Introduction: Make a "Marble" Table From Concrete W/ Torched Wood Base
I entered this in the Furniture Contest - if you like this Instructable, I would greatly appreciate your vote!
I made a coffee table top from concrete, that looks like marble (at least to me). The top is made using a glass fiber reinforced concrete (GFRC) ready-made concrete mix, which is pre-cast in a melamine form. I separated out batches of different colored concrete (from white to dark grey), mixed them together in the form, and then swirled them together by hand to get the marble-like appearance.
I also used an ancient Japanese technique called “Shou Sugi Ban” to make the base for the coffee table. I used a modernized version of shou sugi ban, employing a propane torch to char the outside of the wood. The charred wood is natural way of protecting the rest of the wood, and when finished with a penetrating oil, such as Danish oil, provides a durable surface. This technique works well on any open-grained wood, such as Douglas fir, pine, and cedar. I used inexpensive 4x4 Douglas fir lumber from my local big box store.
Step 1: Materials
- Three (3) 8 ft. 4x4s -- Douglas Fir, Cedar, or Pine
- 3/4" Dowels: http://amzn.to/2yTWrkl
- 4'x8' sheet of melamine for the concrete form
CONCRETE (GFRC) PRODUCTS USED:
• Two bags of Pre-Blended GFRC Mix: https://goo.gl/izSggQ
• 1.5 Bag of AR Glass Fibers: https://goo.gl/c9Vw9x
• Cake Fondant Tool for perfect edges: https://goo.gl/c9Vw9x
• Concrete mixer (HUGE help for GFRC): https://goo.gl/c9Vw9x
• concrete sealer I like: https://goo.gl/c9Vw9x
• Black 100% Silicone Caulk: https://goo.gl/c9Vw9x
• Paste Finishing Wax: https://goo.gl/c9Vw9x
• Concrete Pigment: https://goo.gl/c9Vw9x
OTHER PRODUCTS USED IN THE BUILD:
- Propane Torch: https://goo.gl/c9Vw9x
- Danish Oil: https://goo.gl/c9Vw9x
- RZ dust mask: https://goo.gl/c9Vw9x
- Diamond Grinding Wheel (for grinding underside of table): https://goo.gl/c9Vw9x
- Forstner Bit Set: https://goo.gl/c9Vw9x
- Level: https://goo.gl/c9Vw9x
- Kreg Pocket Hole Jig: https://goo.gl/c9Vw9x
- Herzo Dust Shround for Angle Grinder: https://goo.gl/c9Vw9x
TOOLS USED IN BUILD:
- Angle Grinder: https://goo.gl/c9Vw9x
- Bosch 18V cordless circular saw: https://goo.gl/c9Vw9x
- Bosch 18V cordless Drill & Impact Driver Kit: https://goo.gl/c9Vw9x
- Dewalt Table Saw w/ 32” Rip Capacity: https://goo.gl/c9Vw9x
- Japanese Flush Cut Hand Saw: https://goo.gl/c9Vw9x
- Dewalt 12” Miter Saw: https://goo.gl/c9Vw9x
Step 2: Make the Wood Base From 4x4 Lumber
The base is made of 8 pieces of 4x4 lumber. I just used Douglas Fir. I passed all the 4x4 lumber through my planer on all 4 sides to get each piece down to exact 3.5"x3.5" dimensions. However, this is definitely optional. You can use the 4x4s as is - just do your best to pick out straight lumber. You'll need three (3) 8' 4x4s for this table base.
The eight pieces (shown in the attached figure) are:
4x Legs (A)
2x Outer Stretchers (B) - each of these goes between two legs
2x Inner Stretchers (C)
All your angles will be cut at 36 degrees (note: in the video I misspoke and said 35 degrees). However, I recommend cutting all your 4x4 pieces to rough length (e.g., 4" or so longer than needed) first, then setting your miter saw to the 36 degrees to make the rest of the cuts.
Cut the four legs (A) first. Rather than trying to do the trigonometry and get the length exact, I recommend making one 36 degree cut, then hold the leg so the angled end rests on a table, measure 15" perpendicularly up from table (or whatever you want the height of your base to be), mark it on the leg, then make the second 36 degree cut. Then use this leg to cut the other four legs.
