Making the Countertop Mold

Mold-making is an art — and science — unto itself. Molds can be simple or complex, designed for thick or thin and horizontal or vertical applications, and made from a variety of materials. Molds for precast projects are made in the reverse and negative of the shape you wish to create. The Concrete Countertops book and the Concrete Countertops DIY Instructional DVD tell you how to make a level, watertight mold for a surface that is meant to be finished with a grinder and include detailed mold-making tips, such as creating sink knockouts and incorporating divisions into longer pieces.

At Cheng Design, we prefer thicker (from 2" to 3" thick depending on the project) rather than thinner tops, as we like the sense of mass a thicker top provides. Thicker tops also allow you to include a bailing wire and rebar support system within the piece.

The challenge of a thin concrete countertop is ensuring the reinforcement material lies flat within such a narrow space such that it isn't so close to the countertop surface that it creates a "ghosting" effect (a slight discoloration in the surface of the concrete along the lines of the wire or rebar). When casting a thin countertop, or other thin pieces like back splashes and toe kicks (which you can think of as casting concrete "tiles"), we use wire screen for structural support. For thin pieces, make them in a moderate dimension (not too long and narrow as they can crack and break very easily), and use additional care when handling and installing them.

Yes, we have, on occasion created integral sinks, as have many other fabricators (see our Concrete Countertops book for examples). One reservation to this technique is that constant water flow from the faucet to the concrete surface of the sink will eventually erode the top "cream" layer in that spot, leaving a permanent blemish that will only get deeper over time. Another, is that in a kitchen application, the wear and tear from everyday use (washing heavy pots and pans, pouring wine or acidic juices in the sink, etc.) can significantly impact the finish and overall appearance of the sink area (which is why we favor stainless-steel sinks in kitchen applications. That said, nothing beats a cast, integral sink for looks—it's the long-term wear and tear of the sink that is affected.

Inside standard bagged concrete (like Quikrete 5000) is natural stone aggregate. When you grind the surface of your countertop, you are removing the "cream" layer to reveal these stones, which can vary throughout the country as bagged concrete is regionally produced. Some people trying to achieve the "ground" look are happy with revealing the aggregates that come with the bagged concrete. While others, trying to achieve a truly unique or custom look, will supplement natural aggregate with decorative aggregate (see Decorative Inserts in the Product Catalog) and place them into their mold. This addition is an aesthetic decision and has no impact on the structural integrity or durability of your countertop. For more information on grinding and polishing, refer to "Grinding and Polishing", pages 136-142, in Concrete Countertops.

There is no right answer when it comes to choosing decorative aggregates (see Decorative Inserts) for your concrete countertop. Because this process is completely optional and based on personal aesthetics and preference, feel free to choose the insert or combination of inserts that you desire. Whether you are striving for a subtle monochromatic look, or a standout terrazzo-like finish, you are only limited by your imagination, so have fun! To see Cheng Design's recommended color and decorative insert combinations Click Here.

If you are not grinding the concrete countertop, you are leaving the creamy, aggregate-free layer alone and intact, and finishing with only light polishing. Since you are not grinding to reveal any aggregates, decorative inserts would not be revealed from beneath the surface; therefore, they are not needed with this type of finish.

Terrazzo is an age-old technique of casting bits of stone into another substance and then grinding the hardened surface away to expose the added material. You can add broken glass to the entire mix as though it were another aggregate, or simply scatter glass into the mold and then pour over it. When using the latter technique, add at least 50% more glass than you want to have showing in your finished countertop piece, as much of it will migrate away from the bottom of the mold when pouring and vibrating the mix.

To get around this, we sometimes add a little water-soluble white glue to the glass before scattering it onto the bottom of the mold or, alternatively, use a spray adhesive on the bottom of the countertop mold. Once the glue sets, the glass will be stuck to the bottom of the mold and won't migrate during the pour. Be mindful to use as little as possible and still get "tack."

Our product catalog includes decorative aggregates—bits of semi-precious stone, mother-of-pearl, or tumbled glass—which can be cast into the countertop and then exposed by grinding the surface once it's fully cured, or hardened (see Decorative Inserts). You can grind your countertop a little or a lot, depending on how much of the aggregate you'd like to expose and the aesthetic you're trying to achieve. For more information, refer to "Grinding and Polishing," pages 136-142, in Concrete Countertops.

In Chapter 2, "Building the Mold," of Concrete Countertops, Fu-Tung explains the simple technique of using an aluminum divide. A more sophisticated approach is to shape the metal divide into a "Z" shape that acts as a "key" between the two sections. Another option is to affix a half-dowel on the metal divide to act as the key.

There are many ways to make bullnose edges, none of which are very easy. We have made them by building up a plaster edge with a shaped edging tool (made from sheet metal), then smoothing and coating the plaster with shellac. We have also heat-formed plastic Mylar, laminate and foam edges as well. This and other advanced mold-making techniques are covered in our Advanced Concrete Countertop Design Training.

Generally not recommended. Depends on the condition of the existing countertop.