Plaster (calcium sulfate dihydrate) is commercially available in many different qualities. The type most commonly used for architectural model building is called Plaster of Paris, Alabaster Gips, or Modellbaugips 0 — and combines good workability with a reasonable amount of strength.
Plaster, due to its white color and homogeneity, is an ideal material to study the proportions of a building and to observe how light transitions. Furthermore, it is dimensionally stable, and only a few cheap tools are required to work with it.
VIDEO that shows the “mixing” process
Plaster transitions from a pourable liquid to a pliable solid to a dried mass throughout a couple of days. The most interesting stages happen during the first 45–60 min after mixing plaster and water.
The most practical time for pouring plaster happens right after mixing within the first few minutes. Not much can be done in this state except for pouring.
The plaster starts to get creamy 10-15 minutes after mixing and eventually reaches a toothpaste-like consistency, ideal for extruding profiles or working additively. This phase lasts for about 5 min.
After around 20 minutes, the plaster becomes rigid and very fragile. Avoid moving or vibrating the mold during this period.
The plaster gradually heats up and hardens at this stage. The exact rate depends on the mixture and the size of the casting. It is an excellent time to remove the casting from the mold when the plaster cools down.
This stage starts when the plaster cools down and lasts until completely dry. It is at this stage when most of the post-processing is done, even if the tools get clogged and need frequent cleaning.
After several days, the plaster enters the dry stage and no longer contains water. At this stage, plaster can be sanded or painted.
A few basic techniques are used when working with plaster following the different stages of the material. Each method can be combined to generate various expressive material gestures.
example: pouring directrly in clay
example: plaster over a mesh
example: profile form
example: cutting and breaking off walls