Large Mould For Cactus

 

The technique for making a large mould has already been described in the post on making large moulds. A variation on this technique involves the addition of a sculpted polystyrene section to make the armature for the large mould. The object that was made using this technique was a large cactus.

Additional steps for the large mould

Again the basic technique is a plaster and vermiculite mix over a hessian covered timber armature. The only extra steps to make the large mould for the cactus involves using polystyrene to sculpt a part of the cactus armature that would be too difficult to make using templates. In this case the rounded top of the cactus was sculpted in polystyrene.

Step 1

Making a timber armature

Making the timber armature

Instead of using ply template ribs to define the shape we use thin strips of 4mm plywood. These are stapled between the top and bottom cross section shapes.

You could add additional cross sections if needed but this shape is simple and only needs a top and bottom.

The rounded top of the cactus was a little difficult to make in wood so it was roughly shaped from polystyrene. This was also covered in a vermiculite mix after being glued to the top of the base section after the main sculpting was finished.

Step 2

Covering with hessian and plaster

Covering with hessian and plaster

After the timber frame has been completed it is covered with a tight fitting layer of hessian. Next a thick layer of plaster is applied to the hessian. After this sets it makes a solid base onto which the sculpting mix can be applied.

It is a good idea to do as much rough shaping as you can when applying the plaster base coat at this stage. This will minimise the amount of sculpting mix that will need to be applied afterwards and also reduce the amount of sculpting work that will need to be done later.

Make sure you have thought through the logistics of how you will handle the armature as it will now start getting quite heavy.

Step 3

Sculpting the detail

Sculpting the detail

After the base section has been covered with a thick layer of vermiculite and plaster the finer sculpting can start. You can start before the mix has dried because it is a little easier to work. When all the detail has been sculpted the whole thing is given a light sanding.

Trace out the shape of the top and cut a block of polystyrene about 25mm smaller all around. This is roughly carved to the approximate shape which will then be fitted to the top of the timber frame.

Step 4

Finished composite shape

Finished composite shape

After the base section has been finished the polystyrene block that has been prepared is glued to the top of the base.

A layer of vermiculite and plaster is applied over the polystyrene and shaped to blend in with the bottom section.

The whole thing can be given a light sanding with fine sandpaper and put aside for several days so the plaster can thoroughly dry out.

Once it is totally dry the surface can be sealed with shellac in preparation for fibreglassing.

Step 5

Making the large mould

Making a fibreglass mould

When totally dry the armature is sealed with several coats of shellac. A wax mould release is now applied and polished to get a smooth surface on the large mould.

Since the cactus has undercut issues with the curved rib shapes in its surface a two part fibreglass mould cannot be used. To solve this problem a three part mould is used which will allow the mould sections to be removed easily from the armature.

The picture shows the completed fibreglassing of the first section of the large mould.

Step 6

Completed fibreglass mould

Completed fibreglass mould

The other two sections of the large mould are fibreglassed and, after bolt holes are drilled in the flanges so that it can be reassembled accurately, are removed from the armature.

The completed mould sections are given a sanding to remove any fibreglass splinters from the outside surface. This makes handling the large mould sections much easier and safer. This is an important step because, while not life threatening, fibreglass needles in the hands are very painful and you will be itching for days until they are out of your body.

Next the three large mould sections are bolted together ready for casting. Below is a completed cast of the cactus. The little horns were added after the main body was cast. Because the cactus was going to have lights inside clear surfboard resin was used so the fibreglass would be translucent.

Completed fibreglass cactus casting

Completed fibreglass casting

Making Large Mould Armatures

When big props or scenic elements are required for a stage show or special event a large mould generally needs to be made. Sometimes the mould can be made in-situ from real world objects such as tree trunks, a rock face or architectural elements rather than having the required object sculpted. This can save an enormous amount of time and money.