Cut your two outer stretchers (B) next. To do this, I just propped up two legs with the scrap cutoffs from the legs (which will be angled exactly to support the the legs from the sides), and move the legs until the distance between the tops of the legs is what you want (e.g., I wanted 42"). Then measure the distance between the bottom ends of the legs, and this is the shorter length (e.g., length of underside) for your outer stretchers (B). Then use your miter saw to cut two pieces with 36 degree cuts, so shorter side length is what you just measured.
Now cut your two inner stretchers. To do this, put two legs and an outer stretcher together (again, scraps from the angled cuts work nicely to prop the legs up). Put a rough cut inner stretcher piece next to it, and mark the place for the angled cutoffs, then use the miter saw to cut them.
After cutting the 8 pieces, you'll glue them up. Ideally, you should make a clamping jig for the 144 degree inner angle between each leg and the outer stretcher. However, I just applied glue, used scrap 4x4s to hold them in place by hand, and used screws to hold the legs and stretchers together while the glue dried.
After the glue dried, I added 6" lag screw to outside of each leg (running parallel to the ground) to hold the leg to the inner stretcher. To do this, I first drilled a 3/4" hole so the screw head would be inset, then drilled a 1/4" pilot hole in the inset, and then drove the lag screw in.
I then used 3/4" dowels to fill in the holes and cover the lag screw heads, and cut off the dowels with a flush cut saw.
Lastly I used a belt sander to sand the base stretcher pieces flush and sand down the dowels. Then it was time to burn.
Step 3: Torch the Wood!
This is the fun part! Just follow the directions on your torch to hook it up to the propane tank. It really is pretty safe, but you should have a fire extinguisher handy, just in case.
There isn't a whole lot to it. Aim the torch at the wood, make sure the hottest part of the flame (the tip of the blue part) is just touching the wood, and watch it start to brown, and then blacken. Stop when it gets to a look that you like, and then spray it down with water. (I just used a spray bottle.) Then use a brush to lightly brush off any grey ashy parts.
After it cools a bit, apply an oil finish. I used Danish oil because it penetrates the wood, and hardens inside of it. This made the charred "alligator skin" on the wood harder and more durable. Be prepared to use A LOT of oil. The process really dries the wood out and opens it up, and you are already dealing with a soft open-celled wood, so it soaks up an amazing amount of oil. I did three heavy coats of oil, and still might go back for another.
Step 4: Build the Concrete Form From Melamine
This is a pre-cast design, so the table is being cast upside down.
To make the base form you'll need to do the following:
(a) Cut melamine strips for the sides of your form to the height of your table + 3/4". In my case, the table was 1.5" thick so the sides were cut at 2.25" It is easiest to cut these strips on a table saw, but it can be done with a circular saw if you are very careful to make cuts consistently. Cut all the strips at once, so the height is identical. Then use your miter saw to cut the strips to the lengths you need. Cut each side a few inches longer than the base (e.g., 17" or so for a 14" side of table top). The extra overhang gives you leverage to pull the sides away from the concrete when you demold.
(b) Cut a base piece of melamine to the size of your table, in my case 48" x 21".
(c) Pre-drill holes in your sides, and then screw the sides to your base. I used 1.25" drywall screws for this.
(d) Apply paste wax to the inside of your form and bottom/sides of slab.
(E) caulk the inside edges using black silicone caulk, then run a metal ball tool around the edges to give you a perfect caulk line, let it dry, then remove excess caulk on either side of the line. (See my previous instructable for more detail: https://www.instructables.com/id/DIY-White-Concret... )
IMPORTANT LAST STEP I LEFT OUT: You'll want to make 2x4 stretchers to hold down foam inserts so these can be attached halfway through the pour. See the video, which shows these stretchers in place. To make them, you'll use a similar technique as I used in my previous Instructable to hold the french cleats in place, except you'll want to position them so the foam is inside the form and level with the top of the side walls.
Step 5: Cut Foam Inserts for the Concrete
You will cut inserts from 1/2" pink insulating foam (i.e., Foamular) to go inside the form, so the concrete is only 1" thick (except at the sides, where it is the full 1.5" thick). Just use an exacto knife to score the foam sheet so it is about 3-4" less than the size of the table in both directions. After you score the foam, it snaps easily by hand. If you want it precise, you can also cut the foam with a circular saw or table saw.
