Material_PLASTER -- Wahlfach Modell und Gestaltung FS20


Tobias Peteler, Hannah Kilian

Material Characteristics

While plasters based on limestone, clay and cement are available, this summary is focused solely on model-grade plaster based on gypsum.
By mixing dry gypsum powder with water an exotherm hardening reaction, called “calcining”, is initiated. During this process, the mixture undergoes a transformation from a thick slurry to a stiff but workable paste before hardening entirely into a solid. During the setting process, the cast will expand slightly, while forming a crystalline structure that is no longer water-soluble. Only by burning the plaster at high temperature, the bounded water will be released, and the powder condition can be restored. The different conditions within the crystallization process each offer an opportunity for a distinct working technique.
The real strength of plaster is, that during a single casting and rework process, all three conditions are present one after another. This provides the opportunity to apply all three techniques for one piece, making plaster a versatile model building material that can be used for a vast diversity of applications.


Alternative Names                
Plaster of Paris
Gypsum Plaster

Chemical Formula                
Ca[SO4] ·2H2O


Gets hot during curing! Hazard of burning when in direct skin contact during hardening process.

Technique A: Casting

The proportion of gypsum-powder and water differs for different kinds of plaster. To achieve plaster of average hardness, one can either measure the quantities of water and powder beforehand or apply the “island method”, in which the gypsum powder is sifted into the water at a rapid rate until dry “islands” start to form on the surface.
As a rule of thumb, to prepare 1 liter of plaster for casting, 930g of gypsum powder is mixed into 650g of water.
After a brief, undisturbed sitting time of 1-3 minutes, the suspension is mixed thoroughly with a small rod. Immediately thereafter, the mixture is poured into the casting form. Carefully shake the the formwork to reduce the amount of air bubbles trapped inside the cast.
Right after the casting process, excess plaster should be disposed into a trash container
and the mixing container should be rinsed and
cleaned with water. For details on the stripping of the formwork, refer to Technique C.
Variation: To color the plaster, the gypsum powder can by mixed with pigments, acrylic paint, or tempera color. With an increasing ratio of pigments, the color will get more vibrant. However, to not impair the setting process, the amount of manufactured oxide pigments or natural pigments should not exceed 15% and 20% of the dry plaster weight, respectively.

Tools & Materials                  
Gypsum powder, water and formwork
Clean bucket
Small stirring rod (e.g. spatula or spoon)

Hints & Tips                             
– When using separate core-elements in the formwork, make sure, that they are properly fixed to the form to prevent detachment during casting
– Short intervals of microwaving can be used to
accelerate the drying process

Technique B: Spreading

To apply the technique of spreading, the plaster needs a paste-like viscosity. To obtain this condition, only a brief window of opportunity between the mixing and the hardening is available, called “period of plasticity”. By constantly checking the viscosity, one can determine the right moment between the liquid state and before the mixture turns to brittle to be applied by spreading. In this condition the plaster can be modelled like whipped cream, making it possible to model 3D-dimensional objects without formwork. This technique can be easily applied for modifications or repair of casted objects. When spreading new plaster on already hardened plaster, the residual moisture of the casted piece will determine greatly the outcome of this technique. A plaster surface, that is already considerably dried will immediately absorb the moisture of the spread plaster. The applied plaster will harden quickly and crumble from the applied surface. To allow for a proper bounding between the casted piece and the plaster paste, the surface of the cast should be still saturated with moisture or should be additionally moistened (see Technique C).

Tools & Materials                  
Scraper (e.g. a spatula)

Hints & Tips                             
To apply plaster paste on textile, mixing 1 part plaster paste to 1 part gloss varnish or acrylic gel makes the mixture more “flexible”. This will make the mixture less prone to cracking and it will ensure that the plaster coating will adhere to the textile without any danger of falling off in the

Technique C: Carving

About 45minutes after the setting process started, the exotherm crystallization is completed and the cast will cool down. At this stage, the plaster is completely hardened and the formwork can be carefully stripped. However, the transformation process of plaster is far from finished at this state.
As excess moisture will continue to dissipate out
of the bulk for days or even weeks, the material quality of plaster will continuously change.
Especially in the first hours after stripping the formwork, the surface will contain a lot of moisture. At this stage, one can easily repair imperfections from the casting process, by spreading new plaster on the spot or remove excess plaster by carving or rasping. To maintain the workability of the plaster piece, the surface in question should be moistened on a regular basis. Sawing, drilling, and grinding should not be performed at this stage, but only after the plaster is completely dried. Otherwise, the tools might get damaged.
During the drying process, the color of the cast is slowly changing from a light gray to a pale white.

Tools & Materials                  
Scraper (e.g. a spatula)
Carving tools (e.g. scalpel or razor blade)
Spray bottle with water

Hints & Tips
Especially for larger pieces, make sure, that a sufficient gap underneath the object is established to achieve sufficient ventilation and a proper drying process

Example 1

With the help of this so-called hanging model Heinz Isler finds the perfect stable form for the open-air theatre in Grötzingen.
It is an experimental form-finding process that Heinz Isler uses as a design method. Once the shape is found by the model, it is possible to accurately measure the model, calculate the shape and scale it.
The smooth surface of this model is very appealing. There is no formwork used for production as one might expect. Instead, plaster is poured over a piece of fabric and hung up at the right moment to set. The photo on the right illustrates this hanging process, even though the material in this case is not plaster but epoxy resin.
By hanging the model, it gets a curved shape by itself. The clever thing is that the dry/hardened model is stable when it is turned over. The pressure forces are perfectly dissipated. So, without previous calculations for a perfect curvature, you can create a stable curved roofing.