For smaller moulds it’s common for the armatures to be sculpted in polystyrene, plaster or clay. Polystyrene is the easiest to use but as the size of armatures increase so does the volume of foam required to make them. With plaster and clay it is the weight factor that becomes the big issue as armatures get larger. In most cases the major cost in making several big props or scenic elements is the cost of making the large mould armatures.

Considerations for large mould armatures

Usually a balance is struck when making large mould armatures between a cheap internal support structure and a thinner outer skin of foam, plaster or clay which is then sculpted to the desired shape. The method described below is a “Template” method for making armatures for large moulds. It involves connecting templates of different cross sections of the object together to make the initial armature base.This minimises the quantity of sculpting material required to make armatures for a large mould.

Step 1

Large mould timber frame

Making the timber frame

A rough shape is first constructed using ply templates to define the required shape. CAD greatly simplifies this task by providing accurate template drawings for marking out the individual section shapes. Use a solid base to build the framework as it will become quite heavy as more material is added. It is not a bad idea at this stage to attach four 100mm swivel castors to the base to make moving the armature around easier.

Always consider the logistics of moving any large object around before you start building. A little planning in the early stages can avoid all sorts of grief later in the process.

Step 2

Covering large object frame with hessian

Covering frame with hessian

Once the armature framework is built cover it with a layer of hessian. This can be simply stapled onto the ribs and battons of the framework.

We now have the basic armature over which we apply the sculpting medium. Hessian is the better base for plaster. If we were using clay, chicken wire would be the better choice because the clay can ooze through the mesh which holds it in position. Polystyrene, on the other hand, needs smooth flat ply facets so it can be glued in position. That can get a little complicated with large objects.

Step 3

Applying plaster for sculpting large shape

Covering with plaster

The hessian is now covered with several layers of plaster. It will need to be a thick consistency so it won’t run off the hessian. Make sure it bonds well with the hessian.

This forms the base layer of the armature and will need to be about 30-40mm thick. The basic shape of the armature can be roughly outlined at this stage.

Step 4

Sculpting large object in plaster

Shaping the plaster

Next make up a mixture of plaster and vermiculite. A 50:50 mix is a good starting point. The vermiculite makes the plaster easier to carve once it has set. Straight plaster is like carving stone. The mixture is applied and shaped as required and sanded smooth once it has dried out a little.

It should now be left a couple of days to dry out completely before the next step of making the large mould.

Step 5

Form ready for fibreglassing

Form ready for fibreglassing

After the plaster has thoroughly dried you can give it a very light sanding with fine sandpaper to get a nice smooth finish. Fill any small holes you find. The plaster is now sealed with shellac and several layers of wax is applied to act as a release agent.

Be fastidious when applying the wax release agent. By building up a smooth glazed wax surface now it will save a huge amount of unnecessary sanding work later if you happen to miss a section.

Step 6

Large mould fibreglassed section

Fibreglassed section

Because there were curved surface features on the object to be moulded it was necessary to make a two part mould. This ensures that the fibreglass shell can be easily removed from the armature. Any subsequent castings made from the large mould will be easy to remove as well.

The photo shows the finished fibreglass shell of the first half of the large mould.

Step 7

Completed large mould

Completed mould

Here is the finished two part mould of the large object. Since it is a symmetrical object we only need a mould for one half.

After casting two complete sections the two halves are fibreglassed together to make the finished object.

The making of large objects involves a huge amount of work. The making of the mould is the major expense incurred. If short cuts are taken with the mould it can lead to very expensive mistakes when the time comes for casting.

Plaster Bandage Mould

A plaster mould is the easiest and cheapest of all moulds to make. They are also the heaviest and most fragile. There are two basic ways of making them. The method described here is to use plaster bandage which is normally used to set broken limbs. The other method is to pour the plaster into a mould which is described in the post on Plaster Mould Making.

