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Difficulty: Beginner
Concrete is a wonderfully versatile material for use in all kinds of arts and crafts.
This simple guide shows you how to create unique house numbers, and you can follow the same process to make other bespoke artworks for your home.
Steps
Step 1
First make or buy papier-mache characters of the letters or numbers that you want to use. You'll be placing these in your mould and ladling the concrete around them. Your characters must have a flat front that can be glued flush to the base of the mould. They must be sealed on all sides (no hollow backs) and should have straight sides to make them easy to remove.
Decide on the positioning of your characters, remembering that they need to be mirror-image as you will be looking at the back as you create the mould. They also need to be at least 50mm in from the sides to avoid cracking. Use your tape measure for accuracy, and mark their location. We used our level as a straight-edge guide aligned with the board edge, and also as a spacer to position and align the top and bottom.
We used formply as timber for the mould because it provides a super-smooth finish and separates easily from any cement mix. You can use other timber for extra texture but you will need to let it set longer to ensure that it separates cleanly.
Now mark your board for cuts. The cut list for our mould box (suitable for three 20cm-tall characters) was:
1 x 690mm x 315mm (base)
2 x 690mm x 140mm (sides)
2 x 340mm x 140mm (ends).
Step 2
Clamp board down, ensuring clamps aren’t in the way of your cuts. Fit guide to your saw and adjust to match required cut width. Saw your sheet up to create the pieces of your mould box.
Step 3
On a flat work surface assemble the mould box. The side and end panels sit outside the base and are screwed to it. Pre-drill and add two to three screws to each join. Ensure all joints are closed tightly.
Tip: clamping sections together can help when assembling your box.
Step 4
Apply a generous amount of adhesive to the face of your characters and position them carefully, remembering that they must be mirror-image and read and run backwards. Press down firmly, taking care not to crush them. Carefully wipe excess adhesive from board and place the mould in the sun to allow adhesive to set.
Step 5
Concrete and cement mixes are typically grey, so if you want bolder oxide colours to show through you will need to blend your own mix of sand and white or ivory cement.
If blending your own cement mix first dry-mix your sand and cement in the wheelbarrow. The ratio is three sand to one cement. Once it’s blended add the colouring oxide and continue to dry-mix, ensuring that the colour is consistent.
If you’re using pre-mixed bagged sand and cement or concrete empty the bags into barrow and then blend in the oxide.
Now add water gradually, blending as you go, and bring to a consistency that is just short of pourable. Our mould used a single mix of three 9L buckets of sand with one bucket of cement.
Remember that pre-mixed bagged concrete contains gravel that will change the texture and appearance of the finished project.
Step 6
Fill bucket with mix and use trowel to ladle small amounts into your mould. With gloves on gently push mix in to fill all holes and remove any air pockets. Make sure it’s pushed well into corners too.
Continue to fill the mould, pressing down as you do so. You can use wire mesh to reinforce larger projects. Use mesh with aperture sizes of 25mm to 50mm and ensure it is at least 30mm away from all edges. It is best added when the mould is just above half full.
Once the mould is full use trowel to bring the back to a smooth finish but do not overwork.
Step 7
Cover filled mould with plastic sheeting (a garbage bag is adequate), and leave for at least 12 hours to cure. The plastic helps prevent cracking, which can happen when the mix dries too quickly. Remove screws from the sides of the mould sides once the mix is cured.
Step 8
Once you’ve removed the sides carefully roll your block on to its back. The concrete can still be a little chalky and easily chipped at this stage. Now lift what was the base of your mould off and the face will be revealed. The papier-mache characters will likely stay in the actual block, but you can remove them easily - again taking care with the edges of the block.
Your project is now complete and ready for showing off.
Materials
- Formply (we used one sheet, 1200mm x 600mm x 17mm)
- 50mm timber screws
- Sand and cement in pre-mixed bags or bagged cement and bagged sand to blend your own. Volume suitable to fill your mould
- Cement colouring oxide of your choosing
- Hollow papier-mache characters or designs. We used three characters 20cm tall
- Quick-drying adhesive suitable for both porous and gloss surfaces, such as Selleys Aquadhere PVA Wood Glue Exterior Adhesive.
Tools
- Small power saw with adjustable guide
- Driver drill with bits for screws and pre-drilling
- Tape measure and square
- Quick-release clamps to fit your mould
- 1200mm long level or straight-edge for setting out
- Wheelbarrow or large bucket for mixing concrete
- Spade or large hand trowel for mixing cement
- Medium-sized concreter’s trowel for finishing cement
- Bucket for measuring cement and sand (if making your own mix) and for ladling out concrete mix
- Marking pencil or felt pen
- Personal protective equipment: Eye and ear protection when using power saw. Eye protection when using drill. Eye, hand and breathing protection when mixing dry cement products. Hand and eye protection when working with wet cement mix.
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