Monday, 1 March 2010

Vermiculite Moulds

Vermiculite moulds have versatile applications as custom moulds both for draping and slumping. With care, they have a long life. They are relatively light for their size and strength. As the vermiculite is an insulator it does not store heat, making it useful for large moulds without the requirement for long cooling times.

It is a wet moulding process, so the cast needs to be waterproof. In this example a body cast is going to be used as the form for the mould. The first thing to do is to calculate the volume of material required. For an irregular form such as a torso, I measure the surface distance of the largest part of the mould - the hips in a torso - and the length. This gives an approximation of the surface area of the mould. Then the depth of the mould material needs to be added. All these measurement should be taken in centimetres. The longer the mould and the fewer curved forms, the greater the depth of the mould material needs to be.

In the case of this tall cast, the height of the cast is 96cm and the distance from one side around to the other side of the hips is 45cm. This is an area of 4320 square cms. The depth of the cast was decided to be 5 cm. So the volume of material required is 21,600 cc, or 21.6 litres. This shows the advantage of using the metric system as fewer calculations are required to obtain volumes than when using the imperial measuring system.

Another bit of versatility of vermiculite moulds is that you can make them hard or soft. The softer ones are easier to carve and shape subsequent to the casting of the mould, but are easier to damage.

A soft mould will use a ratio of 6 parts vermiculite by volume to 1 part of cement fondu. A hard mould will use a ratio of 3 parts vermiculite to 1 part of cement fondu. You can of course vary your proportions as you wish within these limits. Any more cement fondu than 1:3 and you are both increasing the cost and the heat retention. Any less than 1:6 and there is the danger that the mould will crack easily and fall apart.

You can incorporate some re-enforcement within the mould if you like, such as chicken wire.

Measure out the vermiculite and cement fondu and mix them when dry. The first picture shows the beginning of the mixing.

The picture below shows the two ingredients fully mixed

You then mix in the water to give a stiff mix. For a hard mould start with water of about 3/8 the volume of the dry mix. For a soft mould start with about half the volume of water to the volume of the dry mix. You will need to test the stiffness of the mix before filling the cast. It should be easy to make a ball in two hands that will stick together, but when pulled at, breaks apart cleanly.

If you can squeeze water out of the ball, then the mixture is too wet. In this case you need to make up another portion of the dry mix and then incorporate that with the already wet mix.

The cast needs a separator to keep the vermiculite and cement fondu grabbing it. Vaseline works well, as the mould mix is a wet process. The separator should be spread liberally and evenly over the mould taking area.

You pack the cast by pressing the wet mix firmly down into/onto the cast to get good compaction and conformation to the surface without surface pitting. Build the mould material up in thin layers to ensure even coverage.

The packed cast should be covered to keep the whole damp. Leave this for at least one day. The mould and its cast should be air dried for at least one more day before taking the cast from the mould.

By this time, if you are careful, the mould and cast can stand while air drying.

Once the mould has air dried for a while, you can take the cast off the mould. It is at this point that any gross defects to the mould can be repaired. Any protrusions can be taken away with a coarse or open rasp or other wood working tool. If there are areas to be filled, a small amount of the vermiculite and cement fondu mix can be added. The mould needs to be wetted and the repaired area covered for a day, and then air dried for another day.

The mould should be kiln cured to set the cement fondu. This needs to be done at at least 540C.

To avoid cracking the mould, you need to soak the mould at 90C for a time - dependant on the size of the mould - before taking it up to the top temperature. I normally use 720C, which is beyond the slumping temperatures for any glass that I may be using in the future. Once the top temperature is achieved the kiln can be turned off and allowed to cool at its own rate.
This curing process is smelly and so should be done in a well ventilated area, or overnight so the smells do not become a nuisance.
This is the cured mould, which is much lighter with the free and chemical waters evaporated.

After firing to cure, the detailed work can be done on the dry mould. This requires finer grained tools than were required for the un-cured mould. When the surface is as desired, the mould can be batt washed. The mix of batt wash that I use is twice as thick as used for shelves to reduce the amount of water that needs to be evaporated from the mould. The drying should occur at 90C until no moisture comes from the kiln ports.

Thursday, 25 February 2010

Body Casting with Plaster of Paris Bandages

The advantage of plaster of Paris bandages to create a body cast are:
· its light weight,
· rapid setting characteristics,
· the degree of detail picked up, and
· rigidity.

Its disadvantages are
· its easy deformation while wet,
· its fragility when dry, and
· it is a messy procedure.

The advantages over other methods of taking a cast from life are such that this is an almost ideal casting medium.


There are some preliminaries that need to be completed before starting. You need to make sure your model knows what will be happening and is comfortable with it. You need to empower the model so she/he can call a halt to the casting if they are uncomfortable with anything in the process.

Make sure the area is warm for the model as the person will be standing without clothing for at least half an hour. To ensure the model is not chilled, use warm water to soak the plaster of Paris bandages. Top this up with hot water as necessary.

Explain the process to the model. If you have some pictures or examples of previous work this helps the model visualise what will happen and what the result will be. There needs to be some rapport between you two and the model needs to know what to expect because some parts of taking the casting are very intimate.

Provide supports as the model will have to maintain pose for up to 30 minutes. This can be as simple as straps to support the arms, or a table to lean on.