Simplicity of plaster bandage moulds

A plaster bandage mould is useful when taking moulds of body parts because it dries quickly and has no harmful solvents. Plaster bandage can also be applied directly to an object without having to make special dams or walls to contain it. To save money it can also be used as a support jacket in lieu of fibreglass when making a latex mould.

Step 1

Plaster bandage mould error

Who forgot the mould release?

For the purpose of description we will take a plaster cast of a foot. You need to put a release agent onto the skin to prevent the plaster sticking on skin or hairs. Even though it’s messy, petroleum jelly is the best to use. Even if hairs get caught in the dry plaster they can slip out without too much agony.

You can see in the photo of an arm mould that it *is* agonising removing a plaster mould if you forget this important step.

Step 2

Get a roll of plaster bandage, dip it in a bowl of water and start laying it on the foot, fold on fold. Rub the bandage to make the plaster soften and mix through the bandage. As the layer gets thicker keep kneading the bandage down and around the foot to ensure the plaster and the bandage get into all the crevices. Repeat this until the plaster and bandage are about half an inch thick. Keep smoothing it with your hands until you feel it getting warm and hardening.

Step 3

Take extreme care not to plaster all around the foot (or anywhere else!) otherwise you will not be able to remove the mould. After it has hardened sufficiently carefully pull the mould from the foot while it is still a little flexible. Feet are a bit soft and flexible as well so you shouldn’t have a problem.

Step 4

You can now make casts from the foot mould. Make sure you seal it with shellac so that mould release doesn’t soak into the plaster. Plaster bandage is a super quick way of making a mould where you don’t need a lot of definition. It also makes a good support jacket for alginate moulds of the face for example. Dental alginate sets quickly (minutes) and captures very fine detail. They are also very weak and need a support jacket. Plaster bandage is ideal because it can be applied directly, sets quickly and has no harmful solvents.

Prop Bookshelf

Set designs can involve having prop bookshelf or a library on stage. Real books can be used but their weight can be an issue. If weight needs to be reduced usually the pages are removed leaving only the spines which solves the problem. While this works for a small number of books in a prop bookshelf it becomes difficult with hundreds or thousands of them. You can buy faux books which are cast sections of book spines about 30 cm wide which are very realistic. They are also quite expensive, uniform in height. They also look two dimensional because they have no depth.

Prop bookshelf with depth

The technique described here produces a realistic looking prop bookshelf with depth. It involves making a fibreglass mould of a selection of books from which the fibreglass casts can be adapted to different configurations. It’s not a cheap option but the flexibility, weight saving and realism is worth the effort.

Step 1

Preparing real books for prop bookshelf mould

Preparing real books for moulding

Buy a variety of old books that fill one bookshelf section of your bookcase. Choose books that have embossed lettering on the spines or at least some sort of detail. Using a hot glue gun fasten them side by side, with the spines roughly aligned, on a board.

Step 2

Preparing the real books

Preparing the real books

The books all have different depths so draw a line corresponding to the depth of the largest book along the back of the books. Fit a vertical piece of wood along this line about 10cm higher than the tallest book. This will be the back of the mould. Fill the gaps between the other books and the back of the mould. You can use bits of ply or polystyrene. It doesn’t have to be too neat as it will be trimmed off the finished castings.

Step 3

Fibreglass mould of prop bookshelf

Fibreglass mould of books

After making sure all the gaps are filled and smoothed with plasticene give the whole thing a couple of coats of PVA mould release. Put a layer of gelcoat on first to capture all the detail on the book spines and finish the mould off with 3 or 4 layers of fibreglass. This is described in more detail in the post on making fibreglass moulds.

After the mould has cured remove the books and you will end up with a prop bookshelf mould as shown in the photo.

Step 4

Fibreglass castings of books

Fibreglass castings of books

Clean the mould thoroughly and prepare it for making casts. You can make as many as you need. Since they will all be the same you can add a bit of variety by cutting casts in half or quarters and fibreglassing them to other section. This mix and mash technique gives several bookcases a more random arrangement of books. If they were all the same the repetitive shapes of the books would look very obvious to the audience.

