Concrete Clock - Secret Lightweight "solid" Concrete Clock for Indoors or Outdoors

Welcome to my Instructable for a Concrete clock, designed to look like a paving slab but secretly, even though its real concrete, it’s lightweight allowing it to be hung on an internal, plasterboard, wall with no fear of it ripping chunks off the wall and smashing to the ground.

It’s rugged looking to give the image of a slab you'd buy at your local DIY or garden store. I love the rough finished appearance and have now permanently mounted it in my smoke house corner in the garden.

The clock is molded in ready to use concrete mix created on top of a wood and polystyrene base, it’s sealed and sits flush to wall. It’s easy to make and you can created one in any shape or size.

Apart from waiting for the concrete to dry you can build one of these in about an hour of actual working time so is easily completed over a weekend or a couple of evenings.

Materials:

1. Ready mix concrete(I used about 1.5kg)
2. 5.5mm Plywood
3. Sheet of polystyrene about 20mm thick
4. Water
5. Eco Resin Plaster/ Resin Crete (or plaster of Paris)
6. Coloured mica powder
7. Clock mechanism with wall hook and hands - about £5 online

Tools - supplies

1. Safety - a face mask/filter and gloves
2. Clock mold - you can make one or use a pre-formed one like mine
3. Box Knife / Stanley Knife - alternatively a hot wire polystyrene cutter
4. Jigsaw - saw
5. Drill + Bits
6. Weighing scales
7. Optional - big benefit a laser cutter

To make a sturdy back for the concrete facia of the clock we’re using plywood and polystyrene.

The mold - I make clocks, a lot, it’s become a vice. So I have a 30cm round silicone mold ideal for making a round clock. However rather than investing in one you can make a mold of any shape using wood. Wood makes excellent concrete forms and is relatively easy to remove, Instructables user Devin Das has an excellent listing showing how to make one here. One important thing to bear in mind is the base of the mold will be the front of the clock.

The plywood - This adds stability to what will be a thin veneer of concrete, cut the plywood into the shape of your mold about 10-20mm smaller than the outer mold. It’s easy to use a jigsaw to do this, I used a laser cutter because I have one and am lazy. You can replace plywood for any wood you have knocking around, just adapt the thickness of the polystyrene and/or mold to accomodate it.

Polystyrene - This is the secret to making a substantial cement item with minimal weight. Cut the polystyrene to the same size as the plywood. You can do this using a sharp knife (carefully and wearing protective gloves) or using a hot wire cutter which makes it simple and quick, make sure to wear a filter mask melting poly.

Once these parts are ready we move on to the concrete.

Ready mixed concrete is easy to use, it comes prepared with the right mixture of cement and aggregates ready combine with just adding water.

Clean your work area and make sure you’re working on a surface that will forgive you some spills and drops of concrete.

Add the required amount of cement to a tub or container, wear your face mask as no matter how gently you tip or spoon out the mix it will send up concrete dust which is not really very good for you.

For our mix we need it to be quite fluid to make it easier to work with and to fill gaps, so add water and stir. You want it to be the consistency of a very thick chowder/soup. This is generally more water than the instructions will state but don’t worry it will dry but may take a little longer.

Using a stick or stirrer, vigorously mix the concrete to ensure you’ve got rid of any clumps and lumps.

Place your mold on a level surface where you will be able to leave it until it sets.

Pour in some of your concrete and using a flat blade or spatula smooth out a layer across the complete mold. This should be about 4/5mm thick which allows a good solid facia to the clock but keeps the weight to a minimum.

Now place your plywood support piece centrally on this initial pour, you don’t have to measure just judge where looks like the middle. Now add your polystyrene on top of this.

Next pour the remaining concrete into the gap created around the two supports, use your scraper/spatula to push it in then smooth out the sides to the same level as the polystyrene.

At this stage you can run round the edge with a thin blade or stick to push in the concrete to remove any air bubbles, I don’t do this as a few bubble holes add that rustic, industrial finish I was aiming for.

