Casting Costanza

Tutorial: Creating plaster casts from 3D Prints

By Scan The World

Costanza Bonarelli by Jonathan BeckScan The World

Casting Constanza

In this tutorial by Abbey Ellis for Scan The World we will teach you how to cast a faithful reproduction of a 3D printed object using plaster of Paris. For our tutorial, Abbey used a 3D printed bust of Costanza Bonarelli, available as open data on Scan The World.  

3D print and final plaster cast of Costanza Borelli's bustScan The World

Abbey Ellis is a 25-year-old PHD researcher at the University of Leicester’s School of Museum Studies and the Ashmolean Museum of Art and Archaeology, Oxford. Abbey’s work considers perceptions of plaster casts’ value and authenticity asking questions such as: “Can reproductions ever be considered authentic museum objects? What values do visitors associate with them?”

3D print and final plaster cast of Costanza Borelli's bustScan The World

Abbey has very precise thoughts when it comes to the digitisation of cultural artifacts: “Digitising artifacts is revolutionising the world of museums and heritage, creating amazing new tools for education and engagement. 

 However, my own work has made very clear the need to ensure that the reproductions that we are making today do not fall into the same trap as historic plaster casts. Many plaster cast collections which came into being in the objects’ eighteenth and nineteenth heyday are very poorly documented. 

(…)Making sure that we produce high quality documentation as part of the process of digitising objects and making 3D printed reproductions will mean that scholars in years to come can continue to understand and appreciate them.”

Portrait of Costanza Bonarelli, Scan the World, From the collection of: Scan The World
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For this tutorial, we have used a 3D model of a bust Scan the World digitised in collaboration with The Statens Museum for Kunst, Copenhagen. The work is a 3D scan of a plaster cast of an original marble by Gianlorenzo Bernini depicting Costanza Bonarelli, housed at the Museo Nazionale del Bargello. You can download and read more about the bust here.

Costanza Bonarelli by Jonathan BeckScan The World

What you'll need

The materials needed for this tutorial are as followed:
- Plaster from Paris (approximately 8 kg used for 40x35cm 3D print)
- Alginate (approximately 7 kg used for a 40x35cm3D print)
- A smooth plastic container big enough for the object
- Washing up liquid
- Old newspapers
- Craft knife
- Blu-tack
- Mastic (for example this one)
- Ready-mixed filler (we used this one for this tutorial)

Preparation for casting - cleaningScan The World

Step 1

After finding the right sized casting container (the object should fit perfectly without leaving too much space), Lubricate the interior of the container with washing up liquid, which will act as a release agent.

Preparation for castingScan The World

Step 2

Fill in any hollows present on the original object with old newspapers to prevent the alginate from damaging it (in our case we covered the base of Costanza’s bust).

Mixing the siliconeScan The World

Step 3

Mix up the alginate powder with water as indicated in the instructions of your product of choice. Be fast and mix vigorously as this type of material tends to set fast.

Filling the mouldScan The World

Step 4

When ready, pour the liquid alginate in the container around the object, trying to hold the object still while pouring.

After this, leave the alginate to set.

Preparing the baseScan The World

Step 5

Once set, run a craft knife around the rim of the container to start detaching it. Flip the container upside down delicately, the washing up liquid will help the mould come out smoothly.

Plaster Casting the BaseScan The World

Step 6

Carefully slice the alginate vertically in order to reveal the original object. The alginate material has now taken the shape of your object. Remove the original object and place the alginate back inside your container to act as a mould for the next steps.

Mixing the Plaster of ParisScan The World

Step 7

Mix up the plaster of Paris ready to be poured into the negative space of the mould.

Filling the mould with plasterScan The World

Step 8

When the plaster mixture is ready, it will usually have the consistency of double cream. At this point you can pour it inside the mould carefully and let it dry.

Removing bubbles from the wet plasterScan The World

Step 9

To avoid air bubbles forming, gently rock and tap the container so that any air bubbles within the wet plaster are forced into rising to the top.

Step 10

Leave the plaster to set for at least 30 minutes.

After castingScan The World

Step 11

Invert the container and carefully extract the mould. 

Attention: if the object is big, it will be very heavy at this point.

Unfinished plaster base of bustScan The World

Step 12

  Carefully peel the alginate to reveal the cast inside. This also reveals two vertical lines running around the object, the so-called “flash lines”.

Step 12

These lines indicate where the two halves of the alginate sat next to each other during the casting process, they can be easily smoothed out afterwards.

Preparing for castingScan The World

Step 13

The 3D printed bust of Costanza Bonarelli is a big object, therefore the bust was split in multiple parts and the above steps were repeated for each part of the bust.

Step 13

  When it came to casting the breast of the figure, the extremely detailed relief had to be covered with some blu-tack. Undercuts on high relief areas such as this can often cause the alginate to get stuck and tear away during the de-moulding process.

Step 13

Using the blu-tack to cover those parts prevented this from happening.

Inverse of the silicone mouldScan The World

Step 14

After casting all of the object’s parts, let them dry overnight.

Connecting two plaster castsScan The World

Step 17

If applicable, stick the parts of your plaster cast together using Mastic.

Filling holes in a castScan The World

Step 18

Finally, smooth out the plaster cast with lightweight ready-mixed filler and remove any existing cracks or bubbles that might form during the process.

3D print and final plaster castScan The World

Now your plaster cast is ready! Leave the object for a few days to ensure the plaster is dry.

Credits: Story

Thank you for following this tutorial kindly directed by Abbey Ellis. All images and text were created by Abbey.

Please share with us your creations!

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The story featured may in some cases have been created by an independent third party and may not always represent the views of the institutions, listed below, who have supplied the content.
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