Step 6: Mix and Pour the Concrete
Check out my previous Instructables on GFRC concrete for more details on mixing. For this project, I used a just-add-water Glass Fiber Reinforced Concrete (GFRC) mix. You mix it by adding 1 gallon of water for every 50 lbs (one bag) of mix. Just adjust the ratio for the size of your tile, factoring in about 10 lbs of mix per square foot of tile (to err on the side of having more than needed).
I used two 50 lb bags for this project. I separated out 20 lbs to use as a face coat (w/ no glass fiber) and the remaining 80 lbs to use for the backer coat (w/ glass fibers). I only used 1.6 lbs of glass fiber for the 80 lbs.
When mixing the face coat (and back coat), add the concrete mix slowly to the water. E.g., add a third of the mix, then mix it up, add another third of the mix, mix it up, and so on. You can add a bit more water (but careful not too much) if you need to get it more flowable. The face coat should be like runny pancake batter.
To get the marble look, I separated out three smaller paint cups full of the pure white mix. I added a lot of pigment to one paint cup so it was really dark grey (almost black). I then added a bit of pigment to the remaining face coat in the main mix bucket, and mixed it slightly so the concrete in the bucket had a swirled look. I then added all three color variants, and swirled it around by hand until it covered the entire concrete form in a thin (.25" or less) layer. Just swirl it by hand until you get a look you like. Then let it set up for 30 minute to an hour (however long it takes for it to firm up, but still be slightly damp to the touch. You want it firm enough the back coat doesn't push through the face coat, but still damp so the back coat will bond to the face coat.
NOTE: There is NO need to vibrate GFRC mix.
Then mix the back coat the same way, except after it is mixed, add about 1 lb of glass fibers per 50 lb bag (or 1.6 lbs for 80 lbs in this case). Add fibers in slowly, e.g., a third at a time. Add water if needed to get the mix so you can pour it right out of the bucket into the form. Pour it in and use your hands to work it into the edges and corners, and make sure it covers everything. Again, there is no need to vibrate. If you made your mix flowable (so it pours easily), it will self-level. Add some of the back coat without any pigment (e.g., pure white), then swirl the pigment in to the rest of the back coat in the bucket, so it is unevenly mixed. Then pour the back coat until it is about 1" high in the form. At this point, add the foam, and screw in the 2x4 stretchers on top of the foam to hold it down. Then pour in the remaining back coat to fill the form.
Step 7: Grind Down the Underside of the Form
Let it sit for at least 24 hours (36 hours if it is under 70 degrees). Then, if your concrete has gone over the edges of the form, you'll want to use an angle grinder with a diamond cup to grind down the underside, before you remove it from the form. It is important to do it before removing the top from the form, so that you can use the sides of the form as a guide to grind the underside evenly all around.
Step 8: Demold the Concrete, Sand and Seal
After grinding the underside down to be even with the edges of the form on all sides, it is time to demold the table top. Just unscrew / and pull the form apart.
If the side walls don't come off easily, use a rubber mallet to gently tap the sides of the form away from the concrete. If you flipped the piece over before demolding, it should be fairly easy to remove the bottom of the form (which is now on top). If it doesn't come off, then use a plastic putty spackle (not metal or anything sharp that could scratch the concrete!) to gently pry the bottom off. If it is really tough, then there is likely a vaccum between the form and concrete. In this case, you can use an air gun attached to your air compressor to shoot air between the form and concrete. This will break the seal created by the vacuum, so you can remove the bottom of the form.
Next given the whole service a quick wet sand, by hand, with 400 grit sandpaper. Just sand until you feel it smooth a bit. You'll feel it when you've removed the texture of the melamine, which is all you need to do.
Then, apply a concrete sealer of your choice. Just follow the instructions that come with your sealer.
Step 9: Put the Table Base and Top Together -- You're Done!
Place the table top on the base, and get it lined up so it is perfectly centered. Carefully lift one side at a time and add construction adhesive to the tops of the legs (basically the same way you'd attach a countertop). Before the adhesive sets, double check and make sure your table top is centered over the base (and adjust if necessary).
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