The concrete is ready to leave alone to dry, what you don’t really want to do is let any remaining concrete dry on your containers, tools and surfaces.

Carefully dispose of any remaining concrete, I used what little I had left to re-grout a couple of paving slabs in our garden.

Rinse all containers and tools under an outdoor tap, don’t do it in the kitchen sink, I find it causes arguments with your partner and blocks your sink.

Whilst the clock is curing… make the details.

You can pretty much choose any kind of numbers, icons or marks to depict the face of the clock, heck use a maker pen if you want, design it to suit where you’ll put it.

I used shapes for the hour markers, I made these using eco resin / resin Crete. Resin Crete produces similar results to plaster of paris but is stronger and waterproof, it’s finish is white so I use mica powder to add colour and swirls to it.

Using scales mix the resin Crete 70/30 by weight with water and add the mica powder. As I mentioned I make too many clocks so have molds for numbers and symbols so poured out four straight pieces for the quarter hours marks and round “pebbles” for the remaining numbers.

Resin Crete dries in only about 40 minutes and I like the result, the black mica powder created swirls and seams to make them look like cement.

I left the concrete overnight and the next day it was solid, hot weather certainly helped. Because we added more water than usual it may take longer than expected, don’t rush it though, its better to leave an extra day than have to start again.

It turned out really well, no residue left and looking great. As I mentioned I wanted a few air pockets and rough patches and did really well, not too many, not to few. With too many air bubbles it can become brittle.

I weighed it at this stage to show how, although it looks really thick and substantial, it weighs much less.

This one totaled 1.3kg, this might sound a lot but it’s not, my kitchen clock which has a metal surround weighs 1.8kg. This weight will not destroy your walls (Disclaimer - I cannot be held responsible for the current state of your walls:)

When building clocks you need the mechanism shaft to be central, of course.

I have tried many, many times to use equipment to identify the exact centre, it’s tricky. I’ve found that using a template allows you to position it by eye which I’ve found every time to be better than using math's. I’ve uploaded my template design in DXF for laser cutting and SVG so you can alter or print it out.

Once you’ve decided on the centre you need to drill the hole, most mechanisms are 5 or 5.5mm diameter.

Concrete is not forgiving when drilling and can tend to break and burst, I’d really recommend using a diamond hole bit to drill it out. Add some water to the centre and starting at a slow speed drill through.

Go all the way through so it comes through the polystyrene (you might want to change to a regular drill bit for the wood when you reach it)

Now place the mechanism into the hole from the back and draw around it, using either your knife or hot stick cutter cut out the shape ensuring you’re leaving room for the hook as well. Now screw the retaining washer and bolt to the front, don’t mount the hands yet.

Using the template mark out the positions for the hours, now using multipurpose glue adhere the markers in position. MAKE SURE 12 O’clock is in line with the hook on the back! Let them dry.

Now, a concrete item will shed dust over time, you may have noticed any time you’ve had to move a cement or concrete planter in the garden, you don’t want this dust in your home.

Mix equal parts of PVA glue and water and mix well. Now paint this over the entire clock, don’t forget the back as well.

Initially it’ll change colour and go darker, don’t panic as it dries you cannot see it. This seals the surface and makes it easier and cleaner to handle.

You should have a solid concrete looking clock now! (We know its not solid)

Add the hands, I discarded the hands that came with the mechanism and went for some basic black hands that I think match its industrial look.

To hang, the back of the clock should be flat to sit flush with the wall so I used some stand off screws that protrude enough to meet the hook hidden within the polystyrene. For internal fixings a nail or raised screw would work, but, I think a traditional picture hook would probably be difficult to get it flush with the wall.

I tested it on an internal wall and it hung easily with no concern of it falling at all. I decided though I’d like it outside near my smokers, under a lip of the wall and flush against the back it will be weather proof although you can add some tape over the mechanism at the back if you think its too exposed.

Enjoy, if you make one let me know.