Step 5

Gold paint applied to spines

Gold paint applied to spines

Painting the casts is the next step. To get the gold lettering on the spines mix up some gold powder in some shellac and paint all the spines paying particular attention to the lettering.

You can now see why it’s best if the original books had embossed lettering.

Step 6

Painted books

Painted books

Now lightly brush different colours on the books taking care not to get paint inside the embossed lettering. Wiping a flat brush over the spine is best because the bristles don’t get into the lettering. Don’t worry about getting it too perfect. A bit of gold here and there actually makes the books look a little worn. Make sure you paint the books in matching positions in separate bookcases a different colour. This helps randomise the look when they are viewed together.

Step 7

Completed prop bookshelf

Completed books

Now mount the prop bookshelf sections into your bookcase. They can be screwed down from behind but every situation is different so choose the method that is best for you.

Making A Silicone Mould

A silicone mould is much more forgiving with undercuts than one made from fibreglass because they are so flexible. A silicone mould is used more for casting urethane foams and resins to make solid objects. They are excellent for making bottles and glasses and usually have a cut down one side to make it easier to remove the cast. A wooden box or tube is used around the mould to support the silicone rubber when it is filled with casting material.

A silicone mould is easy to make but the materials are expensive. You need to balance the size of your object with the size of your wallet. That being said they are robust moulds and can give many impressions. For the purpose of description we will make a mould of a engine cylinder head for a full-size prop replica of a Wright Cyclone radial engine.

Cylinder head silicone mould

The following steps describe the construction of a silicone mould and casting of a prop cylinder head for an aeroplane engine. More construction details for the engine construction can be found here.

Step 1

Silicone mould form of cylinder head

Wooden form of cylinder head

You usually don’t need a release agent with a silicone mould as it easily peels off the object your copying. If the object is porous it is wise to seal it with shellac just to be on the safe side. The object to be moulded is a prop cylinder head for a 9 cylinder Wright Cyclone aeroplane engine. It was made from MDF from a CAD drawing of the engine.

Step 2

Silicone mould box ready for pouring

Box ready for pouring

Build a box around the head as shown in the photo so you can pour in the silicone rubber. Make the base removable to enable removal of the wooden form. Notice the blocks of wood in the corner of the box? This is to reduce the volume of silicone required and hence the cost. Fill the box up with water and then empty it into a measuring container to measure the volume of the mould. This will be the amount of silicone rubber you need to mix. Dry the box out before proceeding.

Mix up the silicone rubber in the correct proportions by weight and mix thoroughly. Use the correct ratio is important with silicone rubber because of its narrow tolerance range. Pour gently so you get a thin dribble of silicone to fill up box. This minimises any bubbles getting trapped in the mould. You can use a vacuum chamber to remove all the bubbles from the mix but that is pretty high end stuff. Leave it for a day to cure.

Step 3

After the silicone has cured remove the base of the box and carefully remove the wooden cylinder head plug by peeling the silicone back. Because of its flexibility you can have small undercuts with silicone but be careful removing casts as it can tear.

Step 4

Cleaning a cast

Cleaning a cast

Polyurethane foam is poured into the mould to make the casts. It cures very quickly and you can remove them after about half an hour. A little light sanding and the head is ready to have the MDF cooling fins attached.

Miscellaneous notes
You can make moulds of bottles or glasses. It is easier to use a PVC tube around the object instead of making a box to contain the silicone. You can then make hollow bottles by melting breakable glass resin and pouring it into the mould and roll the mould around by hand lining the inside of the mould with resin. It is a tricky technique that needs a bit of practice but it’s a good way to make breakable bottles and glasses. Be careful removing the resin bottle when it has cooled as it is very fragile.

