Plaster & Reinforcement, Modroc

Plaster & Reinforcement, Modroc

This page is about Plaster, check out the main project index for the rest.

Plaster is a very basic moulding and casting product. It is derived from Gypsum which comes out of the ground, and supplied as a powder which is mixed with water to set. This article explains why you may wish to use plaster for some applications.

In some form, plaster is probably what lines the walls of your house and has been used to make decorative casts for many years. It is one of the most basic compounds that starts are liquid and sets to a solid. Plaster is a water based casting compound, although safety precautions still need to be observed. Plaster will get hot as it sets due to the chemical reaction that takes place so you should never encase body parts directly in liquid plaster. The only exception to this is modroc – this is plaster impregnated bandage, which contains less plaster and has been used in hospitals for years to set broken bones.

Pictured above are a bucket of Herculite 2 casting plaster, some Modroc, and Some Jute Scrim. In the UK, you will typically find British Gypsum products such as Herculite 2 or Crystacal R plaster, which most suppliers carry. These are mixed with water at around one part water to three parts plaster by weight, although the two vary slightly. In the US you will typically find Hydrocal and Ultracal from American Gypsum which of a similar hardness respectively. Ultracal is in fact a Gypsum cement, which is Crystacal R is not however. There are also harder plasters such as the Alpha range.

Plaster is good beginner material for casting, and you ca also continue to use it for test casts once you have progressed to other resins. Plaster can be brittle however in thin sections, so reinforcement can be used in the form of a scrim fabric typically made from Hessian. Plaster casts are typically also quite heavy and pretty much always need to be quite thick to have any strength.

Although I have cast actual pieces in plaster, it is typically used for rigid mother moulds, or for casting latex pieces. This is because plaster is porous so it can soak up the moisture from the latex to make it skin up inside the mould. Here are some pictures of some plaster moulds for latex pieces that I made for my Alien project:

These moulds were made by pouring liquid plaster over a sculpt. The plaster was reinforced with Modroc and then Jute scrim with more plaster.

Modroc is bandage which is impregnated with plaster powder. You soak the modroc in water and apply it to the item, it will stick to plaster itself so this is a good way to reinforce the mould.

Modroc can also be used to back up Alginate in life casting applications, due to the fast set time of both materials.