Any experience making natural molds

I have a project to make a mold of the surface of a boulder and searching the web all I can find are silicone molds. Anyone know of natural alternatives, even if just single use molds?

Alginate is a possibility, depending on the size of the mold master. Plaster of Paris? Are there any undercuts? How big is the boulder and what will you be casting it out of? With the silicone, repetitive casts makes sense. Are you doing a one off or multiple casts?

Thanks Robert, I need to make a single cast of where my home's beams will sit on the boulder, then a cast of that cast which I will use to cut out of the wood. I'm wondering if a paper machė mold would work for the first cast, then plaster of Paris for the second.

Sounds like a good plan, layer of the papier mache and then plaster of paris.

Back in my model railroading days, I believe folks would use latex to make moulds of interesting rocks (brushing it on in multiple coats), not sure of the cure time.

If they are one-offs, perhaps check out Mr Chickadee's videos on how he scribes them in place, might save you a step or two and get better results?

How about making multi-part break away molds out of adobe mixtures. Perhaps you can oil them so that they last longer and pin them together with wooden dowels, knock them apart with a mallet.