to about 2000 grit, so you may have to use sandpaper up to whatever grit you can get then use diamond pads from there,or a kit like Alpha on a variable speed angle grinder.The reason i say the orbital sander and paper is the ease of use, and eliminating the problem with using so much water polishing.I would grout the stone with sanded but would wash the joints out so the grout barely covered the bottom of the tiles.This will stabilize the stone but you can regrout after the polishing and after you clean the grout joints of the sludge.You don't want the stone to have sanded grout up to the top if you're gonna polish it cause it will pull the sand out and scratch the stone.

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Old 12-15-2008, 07:41 PM   #4
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you can also use steinglanz. Puts a hell of a shine on stone.. On really smooth stone, it may tend to be a bit slippery.

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Old 12-15-2008, 07:57 PM   #5
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Thanks guys.

I've been reading up on polished concrete and I'd like to imagine what I have hear is a little like that -- except with super-extra-large decorative aggregate. In a polished concrete setup, do I worry about sand particles coming loose during the polishing and scratching another part of the surface?

Looking at the grout that's in my floor, it doesn't look sanded. It's been polished flat with the stone. It also looks to me that the grinding was done after the grout as there are places where the grinder has left a curved depression that crosses the grout and stone.

If I want to help the grout hold in the gaps, are there tricks I can do like notching the edge of the stone or making a bevel cut such that the grout would keystone in?

Terrazzo would be done with epoxy. Any point in considering epoxy grout for my project?
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