Before You Start
Taking the drying time into account, it takes about two weeks to complete this project.
We cast a simple, symmetrical object. Not all objects are suited to making two-piece moulds, and you will need more experience to mould complex objects and shapes. If you don’t want to make a mould, cast the cement in any suitable container. We used silicon baking moulds for our small candle holders. You won’t need to insert chicken wire; just follow step 2 in the Pour The Cement instructions.
You Will Need
1 Measure the object you want to use for the mould. Add at least 4cm to the length and 6cm to the width so the mould will be strong enough right round. Make sure the container you’re using is deep enough so there’s at least 3cm of free space right round the object. Mark the diameter of the object very precisely – it’s very important for the two halves of the mould to be exactly the same size. For the mould, you need a polystyrene-foam or plastic container with straight sides – we used the former and made it smaller to fit our measurements (300 x 150 x 140mm) by inserting a loose piece of polystyrene foam. Carefully seal the joins with duct tape, then paint the entire container with petroleum jelly to ensure your mould will be released.
2 There are no fixed rules when it comes to mixing plaster of Paris, but as you work with the medium, you will figure out how much you need for different projects. A rough guide is to mix 1kg plaster of Paris with a litre of water. Always wear a dust mask and protective gloves. You will first pour a bottom layer of 3cm plaster of Paris into your container. Determine the quantity by pouring 3cm water into your container (this will also check if the container is sealed). Use a measuring jug to determine the volume, then transfer the water to a bucket. Measure the same amount of plaster of Paris. TIP Plaster of Paris sets faster if you use lukewarm water. NOTE We used glass bowls to illustrate the process.
3 Gradually sprinkle the plaster of Paris powder over the water. Work slowly and sprinkle a little at a time, making sure large lumps don’t drop into the water. Lightly tap the outside of the bucket to get rid of any air bubbles that might form. The water will become milky as the powder starts to settle.
4 Keep going until the level of plaster of Paris powder reaches to just below the surface of the water, and the powder stops sinking away. Leave this plaster of Paris mixture to stand for a few minutes.
5 Carefully stir the mixture without letting air bubbles form, and make sure there are no lumps. It won’t start setting immediately; just keep stirring. As soon as the mixture starts resembling runny yoghurt, you can slowly start pouring it into the prepared container. NOTE Plaster of Paris generates some heat while it is setting, so don’t be alarmed when you detect a rise in temperature.
6 Seal the object you will be moulding by painting it with a generous layer of petroleum jelly. As soon as the first 3cm layer of plaster of Paris is hard enough for you to press on lightly, you can place the object on top of it. Make sure the diameter you marked in step 1 is exactly parallel with the level of the plaster of Paris, with one edge of the object up against the side of the container, with equal spaces to the sides of the container along the object’s length and other end.
7 Mix another batch of plaster of Paris (rather too much than too little; we used about 11/2 litres of water) and pour it into the mould, exactly to the level of the diameter, as marked.
8 Leave the plaster of Paris to set slightly, then press your marbles or beads into it, taking care to space them evenly. You will be using them as ‘markers’ later, to fit the two mould halves together neatly. Leave the plaster of Paris to set for another few minutes, then remove the beads or marbles, leaving their indentations behind. Set aside this first half of your mould for at least two to three days in a warm, dry place. You’ll know it’s ready when the plaster of Paris no longer feels cool to the touch, or damp.
9 Paint petroleum jelly all over the plaster of Paris, as well as the object. Mix the last batch of plaster of Paris as you did in steps 3 to 5, then pour it over the first half of the mould to cover the entire object and fill the container. This is for the second part of your mould. Leave to stand for at least two to three days, as before.
10 Remove all the polystyrene foam to reveal the plaster of Paris mould (if you used a plastic container, you will simply need to turn it out carefully). Leave to stand overnight, so that it can set. Pull the two halves apart very slowly and carefully – ask someone else to help you by holding one half, while you carefully lift the other half. You can insert the blade of a paint scraper along the joint if you’re struggling to dislodge the two halves from each other. Carefully remove the object from the mould and leave the two halves to dry completely.
Mould/Pour The Cement
1 Paint the inside of both halves of your mould with petroleum jelly. Then carefully place the two halves on top of each other so the markers line up precisely. Use duct tape to join the two halves. (The tape won’t stick to the plaster of Paris, but it will stick to itself, so put a strip right round so the tape crosses over itself and sticks.) Cut off a piece of chicken wire that is slightly shorter than the inside of your mould, roll it up and insert it into the mould. This will strengthen your cast candle holder.
2 Put on your dust mask and rubber gloves. Mix one part cement with two parts builder’s sand in a bucket (not the same one you used for the plaster of Paris). About 750ml was enough for our mould, but you’ll have to judge how much you need for your mould. Keep one part water handy in a measuring jug. When the cement and the sand have been mixed, start adding the water little by little, stirring continuously. Take care not to mix in air bubbles – lightly tap the sides of the bucket to get rid of any bubbles that may have formed while you were mixing. When you’ve added about three-quarters of the water, check to see how runny the mixture is. When it is of a consistency that can be poured (but not too runny), pour it into your mould. Do this slowly so no air gets trapped in the mould; pour it all in one go without stopping, until the mould is filled. Lightly tap the sides of the mould, or move it while you’re pouring, so the air bubbles can escape – ask someone to help you if necessary. Leave to stand for 24 hours to set.
3 Remove the duct tape and separate the two halves. Take care not to damage your mould, so that you can use it again. Carefully remove the cast cement item from the mould. Gently smooth over the joins with your fingertips. If there are any areas you’d like to fill up, you can do so with a thick cement mixture.
4 Leave the cast cement item to stand for about a day in a bucket of water, or spray water over it every few hours – this will strengthen the cement.
Text and images: Ideas magazine