I'd have to say that I never bang my molds, and I seldom see bubbles or voids unless the soap was a really thick batter when "poured". (One pine tar batch comes to mind).
The physics of bubbles rising in a fluid: The rise of a small air bubble is related to the bubble's size, the kinematic viscosity of the fluid (the soap batter), and the acceleration of gravity. Since you have no control over the bubble size and gravity, the only thing you can manipulate is the viscosity of the soap batter.
If possible, pour thinner so the viscosity of the batter in the mold is thinner to begin with.
Once the batter is in the mold, don't bang. Gently shake or vibrate the mold from side to side so the batter just jiggles a bit. It's safer than banging, but the jiggling also loosens the batter more effectively (assuming it's not super gloppy thick). Soap batter is a "thixotropic" fluid, meaning it becomes less viscous (thinner) when it is moving. Jiggling is more effective at thinning out a thixotropic fluid -- try it and see what you think.
Oh, and if you can, pour a thin layer and jiggle, pour another thin layer, jiggle, etc. The bubbles will have less distance to travel so they are more likely to get to the surface, leaving you with a bubble-free batter.
Ever wonder why ketchup "sticks" in the bottle but pours really fast once it starts to move? Thixotropy! Give the ketchup some gentle taps at the neck of the bottle. You want to jiggle the part of the ketchup that needs to flow first. This will work much better than madly banging the bottom of the bottle. My husband never remembers this -- he just gives the bottle to me.
The physics of bubbles rising in a fluid: The rise of a small air bubble is related to the bubble's size, the kinematic viscosity of the fluid (the soap batter), and the acceleration of gravity. Since you have no control over the bubble size and gravity, the only thing you can manipulate is the viscosity of the soap batter.
If possible, pour thinner so the viscosity of the batter in the mold is thinner to begin with.
Once the batter is in the mold, don't bang. Gently shake or vibrate the mold from side to side so the batter just jiggles a bit. It's safer than banging, but the jiggling also loosens the batter more effectively (assuming it's not super gloppy thick). Soap batter is a "thixotropic" fluid, meaning it becomes less viscous (thinner) when it is moving. Jiggling is more effective at thinning out a thixotropic fluid -- try it and see what you think.
Oh, and if you can, pour a thin layer and jiggle, pour another thin layer, jiggle, etc. The bubbles will have less distance to travel so they are more likely to get to the surface, leaving you with a bubble-free batter.
Ever wonder why ketchup "sticks" in the bottle but pours really fast once it starts to move? Thixotropy! Give the ketchup some gentle taps at the neck of the bottle. You want to jiggle the part of the ketchup that needs to flow first. This will work much better than madly banging the bottom of the bottle. My husband never remembers this -- he just gives the bottle to me.
Last edited:
