Blog

Sharing my creative impulses and processes through these stories the last few months has been a wonderful experience.

 

Now, though, I'd like to give you a peek behind the curtain and show you an actual "how-to" behind my work: mold making. Let's go, Toto!

 

So you can understand where mold making fits in, here's an overview of the full bronze production process: 

• The artist completes sculpture in clay (positive*)
• A mother mold is made (negative*)
• Wax is poured into the mold, creating a wax pattern (positive)
• The wax is dipped into a ceramic slurry and coated multiple times creating a shell (negative)
• The wax is melted out and hot bronze poured in
• The ceramic shell is broken off revealing the bronze (positive)
• The bronze pieces are cleaned up and welded back together into the final sculpture (really positive!)
• Patina is added to protect the bronze and add dimension and color to the final sculpture

*Special Note: “positive” is the original sculpture shape and “negative” is the inverse shape.

 

So what is a mother mold?

It is a two-part mold: a rubber/silicon layer and a plaster layer. The rubber layer captures the detail and shape of the original clay sculpture. The plaster layer provides support for the rubber layer to minimize the warping, twisting and buckling of the rubber.

 

A mother mold can be complex--based upon the sculpture being molded. The monumental sculpture that I did of Joseph Antoine Janis was complex… and BIG (nine feet tall).

 

In this photo, Big Joe is half-way through the molding process.

He’s been partitioned and the first layer of white rubber applied.

 

The mold maker had to cut Big Joe up into over 35 pieces that met the size limitations of the foundry.

 

This is Big Joe in pieces and parts. After casting him in metal, the foundry workers weld Joe back together again.

 

A good mold maker determines how the molten metal will flow during the casting process. For example, if the mold doesn’t have proper venting--air cannot escape--forming air bubbles that can obstruct the flow of metal. Pieces and parts of the sculpture will then be missing in the finished piece.

 

In the video below, I used a super simple sculpture to show you how molds are made. A simple disk…the Nebra Sky disk.

 

Nebra Sky Disks 3

 

Why are mother molds used in the bronze production process?

I sculpt in oil-based clay with an internal metal armature (an armature is kind of like a shallow skeleton that helps support the weight of the clay). This does not fare well at the foundry.

 

This is a recent work of mine after the mold was made. I got the original clay back in pieces and parts.

 

If I want to make multiple copies—an edition—of a sculpture, making a mold is the best way to do it.

 

Why? The original clay (the positive) tends to get dinged up badly during the molding process as you can see above. As a result, the mold becomes the “original” negative. It's not feasible for an artist to continue to repair the original clay because they'd come out differently each time and the molding process would have to be repeated anyway.

 

Molds don’t last forever and when the full edition (e.g. 10 nearly identical pieces from the same mold) has been produced, the mold will be destroyed by the artist. This helps to hold the value of the edition for art collectors, because no more identical pieces can be created.

 

Here are the steps I followed for making a mother mold of the Nebra Sky Disk:

Step 1   Create a clay dam around the disk to contain the silicon rubber in its liquid state.

Step 2   Mix the 2-part silicon rubber components—making a green liquid. Pour 2 layers and give them time to set.

Step 3   Create a second clay dam around the silicon rubber to contain the plaster. Pour the plaster and let it set. This is the mother mold.

Step 4   Remove clay original and place the mother mold on the floor. Since I am pouring hot wax, I keep it low to minimize spattering wax.

Step 5   Pour 190 F wax into the mold creating a wax pattern of the original. Let the wax cool and remove it from the mold.

 

The knowledge of how to make molds has been invaluable for me as an artist and I've earned the wisdom to know when I need to bring in a professional through lots of trial and error. A good mold maker is worth their weight... in bronze!

 

To view the video of the process, click the image below.