Author / Modelmaker          
Heinz Isler

Project / Year                          
Open-Air Theatre Grötzingen/ 1954

Plaster and fabric

Hanging model

Example 2

The idea of this model is to make the interior space visible. The inside space becomes an independent object.
The plaster was therefore poured into a formwork.
Walls that are visible from the inside in the St. Peter’s Basilica appear as an outer shell in the
finished cast plaster model. The air inside of the St. Peter’s Basilica becomes a solid volume.
Luigi Moretti creates an abstract and conceptually strong model. It is a game of positive and negative form and of turning the inside out. In the model an inner shell of a building becomes visible, which the viewer in reality isn’t able to see.

Author / Modelmaker          
Luigi Moretti

Project / Year                          
Model of the Inner Spaces of the St. Peter’s Basilica in the Vatican / 1952



Example 3

James Turrell is an American artist who works with light and space. The model belongs to the series “autonomous structures”, in which Turrell
wants to challenge the viewer and his experiences with light. The series originated from the conversion of the Roden Crater (an extinct volcano in Arizona) into an observatory in the 1970s.
In the series, the models are designed for an interior lighting experience. Turrell designed each structure to mimic the crater’s chamber.
In this model the upper hemisphere with its soft
shadows contrasts the cube below with its hard shadows cast by its sharp edges. The only opening in the model, namely the staircase leading into the cube and up towards the cupola, creates a vertical axis of symmetry.
The strength of this model lies in the reduction of the forms. The light colour of the plaster makes the dark shadows more distinct.

Author / Modelmaker          
James Turrell

Project / Year                          
Third Day / 1989



Resources / Literature

Chaney, Charles / Skee, Stanley: Plaster Mold
And Model Making, New York 1986

Corcoran, Kayne Griffin: Autonomous Structures,, 27.05.2020

Creahan, D.: James Turrell: “Roden Crater and Autonomous Structures” at PACE Gallery 2013,, 27.05.2020

Dasilva, Samantha: Modelling Paste VS Plaster,
2014,, 29.05.2020

Ker, Anna Dorothea: James Turrell’s Autonomous Structures Challenge Perceptions of Light,, 27.05.2020

Lüdtke, Burkhard: Modell, Architektur, Design,
Berlin 2002

Schwartz, Joseph: Tragwerksentwurf IV, Isler:
einfache experimentelle Form,, 27.05.2020

Unknown: Light and Shadows, an exploration
of light and space, James Turrell’s “Autonomous
Structures” (1989-2010),, 27.05.2020

Wąsowicz, Magdalena: Inside-Out Architecture,
a game with the multistability of solid and void,, 27.05.2020

Weller, Michael W.: Form-Finding, Force and
Function: A thin shell concrete trolley barn for Seattle’s waterfront, p. 17, Proposal_9-26.pdf, 27.05.2020

Resources / Pictures

Dasilva, Samantha: Modelling Paste VS Plaster,
2014,, 29.05.2020

Gibbs, Ruth: Texturized Paste Art,, 29.05.2020

Grimm, Bernd: Model of the Tempietto di Bramante, San Pietro in Montorio, alabaster plaster, _di_Bramante.jpg, 27.05.2020

Karaçizmeli, Enise Burcu / Novak, Wolfgang:
Architecture within the Natural, 2012,, 29.05.2020

Kerez, Christian: Incidental Space Installation, Swiss Pavillon, Venice Architecture Biennale, 2016, plaster,, 27.05.2020

Le Corbusier: Chapelle Norte-Dame-du-Haut,
Ronchamp, 1950-1955, wood, paint and plaster, 36.8 x 89.9 x 65.1 cm,, 27.05.2020

Malevich, Kazimir: Architekton Gota, 1923, plaster, 85.3 x 56 x 52.5 cm,, 27.05.2020

Máthé, Dóra: B&B in Lierneux (BE) / Scale:
1:50, 2014,, 06.06.2020


Moretti, Luigi: Model of the Inner Spaces of the St. Peter’s Basilica in the Vatican, 1952, plaster, 468092955001129276/, 27.05.2020

Rizzi, Renato: Orphan Ground, Venice Architecture Biennale, 2016, 519391769515795438/, 27.05.2020

Schwartz, Joseph: Tragwerksentwurf IV, Isler:
einfache experimentelle Form,, 27.05.2020

The Art House: Plaster Carving Workshop, 2020,, 29.05.2020

Turrell, James: Third Day, Autonomous Structures, 1989, plaster,, 27.05.2020

Weller, Michael W.: Form-Finding, Force and
Function: A thin shell concrete trolley barn for Seattle’s waterfront, p. 17, Proposal_9-26.pdf, 27.05.2020

Wikipedia: Gips, 2009,, 29.05.2020

Wikipedia: Yeso industrial, 2005, Archivo:Calcium_sulfate_hemihydrate.jpg, 29.05.2020