Shrinking Metal

I’ll be conducting some casting experiments this fall. I’ll be looking at shrinking patterning on bronze and aluminum cast eggs. I chose the egg shape for a little more geometric variety than a sphere. We are pouring bronze, aluminum, and differing percentages of the metals to create different patterns. I have two more versions to make; a 75/25 mix heavy in aluminum and one with strategically place aluminum scrap to create specific tearing.

While I am not able to prove anything scientifically, both shrinkage experiments for the bronze went about half way through the egg shape. I don’t know if there is a physics reason for not going through the entire piece.

100% Aluminum. Notice how the shrinking of the aluminum happens close to the surface.
100% Aluminum. Notice how the shrinking of the aluminum happens close to the surface.
100% Bronze. The shrinking is more focused and has texture because the aluminum tears as it cools. The interior texture was much rougher (like a geode) than the piece with 65/35 as seen below.
100% Bronze. The shrinking is more focused and has texture because the aluminum tears as it cools. The interior texture was much rougher (like a geode) than the piece with 65/35 as seen below.
65% Bronze, 35% Aluminum. I have my doubts that this piece had the original amount of aluminum (the weight was very close to the solid aluminum piece). Scrap aluminum was placed in the egg form before casting. I think a lot of it went out the vents when melted by the bronze. The reddish patina on the interior may be from the quick cooling in a water bath immediately after the piece was poured. Reproducing the red color will be another experiment. Gating was different for this piece, so the shrinkage happened on the top of the egg.
65% Bronze, 35% Aluminum. I have my doubts that this piece had the original amount of aluminum (the weight was very close to the solid aluminum piece). Scrap aluminum was placed in the egg form before casting. I think a lot of it went out the vents when melted by the bronze. The reddish patina on the interior may be from the quick cooling in a water bath immediately after the piece was poured. Reproducing the red color will be another experiment. Gating was different for this piece, so the shrinkage happened on the top of the egg.