Introduction: Fireplace Coffee Table
You can have an indoor fireplace safely in any home- no chimney necessary!
When you rent like me, you’re at the mercy of your landlord when it comes to home improvement. Nothing makes me feel cozy on a cold day like an open flame warming the living room, but there’s no way my landlord is going to spring for that.
Using an ethanol burner (carefully following the safety instructions), it’s perfectly safe to have an open flame without the need for a vent. With the right kind of fuel, there is no danger of harmful fumes tainting your indoor air quality.
The companies that make ethanol fireplace burners also make some very nice stand-alone fireplace fixtures, but for the limited space of my Bay Area apartment, I need my furniture to be multi-purpose. Hence, the Fireplace Coffee Table.
This is the first project I've made using my Wooden Nuts & Bolts process.
Step 1: Tools & Materials
• Woodshop: Table saw or circular saw, bandsaw or jigsaw, drill press
• Random orbital sander
• Shop Clamps
• Bucket & paint stirring stick
• Table saw or circular saw with plastic blade or scoring knife and straight-edge
• Hardwood for the table slabs and wooden nuts. I used 2" thick Ash and planed it down to 1 1/4".
• Hardwood 1 1/2" Ø dowels. I used Red Oak Closet Dowels.
• Wood filler, wood glue, brad nails, liquid nails.
• Linseed Oil, polyurethane.
• Cheng Concrete Pro Formula Mix additive
Step 2: Design
Fusion 360 is my go-to for furniture and product design these days. It's free for students and hobbyists, and there's a ton of educational support on it. If you want to learn to 3D model the kind of work I do, I think this is the best choice on the market. Click the links below to sign up:
This is the first furniture piece I’ve designed using my Wooden Nuts and Bolts idea, so I based the table on 4 threaded wood legs with nuts for rigidity. I designed it to be 2-tier, because my current coffee table with 2 tiers is really practical: you can move all the books, magazines, and empty coffee cups to the lower tier when you want to put a laptop on the top tier to watch a movie- or in this case, when you want to light a fire!
The manufacturer’s instructions call for specific clearances around the burner surface to combustibles. So to play it safe, I designed the table to have a 2 1/2” deep cast concrete surround for the burner insert. I wanted the top of the burner to be flush with the top of the table as well, so I designed the cast concrete part to have a 1/8” thick indentation. I left a 1/16” gap in the model for wiggle room between the burner and the concrete.
Step 3: Wooden Parts
The table measures 16"D X 33"W X 16"H when assembled. To make a 16" wide solid hardwood table, I planed down two 9" wide planks and edge-glue them into a single panel, and cut the parts out of that. If you're not used to cutting out complex shapes designed in the computer, check out my Digital Fabrication by Hand instructable.
Forstner bits will give you clean, consistent holes, and it's important to use a drill-press for this project. The holes must be flush and perpendicular to the surface of the table, otherwise you'll end up with crooked legs. When cutting out these parts, note that the top tier of the table (with the opening for the cast concrete) has threaded holes (meaning you start with a 1 1/4" Ø forstner bit), and the bottom tier without the cutouts has 1 1/2" Ø holes which allows it to slide freely up and down the legs.
For the legs, go to my Wooden Nuts & Bolts instructable. I cut the dowels to 16" segments and threaded the entire length of each leg. This allows the tiers to be adjustable, but I also really like the look of the "wooden bolt" holding everything together. There are 4 legs and 8 nuts- 4 nuts to hold up the loose lower tier, and 4 nuts to tighten against the threaded holes on the top tier.
Once the wood pieces were done, I took them to the table router to get a nice 1/4" filleted edge all around.
Step 4: Concrete: Test
In my first test with casting concrete, I added some expanded mesh because I thought it would mitigate cracking. I've designed rebar layouts many times for building projects, and wrongly assumed that the principals are scalable without limit.
I sifted out the aggregate from my concrete mix so that it would flow through the mesh easily. All the mesh did in the end was hold the concrete mix back and keep it from gaining purchase on the wood. The end result only had purchase in one corner- the rest of it was a crumbling mess filled with air bubbles.
The other problem with the casting test was that I didn't seal the wood in any way before pouring the concrete. As a result, the wood grain soaked up all the water from the concrete and made it swell considerably.
