Learn how to easily color concrete and use silicone to make reusable molds for casting concrete.
Step 1: Supplies + Tools
Plastic Bottle or Container
Hopefully plastic packaging will be replaced with more responsible alternatives in the future, but if it isn't going to biodegrade on its own, you might as well put it to use before it goes in a landfill.
Gluestick or Candle
I used a glue stick but a candle would have been easier to remove.
Quikrete Countertop Mix (in White)
vailable at Home Depot
QUIKRETE® Countertop Mix is a super high-performance mix that is worth the price. It has no large pieces of gravel or aggregate, which makes it great for small intricate pieces such as lamps and vases. It’s easy to work with and comes in different colors. Most stores don’t carry it in-stock so call ahead to your local Home Depot to have it ordered for pickup.
Mold Star® 30
Mold Star® silicones cure to soft, strong rubbers which are tear resistant and exhibit very low long-term shrinkage. Molds made with Mold Star® will last a long time in your mold library and are good for casting wax, gypsum, resins, concrete and other materials.
Available at Home Depot
Hot Glue Gun
Available at Home Depot
RYOBI 18 Volt Cordless Drill
Available at Home Depot
Step 2: Drill a Hole in the Cap
I cut an X into the lid of the cap with a knife to help keep the drill bit from slipping. I then drilled a hole right through the center of the lid. The hole should be a little smaller than the glue stick or candle.
Step 3: Insert the Candle/Glue Stick
Carve the hole with the knife so that it's just big enough for the candle or glue stick to tightly fit in.
Step 4: Mix the Pigment With Water
I mixed the pigment with the water before adding the water to the dry concrete mix. Use clean water and add pigment in proportion to the manufacturer’s recommendations. You can increase the color intensity by adding more pigment and decrease it by adding less.
Step 5: Mix + Pour the Concrete
Mix the colored water into the concrete. Since the concrete is being poured through the narrow mouth of the plastic bottle, I made the concrete a little more watery than usual.
Step 6: Remove the Plastic Bottle
Once the concrete has cured for at least 24 hours, you can remove the bottle. Using a heat gun or hair dryer set on hot will heat and soften the plastic and removal easier. I heated the plastic around the mouth of the bottle and then easily cut through it.
Step 7: Drill Out the Center
I used a glue stick and had to drill it out. This can be tricky since the glue is rubbery can push the drill bit into the edges of the concrete causing cracking. A candle would have been easier to remove.
Step 8: Cut the Bottle Into a Tube
Cut the top off of the water bottle and then made a second cut about 1/2" above the top of the object. This creates a plastic tube that's a little taller than the object.
Step 9: Cut a Band
Cut a 1" wide band from the leftover piece of the water bottle. This band will be used as a support for the mold and the end of the bottle will be used as a cup for mixing the silicone.
Step 10: Make a Bottom
Cut a piece of plastic out of the lid of an old plastic container.
Step 11: Glue the Object to the Bottom
I used a hot glue gun to glue the object to the bottom piece of plastic.
Step 12: Glue on the Tube
I used the hot glue gun to glue the plastic tube cut from the water bottle to the bottom base plate. I applied glue all the way around to make sure it was sealed and water tight.
Step 13: Water Test
Pour water into the mold to make sure the silicone won't leak.
Step 14: Use the Water
Silicone isn't cheap and can't be saved once the 2 parts are mixed together, so you want to mix the right amount. I poured the water into the cup I made out of the other end of the bottle and drew a line so that I would know how full to fill the cup with silicone.
Step 15: Mix + Pour the Silicone
Mold Star® 30 comes in 2 parts that need to be mixed together in equal amounts. I filled to the cup and mixed the silicone before pouring it into the mold.
Step 16: Remove the Object
Once the silicone has cured, cut away the plastic and remove the object. The silicone is flexible, but you may need to cut it a bit to get the object out. Try and cut the silicone in a place that will be minimally visible.
Step 17: Prepare the Mold
Once you remove the original object, you'll be left with a detailed mold that's ready to be cast into a concrete copy. Slide the 1" wide plastic piece of water bottle over the silicone to hold the mold tightly together.
Step 18: Mix + Pour the Concrete
I mixed and poured Quikrete Countertop Mix into the mold and made the mix a little more watery than usual so that it would pour easily into the silicone mold.
Step 19: Vibrate the Mold
Tap and vibrate the mold to get out the air bubbles. I didn’t tap it long enough and my concrete piece had a few bubbles but still looked good.
Step 20: Remove the Concrete Copy
After letting the concrete cure for 24 hours, I pulled off the plastic sleeve and removed the concrete copy. I also cleaned the mold with a toothbrush so it would be ready for another copy. The molds are quite durable and you can use them over and over as long as you keep them clean.