Casting a wall clock in concrete takes an afternoon of mold work and a weekend of curing, and the result looks like a piece from a design studio. Concrete rewards attention to the basics: mix proportioning, consolidation, and curing. The mix design principles that govern large-scale work, including pervious concrete placements used in modern paving projects, apply in miniature here, so the techniques transfer directly from driveway scale down to a 12 in clock face. A beginner can complete this project with a bag of mix, a form tube, a scrap of melamine, and a few hand tools, and the total cost stays well under the price of a comparable store-bought clock.
Mix Design and Materials for Small Castings
Choosing a Mix
A standard all-purpose concrete mix is the right starting point for a small casting. It is sold by the bag at any home center, often for less than specialty casting mixes, and it produces a surface that is fine enough for a clock face. The 3/8 in stone in standard bag mix works in a 3/4 in thick pour; if you want a smoother finish, use a finer aggregate casting mix instead.
Batch Consistency
Mix to a stiff but workable consistency, about the texture of damp sand that holds its shape. Excess water makes the mix easier to pour but weakens the cured piece and raises shrinkage, which is how thin castings crack. Add water gradually and stop as soon as the mix comes together.
Workability for Thin Molds
A slump of 2 to 3 inches is the working range for a thin mold. Test by lifting the trowel: the mix should hold its shape briefly, then slump slowly. If it runs, add dry mix; if it crumbles, add water in small amounts. Let the mixed batch sit for a few minutes before the final troweling so the dry particles hydrate evenly.
Finishing the Surface
The surface of a small casting is finished by hand, but the goal is the same as on a big slab: a dense, closed surface. The ride-on concrete trowel technology that finishes large floors mechanically does the same job as hand troweling on a clock face, so the principle, consolidate the surface early and float it smooth, holds at any scale.
Tools and Safety
- Mixing bucket and trowel for the batch
- Rubber mallet or vibrating sander for consolidation
- Gloves and a dust mask for handling dry mix
- A level and a straightedge for the form and the strike-off
- Work outdoors or in a ventilated space while mixing
Strength, Curing, and Reinforcement
Why Thin Concrete Cracks
Concrete shrinks as it cures, and a thin, unreinforced section cannot resist the shrinkage stress. The first version of this clock broke for exactly that reason: no reinforcement and a pour too thin to hold together. The fix is not a thicker pour, it is reinforcement.
Reinforcing with Wire Mesh
Cut a piece of 1/4 in wire mesh to fit inside the mold and place it mid-pour: pour half the concrete, set the mesh, then pour the rest. The mesh carries the tension that the concrete cannot, and it turns a brittle disk into one that stays intact even if it gets dropped.
Curing and Strength Gain
Curing is where strength comes from. Concrete gains strength over weeks, reaching most of its design strength within 28 days, which is why the cube compression test used to verify concrete strength in the lab is performed on cured samples at set ages. For a clock, two days in the mold is enough to demold cleanly, but the piece keeps hardening for weeks afterward.
| Property | Value | Purpose |
|---|---|---|
| Water-cement ratio | 0.45 to 0.55 | Strength with workability |
| Slump | 2 to 3 in | Fills a thin mold |
| Mesh reinforcement | 1/4 in wire mesh | Prevents shrinkage cracks |
| Time in mold | 24 to 48 hours | Clean demolding |
| Full strength | 28 days | Continued strength gain |
Water and cement react chemically during curing, and the reaction needs moisture to continue. Keep the mold out of direct sun while the concrete cures, because rapid drying pulls water out of the surface faster than the interior and causes surface crazing, the fine web of cracks that shows up on a fast-dried face. Covering the mold loosely with plastic slows the drying and produces a stronger surface.
Building the Mold and Pouring the Clock
Making the Motor Recess
The clock movement needs a recess in the back of the casting. Cut a scrap piece of melamine oversize and bevel it at a 10 degree angle; the taper lets the piece release cleanly from the cured concrete. Glue the recess block to the base of the form with hot glue, and apply a light release agent to the mold surfaces so nothing sticks.
Setting the Shaft Hole
Drill a 1/4 in hole through the center of the melamine base and stand a 1/4 in dowel in it. The dowel forms the hole the clock shaft passes through. Use a compass to draw the circle of the form on the base; the compass point leaves a center mark that guides the drill bit.
Pouring the Face
Cut a 4 in slice from a concrete form tube for the mold wall and hot glue it to the melamine base, lining the tube up with the compass line so the circle is true. Set the whole mold on a level surface, because a tilted form produces a face of uneven thickness. Pour half the mix, set the mesh, pour the rest, then strike the back flat with a straightedge. The clock is one of many DIY concrete projects that homeowners can cast at home, from planters to bookends, using the same mold-and-pour routine.
Finishing and Decorative Options
Surface Treatment
After demolding, sand the edges lightly and seal the face with a concrete sealer or paste wax. Sealing deepens the color, protects against stains, and makes the surface easy to dust. Wet-sand with fine grit for a smoother face if the mold texture bothers you, and leave the form marks in place if you want the industrial look.
Color and Pattern
Concrete does not have to stay gray. Integral pigments go into the mix before pouring, acid stains react with the surface after curing, and paint works on a fully cured piece. The color systems used for decorative concrete floor and wall tiles demonstrate the range, and the same products work on a cast clock face. Test any stain or pigment on a scrap casting first, because the final shade depends on the mix and the cure.
Consolidation and Defect Prevention
Removing Air Bubbles
Fresh concrete traps air, and trapped air leaves pinholes on the face and weakens the edges. Tap the mold with a rubber mallet or run a vibrating sander against the form after pouring so the bubbles rise and pop at the surface. A few pinholes can be filled with a slurry of the same mix after the surface stiffens.
Consolidating Tight Sections
The same principle applies at every scale. In structural work, crews consolidate concrete in congested reinforced members so the mix fills around every bar, and on a clock face the job is the same: vibrate until the bubbles stop rising. Under-consolidated concrete leaves voids, while over-vibrating a small mold separates the fine paste from the aggregate, so a few seconds of vibration is enough.
Using a Vibrating Sander
A vibrating sander is the easiest tool for the job on a small mold. Hold it against the outside of the form tube for a few seconds at a time and watch the surface: when the air stops bubbling up, the mix is consolidated. A rubber mallet tapped around the form does the same work if you do not own a sander.
Demolding, Mounting, and Repair
Pulling the Cast
Let the clock cure for two days, then drill out the dowel and pop the melamine base free. The beveled recess piece comes out with a light tap. If the shaft hole is not perfectly round, a painted fender washer covers it neatly when the movement is installed. Choose a clock kit with a quartz movement and a shaft long enough for the casting thickness plus the hands.
Mounting the Movement and Hanger
- Insert the clock kit movement through the shaft hole from the back.
- Attach the hands to the shaft on the front face.
- Epoxy a picture hanger to the back of the casting.
- Hang the clock, set the time, and install the battery.
Repairing a Broken Cast
If the first attempt cracks or breaks, keep the pieces: a fresh layer of mix can be poured over the old surface once it is clean, roughened, and dampened. Pouring new concrete over an old concrete surface works when the old face is prepared properly, and a repaired clock face can be sanded flat and refinished to look new. Between the mesh reinforcement and a patient cure, the second attempt will be the one that stays on the wall.