Wednesday, 8 October 2014

#572 The studio in Canada: making the "First Season Promise" mold

This summer and early fall while at the island studio on
Lake of the Woods in Canada, I created a sculpture
entitled "First Season Promise".   The focus of this
blog is making a rubber mold of the sculpture.

 For more information about creating
"First Season Promise",  please go to
posts # 562 and # 563, posted on September 3
and September 7, 2014 while at the island studio in Ontario.

Below, is are images of the completed clay model of "First Season Promise".


Before the sculpture can be cast in bronze, a rubber mold must be made of the original clay model.
After the mold is made, the original clay model is typically dismantled and clay is returned to the clay box for future use. 

To make a mold of a clay model sculpture for the foundry to use in the lost wax process of casting in bronze,
most mold makers employ recent technology and use silicone rubber.  We use Polytek and typically buy
5 gallons at a time . . . although it is available in a 1 gallon container.  This product does not require
a release agent and can be thickened to desired consistency with TinThix TenSil liquid additive.

Below, are images of Polytek TilSil 80-30 RTV Silcone Rubber, parts A and B.
The small white container is 70 series TinThix TenSil liquid thickener additive.

Trish is an excellent mold maker and has made my molds for many years.
Below, she carefully measures part A and part B and applies the first coat of rubber -  called the "print coat" -  to the clay model. The first coat is thinner than the other four coats, has no thickener in it, and picks up all of the surface detail.

It takes approximately 2 to 6 hours for each coat to set up . . . this cure time is dependent upon temperature and humidity.  Note the oval clay "dam" she has made to keep the thin rubber contained while it cures.

Show below,  the first coat of thin rubber has cured and set up.

Thickener is carefully measured and added to the second coat and the second coat is applied.
Each coat requires a new brush as the rubber sets up in the brush in use and must be discarded.
We use natural bristle 1" industrial grade chip brushes available at Harbor Freight.

Below, the second coat has been applied and has cured . . . it is now ready to be shimmed.

Before the third coat of rubber can be applied, the piece must be shimmed.
Shown below, is the shimming process and the application of the thick shim coat.
Shimming is necessary to part or separate both sides of the mold after wax has been
poured into the mold by the foundry.  After the wax hardens in the mold, the mold is opened and the wax replica of the sculpture is removed.  The casting process then proceeds.
Note, the "seam-marks" where the mold parted will be taken off of the wax replica by the foundry.

Below, waxed paper cups are carefully trimmed to follow the contour or edge where the mold will open or part.
Note: Be sure to use waxed-coated paper cups . . . the rubber will not release from unwaxed cups.

Below, clear plastic "keys" will hold the two sides firmly together while wax is poured into
the  mold at the foundry . . . the keys are taped and held in place by the third coat of rubber.

The wax replica of the artist's clay model is so exact that it can include the artist's fingerprints!

Below, the thick shim coat - or third coat - is being applied.

 Shown below all five coats have now been applied and the mold is ready to be taken off the island and go back to the Lander studio for a plaster jacket to be made . . .
the jacket will hold the rubber mold firmly in place while wax is poured into it.

Note: We do not make plaster jackets at the Canada island studio due to transporting weight, the mess, and
mainly because the algae bloom in the lake water keeps the plaster from setting up and hardening properly.

Making the plaster jacket will be posted in a future blog.
We will also follow the sculpture throughout the entire bronze casting foundry process in upcoming posts.

Molding and sculpture supplies are available at Sculpture Depot in Loveland, Colorado.


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Blog, text, photos, drawings, and sculpture . . . © Sandy Scott and Trish


  1. Thanks for this post Sandy and Trish, very interesting. looking forward to following along on First Season Promises journey. My first sculpture is at the foundry being cast, very excited, I have butterflies fluttering around in my belly! seven more sculptures awaiting their turn, cant wait to meet them in bronze. Its been a long journey and I'm still experimenting with clays. But very exciting. Take care. Kind regards Elizabeth from Australia

  2. Elizabeth, Thank you for your interest and your comment. Hope all goes well for you . . . please stay in touch. Upcoming posts will include more about the foundry adventure and casting in bronze.