Modelling with plaster

What is plaster?

Gypsum is a mineral that occurs in nature. In geology, gypsum is also known as gypsum spar. The chemical name is calcium sulphate dihydrate. Originally, gypsum is white to colourless. Due to admixtures of other substances in nature, gypsum can also occur in a wide variety of colours. There are variations of gypsum all over the world.

Plaster is a very soft mineral with a  hardness of around 2. To produce gypsum as a building and sculpting material, it is finely ground into a raw gypsum and then "burnt". During this process, some of the water of crystallisation (or hydrate water) escapes and a hemihydrate (or hemihydrate) is formed. At around 110°C, burnt gypsum (alpha-hemihydrate gypsum) is produced, at 130°C to 160°C stucco gypsum (beta-hemihydrate gypsum) is produced. Due to the different burning conditions, alpha- and beta-hemihydrate gypsumalso differ in their properties: Alpha-hemihydrate gypsums consists of compact crystals without cracks and craters in the surface. Due to its denser structure, alpha-hemihydrate gypsum has a higher density and therefore a higher bulk density. Accordingly, less water is required for mixing, which means that this gypsum has a significantly higher strength - up to around three times that of beta-hemihydrate gypsum. In addition, alpha-hemihydrate gypsum hardens very quickly.

Beta-hemihydrate gypsum
has a porous structure and therefore a high water absorption capacity.
With a mixture of both types of gypsum, the hardness and processing properties of a gypsum can be adjusted very precisely depending on the proportions used.

In addition to nature calcium sulphate, i.e. gypsum, is also produced in industry, e.g. as a by-product in the production of various acids. Flue gas desulphurisation also produces gypsum, which, provided it is not too contaminated, can be used in the building materials industry, for example, or further processed for other uses. Gypsum therefore does not need to be produced separately.

Natural plaster contains no additives, which means it is ecologically safe, non-toxic and non-corrosive. One such natural plaster is alabaster molding plaster from KRONE. It is very finely ground and has a high degree of whiteness.

How plaster is processed

To make a plaster compound for casting or modelling, first pour the mixing water into a suitable container.

Then allow the plaster powder to trickle into the mixing water by hand in small portions. The dry plaster will sink to the bottom of the container. When enough plaster has trickled into the mixing water, small islands will form on the surface of the water.

Then briefly let the plaster soak in (1 – 3 minutes, depending on the type of plaster), then stir vigorously, preferably by hand again or with a very slow electric stirrer: As few air bubbles as possible should be stirred in, while at the same time creating a uniform mass without lumps.

After mixing, the plaster mixture must be processed immediately, as the plaster can set very quickly depending on the type of plaster. For pouring, the mixture is mixed with more water. You can find more detailed information on the proportions and setting times for the respective products.

Modelling with plaster: building a sculpture out of plaster

If, for example, you want to build a figure out of plaster it is advisable to first build a reinforcement i.e. a basic framework around which you will place your plaster figure. The framework should be made of metal that is as rust-free as possible, e.g. rods joined together. For larger sculptures, it is also helpful to work with wire mesh and thus already create the basic shape.

 For the first layers of plaster you can then use plaster bandages or jute strips that you dip in liquid plaster. In this way, you create a base to be able to apply further layers of plaster without fibre reinforcement.

The inner layers should be made of as hard plaster as possible. If the outer layers are made of asofter plaster, you can easily work on the mould after it has hardened, e.g. with rasps and files.

Modelling with plaster: carving from a cast block

You can also work with plaster using abrasive techniques such as a stone sculptor: To do this, first cast a block of plaster in a square or even cylindrical mould. If the block of plaster has not yet dried, you can carefully work it with a hatchet or carving iron and chisel. Once the block is completely dry, it can be easily shaped and finished with rasps, files and sanding grids. You can also cut the plaster block with a saw.