Purchasing Moulds

Moulds may be purchased from Yard Art as quality, re-usable latex, or as latex and fibreglass support or jacket (mother mould).

It is advisable to get moulds with good fibreglass outers or mother moulds, particularly larger ones as they are tough, free standing and do not require any extra work on your behalf.

However, to save cash you can purchase your latex-only mould and with a bit of kiwi ingenuity, make your own mould support using materials such as fibreglass, plaster, plywood, polystyrene or cardboard.

Larger moulds must have good outer support of some description or they will bulge, sag and generally be unable to support the weight of the the concrete when you pour it into your mould.

I am concerned here with making ornaments out of concrete, but latex moulds can be used to make ornaments out of many products including plaster, wax, resin, chocolate or even ice (if you have a large enough freezer!)

For small 3 dimensional moulds (gnomes, animals etc.) without an outer support, make a fibreglass jacket using epoxy resin and fibreglass matting, layering up to make two separating halves (or more depending on the complexity of the design).

Casting

Making cutout support for your latex mould (For small moulds not requiring jacket for support)

Find a strong cardboard or polystyrene box (or you can jigsaw out of ply). Take a rough trace of the base of your latex mould onto the underside of your cardboard, polystyrene or plywood, by folding back the rim of the latex and tracing the shape as close as possible with a marker or pencil.

Your trace should be a little larger than the mould base for fitting. Cut out the shape with a craft knife or jigsaw. Place your latex moulds into the hole shapes and number the mould and its corresponding hole for later use. If the latex has a demoulding slit you can hold it together with large bulldog clips.

Making fibreglass outer mould (For larger moulds needing jacket for support)

Fill your latex mould (it’s a piggy in the picture) with screwed up newspaper so for some strength so it doesn’t distort while you are working on it. Cover the outside of your latex mould with a mould release such as petroleum jelly or wax so the fibreglass doesn’t stick to the rubber. Find the natural centre and draw lightly around the rubber mould (being aware of any nasty undercuts which may not demould easily) and make a rough “wall” using cardboard and brown tape roughly 50mm high all round as pictured.

You will be making two solid fibreglass halves so they need to be able to separate easily from the latex mould when it is filled with hardened material such as concrete. Then using 2-part fibreglass epoxy resin mixed with hardener (see label instructions) start to build up one side of your mould cutting and dabbing the matting into details and recesses. Work from the top to save resin running away and make the first half of your fibreglass mould.

Allow to dry overnight and remove from the latex mould to ensure it is not sticking. Trim the top of the “wall” to remove any rough bits and patch up any areas you missed.

Next put your new fibreglass half mould back onto the latex and start on the other side, remembering to cover with petroleum jelly including the back of the “wall.” When nearly dry, separate the two pieces, trim the edges and sandpaper all over to remove any rough bits and spiky bits of fibre matting. When finished you can join the two halves together by drilling and fitting bolts through the “walls” or simply use bulldog clips to hold them together.

Plaster moulds can be made in the same way (see pic above). When you pour you can make a cutout as described above or simply stand your mould upright in a bucket or bag of sand to keep upright.

For stepping stones, plaques etc (2 dimensional moulds) you can easily make a plaster of paris support mould by placing the latex mould face down on your bench (fill under with newspaper etc for support) and work plaster over the back of the rubber to a thickness of 10 or 20mm leaving yourself a plaster rim wider than the latex rim and a flat base so it can be reasonably level when you pour. The plaster mould should dry overnight and be ready for use the next day.

Pouring and demoulding your ornaments

Position your moulds and supports so that they are level, spray the insides with a thin layer of silicone oil (protects your moulds and helps with demoulding) or use weak detergent and water to assist with pouring and demoulding when cured.

Prepare your material for pouring.

Concrete can be mixed in a mixer or by hand in a wheelbarrow, or simply in a bucket with a trowel at the kitchen table. I use three parts coarse sand or builders mix to one part cement. Or you can buy premix bags and just add water. Mix to a creamy consistency, not too thin, not too thick, just right (If it’s too thin you’ll end up with fine air holes and sandy streaks, if it’s too thick you’ll end up with large holes to fill).

This may take a little trial and error to get it right. If you want to add coloured oxides to your mix, add to the water before mixing.

At this stage you can add plasticiser for pliability and other admixtures to frost proof, accelerate or inhibit the curing times. For more on this, see http://www.wikihow.com/Use-Cement-Additives

Use a plastic jug or similar to slowly pour the mixture into your moulds. I pour in three stages, removing air bubbles after each stage by tilting, jiggling, tapping the sides and poking inside with a stick taking care not to damage your mould or spill concrete everywhere!

Finish off just below the rim and jiggle to level the surface and smooth over with a scraper or trowel. Pour off any excess water if it’s too wet. Otherwise leave to set for 24 hours. Demould and store your ornaments in a dry place for at least 10 days before painting. Your moulds should be cleaned with detergent and water and stored in a cool dry place for next time.

Painting

For full colour finishes, spray or brush black acrylic paint all over your ornaments.

When dry, add acrylic colours over the black, for effects use a “dry bush” technique leaving black in the detail. It’s that easy.

Other finishing techniques

There are many ways to finish or paint your ornaments. Coloured oxides can be used in the concrete mix or added later by dipping or painting after demoulding.

Stone paints, cement wash, bronzing, iron rust, patina green finishes are all possible (products available from Porters Paints). Some ornaments look great with an acrylic clear glaze applied directly to the plain concrete or after painting.