Making A Latex Mould

A latex mould is a nice compromise between a fibreglass mould and a silicone mould. They take a little longer to make but they are more flexible than a solid fibreglass mould and much cheaper than silicone rubber. A latex mould will deteriorate over time since it is a rubber compound but this is generally not an issue. As with fibreglass moulds you can have several sections bolted together. Small undercuts are not a problem with latex because of it’s flexibility.

Step by step latex mould making

For a description of latex mould making we will make a two part mould of a deer head. The following steps are the general ones to follow regardless of what you are moulding.

Step 1

Latex mould flange

Fitting a flange

Build a flange around the middle of the head where the mould will split. Fill all gaps with plasticene so that there is a clean right angle join between the object surface and the flange. This minimises any join marks when the two halves are bolted together.

It’s a good idea to place a few small 12mm diameter domes along the flange. These will form alignment keys when you make the second half of the mould. This ensures both halves line up exactly when bolted together.

Step 2

Ready for latex application

Ready for latex application

Paint several successive coats of liquid latex over the urn and flange. It is important that these initial coats are not too thick and that they dry properly between coats. If you rush the process the latex will lift off the head in places.

Once you have a nice thick layer of latex over the head it needs to be reinforced. Kitchen wipes are ideal for this. Cut up some Chux superwipes into suitable strips and apply them to the latex skin with more liquid latex. Again do one layer at a time and allow to dry completely before applying the next layer.

Repeat this process 3 or 4 times until you have a tough reinforced skin over the deer head.

Step 3

Completed latex mould covering

Completed latex covering

When it has dried give it a couple of coats of PVA mould release and leave to dry. Next lay a fibreglass jacket over the top of the latex. Get a thin piece of plywood about 1000mm by 600mm and put it next to the mould. Tear up some fibreglass matting into 120-150mm squares and lay them out on the plywood. Using a 2 litre plastic ice cream container AND wearing safety goggles mix up about 600ml of unwaxed resin with MEKP catalyst. It is usually about 50:1 but I like to use a little extra catalyst to speed up the reaction. Don’t put too much in because the chemical reaction will be quite brutal and get so hot it will start fuming and could burst into flames. You should be aware of this catalytic reaction with resins. It is worth doing a bit of experimenting with different ratios so you get a ‘feel’ for the dynamic of the material your using and learn to respect it.

With a 50mm brush paint the resin generously onto all the squares of fibreglass matting on the plywood. As the glass matting absorbs the resin it will go from a white colour to the colour of the resin. Now apply a light coat of resin over the latex mould. Pick up the first square of wetted fibreglass matting and lay it down on the latex. Lightly dab it down with the brush to remove air bubbles. Do this with the rest of the matting in quick succession. It’s already soaked through with resin so you can work very quickly.

Once all the matting has been applied you should still have time to dab more resin with the brush and smooth the surface. Try to get all the matting totally smooth with no needles of fibreglass sticking up. Take care doing this now and you will find it easier to lay the next layers. Fibreglass splinters can be very painful so try to avoid them in the first place.

Timing is everything when working with resin. Take note of the consistency of the resin as you are working. When it is about to go off you will notice it getting a little thicker. Finish whatever your up to ASAP because once the resin starts to gel is of no use. Repeat this step until you have 2 or 3 layers of fibreglass over the latex mould. Leave it to cure for a couple of hours.

Step 4

Fibreglass jacket over latex mould

Fibreglass jacket over latex

Remove the timber flange and prepare the other side of the head in the same way repeating Step 2 & 3. Make sure you put mould release on the flange of the half you just finished so you don’t stick the two latex halves together.

When finished drill two 9mm holes through the flanges on both sides of the head. These are for bolts with wing nuts to hold the halves together while the fibreglass cast is curing.

Step 5

Finished latex mould

Finished mould

Remove the fibreglass jackets and carefully remove the latex moulds from the deer head. Wash any residual release agent from the surface of the latex and fit them back into their respective fibreglass jackets. The latex mould is now ready for making casts.

