How do I use Plaster?

A good way to judge how much plaster you might need is to imagine how much plaster powder it will take to fill your mould then add half that amount again. Once you have your dry plaster in a good clean, rubber, mixing bowl, add cool (not cold) water until it almost covers the plaster powder. This usually works out at about 2 parts plaster to 1 part water. The rubber bowl allows us to break up the plaster once it has set so we can empty it in the bin (not down the sink as it will block the drains quickly.)

At this point you can use your hands or a mixing stick to mix the plaster if you only need a little. You can also use a mechanical whisk, which is quicker and allows you to mix more plaster at once, ideal for larger castings or if you are short of time.

Once your mixture is complete you may pour it into your mould. As always, when pouring any material into a mould, pour from the lowest point, up. If you have mixed you plaster with a mechanical whisk, you are likely to have quite a lot of air in the mixture. To remove these bubbles form the casting we must tap and slap the sides of the mould with both hands. This dislodges the bubbles from the castings surface and encourages them to rise and pop.

As the plaster sets over the next 20-30 minutes, you will notice an increase in heat until it is completely set.

Tip: A good way to tell when you can remove the casting from the mould is to press a finger on the surface and drag it towards you if you see water appear at the surface then disappear, it will need a little longer to set fully.

How do I use Mod-roc (plaster bandage)?

Before you start it is important to check that the mod-roc has not been contaminated with water previous to use, or your layers will not bond together properly. This will compromise the integrity of the structure and will not pick up enough detail from your form to be moulded.

If it has been contaminated, you can feel small hard beads of set plaster on the fabric of the mod roc. However, most suppliers seal their mod-roc in airtight packets to prevent this problem.

First, you must prepare your subject to be moulded. You must apply a good amount of release agent such as petroleum jelly, or similar to prevent the mod-roc sticking to the subject. This can especially be a problem if you are working on a live subject!

Next, cut the mod-roc into strips about 15-30 cm long. Remember you will need at least 3 layers of mod-roc, so prepare plenty of strips.

With a bowl of clean warm water, totally submerge the mod-roc in the water for around 5 seconds handling the strips gently, wipe off excess water with the back of your hand and lay over your subject surface. Smooth each strip down with your fingers rubbing the surface over several times until the gaps in fabric have disappeared. As you wet and add each layer smooth down until it blends with the one underneath. Build up the layers using the same method until desired thickness is achieved.

The thickness needed will depend on what it is you are doing, so always check with your technician how thick it needs to be to suit your purpose.

Alginate, mod-roc, and plaster are available from:

Alec Tiranti sculpting supplies

Prices vary:

£8.00 for 500g of alginate

£10.00 for 25kg and plaster

£2.00 per roll of mod-roc

Plaster & Mod - Roc

     Dry state Mod-roc

Single part open mould-making

What is Plaster?

Plaster is a generic term used to indicate that the material is plaster based. There are many forms plaster can take, ranging from use in interior decoration features and smoothing walls to plaster bandage strips used in hospitals to help set broken bones.

The plaster we use for casting is called ‘Fine casting plaster’. This is a particularly fine grade of plaster that produces fine detail casts.

Though there are stronger versions of this casting plaster such as ‘Herculite’ that is readily available. This extra strength plaster is best used for thinner profile castings and large or load bearing castings.

Like many other compounds, plaster has compatible agents that can be added to the mix to chemically slow down (retard) the curing process allowing the user more working time with the material. Also, the same rule that applies to most thermosetting polymers (like resins), the speed at which plaster sets can be accelerated by introducing heat (mixing it with hot water) and retarded using cold water.

Plaster castings can also be stuck together using wet plaster allowing us to make large sculptural, complex forms such as the human form. However, both contact surfaces of set plaster must also be wet or it will not stick properly.

Fine casting plaster is also very good for carving once it is fully dry using: saw, chisel, knife, surform (like a cheese grater on a stick) and sandpaper.

 Plaster casting taken from an alginate mould
 Mulit-part casting taken from several alginate/ 
      mod-roc moulds, assembled post-casting
 Plaster casting taken from an alginate bowl
Exploring positive and negative space casting
      Plaster mixed with iron powder, rusts
 Combining 3 plaster casting’s in plaster slab
     Triple hand family plaster casting
   Sculpted plaster base with life cast
 Plaster cast from a 2 part mod-roc mould
   Full head casting
  Plaster body cast