Modelling with plaster

What is gypsum?

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

Gypsum is a very soft mineral with a Mohs hardness of about 2. To produce gypsum as a building and sculpture material, it is finely ground into a raw gypsum and then "fired". During this process, part of the water of crystallisation (or hydrate water) escapes and a hemihydrate (or hemihydrate) is formed. At around 110°C, fired gypsum (alpha-hemihydrate gypsum) is formed, and at 130°C to 160°C, stucco gypsum (beta-hemihydrate gypsum). Due to the different firing 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 greater density and thus also a higher bulk density. Accordingly, less water is required for mixing, which means that this gypsum has a significantly higher strength - up to about three times that of beta hemihydrate gypsum. In addition, alpha-hemihydrate gypsum hardens very quickly. Beta-hemihydrate gypsum has a porous structure and thus a high water absorption capacity. With a mixture of both types of gypsum, the hardness and the processing properties of a gypsum can be adjusted very precisely, depending on the proportions.

Besides in 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 e.g. in the building materials industry or further processed for other uses. Gypsum therefore does not have to be produced separately.

Natural gypsum contains no additives, which means it is ecologically harmless, nontoxic and not corrosive. Such a natural plaster is, for example, the Alabaster model plaster from KRONE. It is very finely ground and has a high whiteness.

How is plaster processed?

To make a plaster mass for casting or modelling, the making water is first poured into a suitable container.

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

Then allow the plaster to briefly sump in (1 – 3 minutes, depending on the type of plaster), then stir vigorously, preferably again by hand or with a very slow electrically operated stirrer: As few air bubbles as possible should be stirred in, and at the same time a uniform mass without lumps should be produced.

After mixing, the plaster mass should be processed immediately, as the plaster can set very quickly depending on the type of plaster. For pouring, the mass is mixed with more water. More detailed information on quantity ratios and setting times can be found with 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 an Armour , i.e. a basic framework around which you create your plaster figure. The framework should be made of preferably stainless metal, e.g. of interconnected rods. For larger sculptures it is also helpful to work with wire mesh and thus already create the basic shape.

 For the first plaster layers, you can then use, for example, plaster bandages or jute strips, which you dip into liquid plaster. This way you create a base to then apply further layers of plaster evenwithout 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 work on the mould very easily after it has hardened, e.g. with rasps and files.

Modelling with plaster: carving from a cast block

With plaster you can also work with abstracting techniques such as a stone sculptor: To do this, first cast a block of plaster in a square or even cylindrical mould. If the plaster block is not yet dry, you can carefully work it with a hatchet or carving iron and whip. Once the block has dried through, it can be shaped and worked very well with rasps, files and sanding grids. With a saw you can also separate the plaster block.