Making Fibreglass Moulds

Fibreglass moulds can be a one piece mould or, if the object has a complicated shape, made in two or more separate pieces bolted together to make the final mould. This makes removal of casts from the fibreglass moulds possible. There have been many unhappy souls who have made a cast in fibreglass moulds only to find they cannot remove them. There is no flexibility in fibreglass moulds so things will get permanently stuck if there are any undercuts.

Main points of fibreglass moulds

The aim is to make the surface of fibreglass moulds as smooth as possible. If the surface quality of the mould is like a mirror then the cast object will be the same. The extra care taken in making the mould will reap rewards when casting later.

Mould release agents or wax should always be applied to moulds before laying fibreglass matting. The following steps are a general guide to making multi-section fibreglass moulds.

Step 1

Marking the fibreglass moulds

Marking the mould sections

You must carefully plan where you want fibreglass moulds to split. In the photo you can see where lines have been drawn on a statue defining mould sections where there will be no undercuts. It is sometimes easier to lay the object your working on in a horizontal position. This is sometimes not practical. Start by building a flange along the lines that define a mould section. Plasticene has been used because it’s easier to work and get a clean right angle join between the statue surface and the flange surface. You can see in the photo that the first mould section made was the buttock area. When making fibreglass moulds it is a good idea to place a few small 12mm diameter domes along the flange. These will form alignment keys when you make the adjoining mould section. This ensures mould sections line up exactly when finally bolted together.

Step 2

If the surface is porous seal it with several coats of shellac. Give it a light sanding with steel wool between coats. Apply a wax release agent and polish it well to get a glossy finish. You may need to repeat this several times to get a smooth surface. Pay particular attention to getting the release agent on the flange.

Step 3

Wearing safety goggles mix up a quantity of gelcoat in a 2 litre plastic ice cream container and apply an even layer over the section and its flange. Make sure you fill the right angled join between the flange and the object. Fibreglass matting doesn’t fill sharp corners or fine detail very well so make sure these are all generously coated. Once the gelcoat has gone off your ready to lay down the first layer of fibreglass matting.

Step 4

Get a thin piece of plywood about 1000mm by 600mm and put it next to where your working. Tear up some fibreglass matting into 120-150mm squares and lay them out on the plywood. Using a 2 litre plastic ice cream container AND wearing safety goggles mix up about 600ml of unwaxed resin with MEKP catalyst. It is usually about 50:1 but I like to use a little extra catalyst to speed up the reaction. Don’t put too much in because the chemical reaction will be quite brutal and get so hot it will start fuming and could burst into flames. You should be aware of this catalytic reaction with resins. It is worth doing a bit of experimenting with different ratios so you get a ‘feel’ for the dynamic of the material you are using and learn to respect it.

With a 50mm brush paint the resin generously onto all the squares of fibreglass matting on the plywood. As the glass matting absorbs the resin it will go from a white colour to the colour of the resin. Now apply a light coat of resin over the gelcoat and then pick up the first square of wetted fibreglass matting and lay it down on the gelcoat. Lightly dab it down with the brush to remove air bubbles. Do this with the rest of the matting in quick succession. It’s already soaked through with resin so you can work very quickly.

Once all the matting has been applied you should still have time to dab more resin with the brush and smooth it all down. Try to get all the matting totally smooth with no needles of fibreglass sticking up. Take care doing this now and you will find it easier to lay the next layers. Fibreglass splinters on fibreglass moulds can be very painful so try to avoid them in the first place.

Timing is everything when working with resin. Take note of the consistency of the resin as you are working. When it is about to go off you will notice it getting a little thicker. Finish whatever you are up to ASAP because once the resin starts to gel it is of no use.

Step 5

While the first layer is going off tear up more squares of fibreglass matting and lay them on the plywood as before repeating ‘Step 4’. Keep repeating Steps 4 & 5 until you have a uniform covering of fibreglass matting about 3 layers thick over the section. Leave it to cure for several hours.