You should provide showering or bathing facilities for the model to enable the cleaning of the remaining oils and Vaseline, as well as the drips of plaster of Paris that will be on the skin outside the casting area. This will help warm the model up again, if the temperature has not been warm enough.

You should consider what clothing you wear too. The plaster of Paris does not come out of cloth at all well. So what you use should be of a nature that can be thrown away or reserved for future casting projects. I have found bare feet provides the most secure footing.

You need some waterproof floor covering to reduce the amount of clean up after casting. If the waterproof floor covering has some texture that is better than smooth plastic sheet. The floor covering becomes very slippery when wet.

Taking the Cast

You will of course have a good idea of the pose you want for the cast. But you must try it out with the model to ensure you really like the pose and make any alterations. You also need to determine if the model can hold the pose for the required amount of time. Then the various props or supports can be put in place for the model. Most of this can be done clothed.

The model needs a separator between the skin and and the plaster of Paris/modrock. Olive oil has staying properties that baby oil does not and this is what I use now. Ensure that it is applied liberally – so don’t use extra virgin.

Olive oil is fine for arms, and torso of female models. Male models may have significant amounts of hair on their arms and chests. This will be difficult to separate from the cast, so you really need to find male models with little hair. Where there are significant amounts of hair, Vaseline is the best I have found. It needs to be put on very liberally, providing the surface on which the plaster of Paris is placed. So, enormous amounts of Vaseline need to be placed on pubic, armpit, and chest hairs. It should coat the area providing a smooth surface with few hairs poking through.

I obtain plaster of Paris bandages from hospital suppliers as they are cheaper than from art stores, although you have to buy in larger amounts. Cut the bandages into ca. 300mm lengths for ease of application. Longer strips tend to fall off the skin due to the weight of the unattached piece. For detailed areas or places where there are creases, shorter pieces are needed. These are usually available as off cuts from the end of the roll when you are cutting the longer strips. If not, you need to cut some shorter strips in preparation, as once you start you need to work rapidly as you will not have time to pause to cut more strips or make them smaller.

Mark the limits of the cast in oil pastels on top of the oils. This provides a guide on the limits of where the plaster of Paris bandages need to go. This comes off the skin easily and additionally leaves a faint mark on the cast for future reference on where the cast should be filled to replicate the pose you decided upon.

Lay strips one at a time and overlapping from the top to the bottom of the area. Doing it in reverse makes it difficult to keep the bandages against the skin. Also place the beginning strips so as to form a hook or hanger to keep the cast on.

Ensure you are overlapping the strips by laying them in different directions, even after the area seems completely covered. Also make sure you have extra thickness at the edges of the cast to assist in taking it off. Three layers of plaster of Paris on the main areas is usually enough, but 5 or 6 at the edges is a good idea.

You need to work systematically as once the plaster of Paris has cured, you cannot attach further strips. It takes only about 5 – 8 minutes for the strips to cure and become stiff.
Work smoothly and rapidly to minimise the length of time the model has to maintain the pose.
Even though you need to work rapidly and systematically, you must not go so fast that you fail to eliminate bubbles, or adequately cover all the area with several layers. You should ensure that you smooth the plaster of Paris through the bandage weave to the skin. If you watch what you are doing you can catch the bubbles that often form under the bandage. Smooth the bubble to the edge of the bandage where the air can be gently forced out.

Another care needs to be taken to avoid pressing too hard on the wet plaster of Paris bandages, as you will leave dents that will not be pushed back out by the skin of the model. It always is a fine balance between too little and too much pressure.

As you cannot see whether you have enough layers on after you have laid down the first, you need to work in a systematic fashion, such as alternate diagonals or horizontally and vertically. This together with overlapping each bandage, helps to ensure you have enough layers.

When you have finished applying the strips over the area, you can begin the clean up while the lower part of the cast sets.

Taking the cast off is a delicate process. Even on an apparently hairless skin, there are many small fine hairs that stick to the cast. This resists the removal of the cast. The areas where Vaseline had to be applied will be even more “sticky”. First break the seal between the skin and the edge of the cast all around. Then lift the cast from the skin. If you are doing a front or back torso, lift the cast upwards as this is more comfortable for the model. If possible, get the model to help in this process.

Although the top of the cast sounds firm when tapping it, it still is delicate. You have to be careful when handling the cast while it is wet. You can place the cast on a soft surface with the hollow down, so the form is not crushed or you can hang it from a string round the cast.

These two pictures illustrate the finished and dried plaster cast, inside and outside.

Even when completely dry, the plaster of Paris cast is fragile. It needs some protection. A simple protection is oil based varnish – not the flexible ones like polyurethane. This also provides some additional strength. It protects against water based mould materials too.

Following these procedures will provide you with a one-use life cast with lots of detail captured.

Saturday, 20 February 2010

Strip Cutting

Cutting strips is repetitive, but requires accuracy. This can be achieved with expensive tools that do the job very well. It can also be done with only a few tools – most of which you already have.

This photo shows some of them.

The adjustable try square is really useful, as once it is set, you can be sure all the distances will be the same. The distance should be the width you require plus half the width of your cutter head – usually 2mm.
You put the nails into the bench or a board along a straight edge as measured by the try square. Check them both after hammering them in.

Place the glass edge along the edge of the board/bench. Use the normal cutting square to place against the nails. Then draw the cutter along the straight edge.

Move the glass out to the edge of the bench and break the strip off.

The accuracy of this method is dependent on the way you line up the glass along the edge of the cutting surface.