How to Make Concrete Molds from Household Items

Almost anything that holds a shape can work as a mold box. Plastic food tubs, deli containers, yogurt cups and takeout trays all do the job because the plastic flexes when you pull the casting out. Glass and metal have no give at all, so a concrete piece tends to lock in and chip on the way out. Cardboard boxes and offcuts of PVC pipe work too, as long as you tape the seams and line them.

Two containers get you started here. One shallow tub holds corn starch, the other stays empty as a mixing bowl. Add a caulking gun, a tube of 100% silicone caulk and a box of corn starch and that is the entire shopping list.

Load the tube into the caulking gun, cut the nozzle, and run the whole tube into the empty plastic container. Check the label before you buy: you want 100% silicone. Siliconized latex and paintable sealants never cure into rubber, and you will not find that out until the batch is ruined.

One standard 10-ounce tube makes a slab large enough for coasters, planter feet or a set of small press molds. Open a window while you work. Curing silicone gives off a sharp vinegar smell that lingers in a closed room.

A few drops of pigment turn the clear caulk opaque. That matters more than it sounds. Once the corn starch goes in, color is the only way to read whether the mix is done - streaks of clear silicone mean you have more kneading ahead of you.

Silicone pigment works, and so does a dab of acrylic craft paint. The water in acrylic paint speeds the cure a little, which is handy on a small batch and a nuisance on a large one.

Work a wooden stir stick through the caulk until no clear ribbons are left. Scrape the walls and get into the bottom corners of the tub, which is where the unmixed stuff hides. Give it a minute or two of steady folding.

Sort the color out now. As soon as corn starch touches the silicone you have five to ten minutes of working time, and you do not want to spend any of it chasing streaks.

Scrape the blob out of the mixing tub and drop it into the shallow tray of corn starch. The corn starch is doing real chemistry here, not sitting there as a dusting powder. It drives the cure and takes the caulk from sticky paste to firm rubber in a couple of minutes.

Bury the blob. A heavy coat on every surface is what stops it grabbing your gloves before you get a chance to work it.

Push, fold and press the blob down into the powder, then do it again as the corn starch keeps disappearing into it. The texture shifts from tacky goo to something close to modeling clay, and the change happens quickly once it starts.

Wear the gloves. Uncured silicone bonds to skin and soap will not shift it. The rough ratio is a few tablespoons of corn starch per tube of caulk, but ignore the numbers and keep working powder in until the surface goes dry to the touch.

Flatten the putty onto a baking tray or down into the base of a plastic tub. Go at least half an inch thicker than the deepest part of the object you want to copy, so the rubber has enough body to hold its shape when wet concrete pushes against it.

A flat tray gives you a true, level base and an open press mold. If you would rather have a mold with walls, line the inside of a food container instead and work the putty up the sides with your thumbs.

Press the shape you want to copy down into the slab until the rubber wraps every face that needs to show. The LEGO bricks and wax pieces here are just handy props. A shell, a leaf, a drawer knob, a cookie cutter or a carved wooden shape all leave a clean impression.

Smear a thin film of cooking spray or petroleum jelly on the master if it has deep undercuts or a porous surface. Then walk away. A shallow press mold is set in ten to twenty minutes. Anything thick wants a couple of hours before you touch it.

Bend the rubber back and the master drops loose. That flex is the entire reason to make a mold in the first place - concrete has no give, so the mold has to be the part that moves. What is left is a clean cavity with every detail of the original in it.

Now use it. Mix your concrete on the loose side, fill the cavity, tap the tray on the bench to float the air out, and pull a finished piece the next day. Rinse the mold in warm water between pours and one batch of putty will carry you through dozens of castings.