Introduction: Concrete and Glass Coaster for Valentine's Day

Recently, I have been playing with concrete as a medium. When I stumbled upon this tutorial, I figured that it would make a nice Valentines Day gift for my wife. The original tutorial is pretty good, so my main contributions in this Instructable is how to make the heart shape, and some tips on the process.

This project took me way, waaaaay longer than I expected, since I had to try out different things and learn some new techniques. I'm sure there's an easier way to make these :)

Note - I'm missing some photos for this instructable, so I'll do my best to describe the process when needed.

Step 1: Materials

Concrete layer

  • Some concrete. I use Quikrete 5000
  • A sieve (optional). I use it for removing the small rocks from the Quikrete mix
  • Mixing container, a stick for mixing
  • Protective equipment - gloves, mask, and safety goggles. Seriously, at least wear a mask. You don't want concrete dust in your lungs
  • Glass. Either empty bottles or glass marbles. I got glass marbles from Michael's (nice large ones, with a flat bottom)
  • A hammer
  • Heavy cloth. You'll use it to prevent the glass for scattering when you crush it
  • Coaster mold. I used a 4" silicon mold, but you can use any mold (or make one yourself!)
  • Heart shape cookie cutters in various sizes. Again, I got mine from Michael's
  • Vaseline (optional). Use to secure the broken glass pieces to the mold
  • 60 - 80 grit sandpaper

Coating (optional, but recommended)

  • Clear Epoxy. I used EasyCast
  • New golves (this expoy stuff is sticky)
  • Wide tape
  • Varnish
  • Brush (for applying the varnish)
  • Sand paper of different grits (coarse to fine)

Step 2: Glass and Mold Preparation

First, you'll need some broken glass. Wrap your glass pieces in a heavy cloth and start hammering. You'll want a nice mix of small pieces and large pieces, so make sure you crush them too thin.

I actually found the glass marbles to be hard to crack - I had to crush them 3-4 marbles at a time. Also, crushing them on a hard, stable surface makes life easy (concrete floor = good, wooden table = bad).

Next, get your mold ready. Clean it up, and if needed - apply some release agent. For the silicon molds that I use you don'e really need and release agent.

Place the cookie cutter in the center of the mold. I tried several different combinations of heart sizes (and a heart within a heart, which is my favorite now).

You can optionally apply Vaseline where the glass goes to hold it in place (see photos). This really depends on the effect you want to get - the Vaseline will not only keep the glass pieces in place, but also prevent the concrete from going in between (and sometimes over the glass). If you choose not to use Vaseline, you'll might need to so some sanding afterwards to get rid of a layer of concrete and "reveal" the glass.

I ended up liking the version without the Vaseline better. Check the photos and see which result you like.

Step 3: Preparing and Adding the Concrete

Put on the protective gear (mask, goggles, gloves).

The, take some concrete out and (optionally) use the sieve to remove larger rocks. You don't have to do it, but for a narrow object like a coaster, removing the rocks will make it easier to work with. It's also much easier to hand-mix the concrete without the rocks.

Add a little bit of water and mix. There is no "right" water to concrete ratio (well, there probably is, but I don't know it), but you basically just don't want to add too much water, so add a bit of water every time, mix, and add more if needed. More water will make the concrete easier to work with, but will take longer to dry and will make it more fragile. I like to make the concrete feel like putty: dry enough do you can pick up chunks of it by hand, but wet enough so if you tap the container it will flow and get the shape of the container.

Using a spoon (or better yet, just your hands), fill the mold with concrete. I started with the area around the cookie cutter (so the concrete actually holds the cookie cutter in place), tapped and vibrated the mold (to air bubbles out), and then covered the glass area with concrete. In generate, tapping and shaking the concrete in different stages will give different results. You'll just need to experiment and see what you get.

You can now carefully remove the cookie cutter. Place your finger on the concrete covering the glass pieces to ensure they don't move anywhere.

You can now either use your finger to move around the concrete and flatten it, or tap/shake/vibrate the mold. Tapping and shaking at this stage will cause the concrete to go in between and under the glass, especially if you didn't apply vaseline (again this is fine, but it will give a different effect and will require some sanding).

Find a nice horizontal surface and let the concrete dry for a day or two. Remove from the mold once it's dry.

Step 4: Finishing

Ok, so there several different things you can do here. But first, if you used Vaseline you need to clean it off. Use hot water + soap + old toothbrush and then let it dry for a day.

No matter what you decide to do, you will probably want to sand down the piece using coarse sand paper - it will remove some of the concrete that covers the glass, and will bring out some of the small stones in the concrete.

If you got concrete covering the glass, you'll just need to do more sanding until you reveal enough glass. Don't worry about scratching the glass - you'll hardly see it after applying a coat of varnish, concrete sealer or epoxy. I actually cheated and used a air sander to wet-sand the back and front. Saved a lot of time.

I think that using resin here is a good option - it gives a nice, clean look, keeps the glass for falling off and protects from sharps edges. An alternative would be to sand the piece and only apply sealer. I'm going to explain how to use resin.

First, prepare the concrete piece by putting tape around the edge - it will keep the resin from spilling. You should also check the bottom of the piece for holes and tape/seal them.

Now, prepare the resin. Preparation depends on the specific type of resin you got, but the reason I liked using EasyCast is because it uses 1:1 resin to hardener ratio. Just follow the instructions and prepare the resin. IMPORTANT - do anything you can to prevent air from betting into the resin. Pour the materials carefully and slowly, and the mix it slowly. I ended us using a vacuum chamber to get rid of the bubbles.

Pour the resin slowly so it covers the piece. Then let it cure for at least 24 hours (more is better).

Now for the tricky part - sanding. Because of the surface tension, you are likely to have sharp pointy edges of resin that you'll want to get rid of. One way to get rid of them is to carefully sand them off using coarse sand paper and then use finer grit to smooth it. Afterwards, you can apply some varnish to make these areas more transparent.

If you feel more adventurous (I did), you can use a router with a 1/4" roundover bit to smooth the edges. I only had a hand-held router, so I had to improvise. I found a nice thick plywood piece (has to be thicker than the coaster) with a large hole cut in the center, and placed four screws on it to hold the router from moving back (you still need to hold it with one hand). I carefully set the bit to the right height so it will remove the pointy edges and create a round edge. I tested the settings first, adjusting the height and then just rotated the coaster until I got the entire edge rounded. Afterwards, I used some fine sand paper to remove and remaining imperfections and sticky areas (residues from the tape), and applied varnish on the edge (but NOT on the top surface of the resin) to make the edge transparent.

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