I talked to the nice folks at Cheng Concrete in Oakland, and they set me straight. They make amazing cast concrete countertops and furniture among other things and have a line of products available online that will allow you to cast concrete for just about any application in just about any color.
First, they confirmed that the mesh isn't doing anything to reinforce concrete at this scale. The other principal they pointed out is that the aggregate I sifted out is structural, and plays a role in keeping everything together, which is important even at such a small scale.
They recommended first sealing the wood to make it waterproof, then pouring the concrete. They also recommended their Pro Formula Concrete Mix- this helps mitigate cracking, increases curing time, and has the added bonus of coming in a bewildering array of colors. I went with Charcoal because I thought the contrast would be nice.
Step 5: Concrete: Mold
Having designed everything in Fusion 360, it was easy to make line work for all the parts of the mold. The mold is basically an 1/8" thick acrylic copy of the burner that is 1/16" thicker than the burner itself. This allows for an offset to give you some wiggle room to put the burner in.
I used acrylic for the positive mold because it's rigid and makes for a very smooth cast concrete surface that doesn't require grinding or machining. With acrylic cut to size and welded with plastiweld, it's easy to get solid, air tight seals on all the parts.
Tap Plastics has a great tutorial on how to use acrylic cement to weld acrylic sheet together. For this mold, I cut the parts to size using the layout attached in this step, then built the box so that it was enclosed on all sides. Having no plans to re-use this mold, I decided to make it one monolithic piece, then break it apart with a rubber hammer and putty knife later.
I sealed the inside of the cutout on the table top with several coats of polyurethane to keep the water from wicking into the wood and making it swell.
Next, I aligned the cutout on the table top with the mold positive- this was easy to line up since I cut the bottom surface of the mold to the same size as the table top. I clamped the top down with more clamps than I thought I needed to get a tight seal.
Then, I mixed the Pro Formula into the concrete mix. I used 7LB of concrete mix, because I calculated that would fill the space about half way, which would be plenty. After stirring with a paint stirrer to get all the dry clumps out, I poured the mix into the cutout and let it cure over the weekend. 1 bag of Pro Formula mixes into 120 LB of concrete mix, so you have to do the math an measure the right amount for a smaller volume of mix. I have enough Pro Formula leftover for 20 more concrete cast projects after this.
Step 6: Concrete: Results & Reinforcing
The concrete came out beautifully for the most part. The top surface was very smooth without any break-outs or cracking.
Having sealed only the cutout though, there was a small amount of wicking in the top surface of the table from the edge of the concrete. Sealing the entire piece before casting would probably fix this. One of the shop guys also suggested just using vaseline around the edge of the cutout, which seems like it would work.
I added some joining dowels to the inside edge of the cutout before I poured the concrete so that the cured concrete slab wouldn't rely completely on the inside edges for structural support. Once the concrete was cured, I also added some extra wood structure to the underside with liquid nails- better safe than sorry!
I also used Concreteworks' concrete polish for the finished surface: wax-on / wax-off with a rag and this was done in 20 minutes. The result is a smooth, shiny concrete surface that would be perfect without the wicking at the edges.
Step 7: Assembly
- Thread one nut half way down the length of each leg. These positions should be consistent, since they will set the height of the lower tier.
- Insert the leg into an unthreaded hole on the lower tier.
- Thread another nut 1 1/2" down the top of the leg.
- Place the top tier and screw in the leg until its top is flush with the table top.
- Tighten the second nut against the bottom of the table top until it's rigid.
Repeat these steps for each of the other 3 legs. This is a good time to choose the pieces that came out the most perfect for the most visible placement.
The great thing about making this table with wooden nuts and bolts is that it's endlessly adjustable. Flip the table to its upright position and adjust the nuts until everything is flush and level.
This construction is ROCK SOLID. It's got serious lateral stability and can take a lot more dead load than it needs to. The wooden nuts & bolts scheme makes for a very robust finished product.
Step 8: Light It Up and Relax
This is a seriously versatile piece of furniture. The ethanol burner closes with a steel tool that's included, and after 15 minutes it's cool to the touch. You can use it as a fully functioning coffee table with a level surface 95% of the time, then clear off the top, pour in some fuel, and read a book by the fire on a cold night.
The burner is guaranteed by the company and comes with a complete set of safety literature. Follow the instructions and use common sense, and this is an awesome addition to the home you don't own.