The recipe for rocket sauce, an inexpensive, effective release for silicone molds

I made a big, big mistake the first time I made a complex two-part silicone block mold. I used an inadequate mold release. When you pour fresh silicone on cured silicone without properly applying mold release to the cured silicone, you end up with an even bigger block of silicone, practically inseparable, with your original sculpture, fossil or whatever trapped in the middle.

I thought what I did with that block mold was sufficient. I finished one side, turned it over, then used the parfilm release I had, a spray that is fine for making castings but not adequate to prevent silicone from adhering to silicone. What I should have done, and what I did do for the next several molds I made, was rub on a thin layer of petroleum jelly (Vaseline).

When I went to work for the paleo lab, I discovered "rocket sauce". Other shops call it "slime" and probably a few other names. It's a simple combination of petroleum jelly and mineral spirits or naphtha, easy to make, easy to apply. Nowadays I generally use it only on silicone, but it's also useful to rub a little, very little, into particularly tight areas on molds and even onto the surface of some fossils.

Mold makers and casters make note: Rocket sauce is only for silicone rubber molds. Do not use it on latex molds, ever. Petroleum jelly breaks down latex, which damages the mold.

The ingredients: Petroleum jelly (generic brands are fine),
mineral spirits or naphtha, and a sturdy jar.

Put petroleum jelly into the jar, followed by mineral spirits
or naphtha. The higher the ratio of liquid to petroleum jelly,
the thicker the rocket sauce will be. I like it to be kind of thin,
so I've made this batch at about a 50-50 ratio by volume.
The mixture is going to be heated, so don't fill it to the top
or it may spill over when it expands.

Place the mixture on a warming surface, while the surface is still cold.
 I'm using a hot plate that can be set at low heat. It also works
well to put the jar into a small pot of water, which can then be placed on
a gas flame. Make sure the jar's lid is off so that pressure cannot build up.
Turn the heating element to its lowest setting to start out; you can turn
it up later if the mix takes too long to liquefy (say, more than half am
hour). Setting the temperature too high could break the glass, as could flame.
 

Petroleum jelly will get thin as it warms up.

It helps to occasionally stir the mixture. I'm using a toothpick.

The mixture combines into a yellow liquid when it's hot. Turn  off
the heat but don't touch the jar until it's had time to cool.

The rocket sauce will thicken as it cools. This batch was still warm.

Always label your chemicals. I used a Sharpie on the jar.

Put on the lid after the rocket sauce mix cools to prevent the mineral spirits or naphtha from evaporating. Apply it to molds with a brush or fingertips.