Step 6

Plasticene flange for fibreglass moulds

Plasticene flange

Now remove the plasticene flange from around the section you just finished. Extend the flange of the just completed section along the lines of the next section you want to make by using plasticene as before.

You can see in the photo that the back section has the plasticene flange running down the statue spine. The alignment domes are clearly visible around the arm and the flange of the buttock section forms the support for the lower flange of the back section. The secret to making good fibreglass moulds is to make accurate, well fitting flanges on the mould sections.

Step 7

Repeat Steps 2 – 5 taking care to wax the flange on the finished sections. When you’ve finished fibreglassing all the mould sections drill 10mm holes through the flanges of each mould section. These are for bolts with wing nuts to hold the sections together when your making a cast.

Step 8

Completed fibreglass mould

Complete body mould

When the fibreglass has cured separate the fibreglass sections from the object. They should pry off with a screw driver between the flanges. You can also use compressed air to blow between the fibreglass and the object to help separation. Once separated give the outside surface of the mould and the edges of the flange a light sanding to remove any sharp protrusions. You can, if you like, mix up a little more resin and paint the sanded sections with resin. When it cures it gives the mould a smooth glossy finish. This makes fibreglass moulds much easier to handle.
Give the mould sections a good wash to remove release agent (or clay in our example) from the mould. If there are any imperfections they can be sanded smooth with fine wet’n’dry abrasive paper.

The head of the fibreglass body mould was a separate silicone mould because of its detail. Fibreglass moulds may need bracing if some of the sections are quite large and not supported by other mould sections.

This large statue was built to support a large armillary sphere in the opera Otello. More information on the armillary sphere that was fitted atop this statue can be found on this page.

Making Plaster Moulds

Plaster moulds are the easiest and cheapest of all moulds to make. They are also the heaviest and most fragile. There are two ways of making them. The first is to make a waterproof box around your object, pour in the plaster mix and wait for it to set. The second uses plaster bandage which is normally used to set broken limbs.

Steps in making boxed plaster moulds

Following is a step by step process in making boxed plaster moulds. They are very straight forward and require no special equipment or chemicals. All that is needed to make plaster moulds is plaster of Paris and some scrap timber to make the box section to hold the plaster mix until it sets.

Step 1

Fasten your object to the middle of a smooth, flat piece of wood. Make a waterproof timber dam around it a couple of inches higher than the object. Use plasticene to fill all the undercuts on the object and any gaps between it and the base board. Take extra care doing this because it makes for a better mould. This is very important otherwise the mould will break when you remove it from the object your moulding.

Step 2

When your satisfied nothing will obstruct the removal of the object from the mould you need to seal it and apply a release agent. The best sealer to use is shellac. Two or three coats should suffice. When its dry apply the release agent. You can use wax or PVA but the best of all is petroleum jelly. Nothing sticks to it and its cheap.

Step 3

Now mix the plaster. This is the messy bit. Start with a bucket of water and add a little plaster and thoroughly mix it before adding a little more. Make sure its all completely dissolved before adding more. Continue adding plaster in this way until it becomes a smooth creamy paste with the consistency of oil.

Now pour the mixture over the object in a thin dribble. This minimises bubbles getting trapped in the mould. If the mould is not very thick you can lay some Hessian on top of the mould and add more plaster. This reinforces the plaster and prevents it cracking. The plaster will start going hard very quickly and will get quite hot as it sets.

Leave it for a day or so to cure and then carefully remove the baseboard. If you did everything correctly the object will plop out of the plaster and you will be left with a nice impression.

Step 4

Plaster moulds are great for casting latex objects. You just pour in the liquid latex and wait for it to dry. Then they just peel out. Easy. It’s a great way for making decorative trims etc. If your casting with latex leave the mould as raw plaster. That way the latex can cure faster by ‘breathing’ through the plaster. If other casting materials are used seal plaster moulds with shellac and use a release agent.