Molding Question For Harry

[Boesse]

Hey Harry,

I remember a while ago you mentioned some sort of silicone substance you used for molding. I'm still using paintable latex, and am interested in a method that takes less time. I'm not sure how more/less expensive your method is. Care to elaborate on it, for my sake and for everyone else's?

Thanks,

Bobby

[Harry Pristis]

Hey Harry,

I remember a while ago you mentioned some sort of silicone substance you used for molding. I'm still using paintable latex, and am interested in a method that takes less time. I'm not sure how more/less expensive your method is. Care to elaborate on it, for my sake and for everyone else's?

Thanks,

Bobby

You can buy the essential components for casting smaller fossils at Home Depot or Lowes.

What you want is GE Silicone Silicone I clear caulk (about $4.50 per tube).

DO NOT USE Silicone II caulk. If you don't get the vinegar odor from the exposed caulk, you've purchased the wrong stuff!

Thin the caulk with toluene for a first coat, if you wish.

Coat the fossil with a release agent. Vaseline or PAM works. Blot any excess. Plug small foramina with waxed dental floss. Plug a big hole with a wad of Saran/plastic wrap.

You can use a sand (uncooked rice) box covered with SaranWrap to hold the fossil for the first (and second, if you've applied a thin coat first) application of silicone. Work carefully to avoid creating air bubbles.

If I detect an air bubble in the uncured, translucent silicone, I perforate it with a sharp tool, then collapse the bubble with a wet finger-tip.

You can work this silicone caulk with a wet finger-tip -- smooth it, thin it, patch it. The moisture actually speeds the cure a little. It is non-toxic as long as you're not sensitive to acetic acid (the vinegar odor). Allow each coating to cure before applying another. Pay attention to the maximum thickness instructions on the caulk tube.

Spread some extra silicone in a wider margin (say .75 to 1.0 inch) along the least crucial side of the fossil. (For a fossil jaw, this is likely to be on the underside of the jaw, away from the teeth.) This margin will become the opening and sealing lips of the mold. Allow it to cure.

Coat this margin of the silicone caulk skirt with petroleum jelly or other release agent. The margin will become the opening of the mold; don't forget to apply release agent to this first lip. The part of the silicone which you do not coat with a release agent will merge with subsequent coats on the other side of this "pocket mold."

Apply one or two coats of caulk to the second side, the exposed side of the fossil and the silicone skirt. Allow to cure.

DO NOT REMOVE THE FOSSIL YET. You now have a fossil snugly coccooned in silicone rubber. It has a silicone rubber shelf that sticks out from the body of the fossil. This shelf will split open (if you've applied the release agent properly) later to remove the fossil and eventually, the casts.

Float this coccoon in a tray of wet plaster of Paris, embedded about half way. Plastic food containers from the dollar store work well. Allow the plaster to cure.

With the coccoon still in place, coat the margin (should be a margin all around the coccoon) of the plaster mother-mold with petroleum jelly or PAM.

Pour another batch of plaster of paris over the exposed side of the silicone coccoon. Allow the plaster to cure.

Pop the two halves of the plaster mother-mold apart (if they don't come apart, you've not used the petroleum jelly correctly).

Pop open the silicone pocket mold (the lips/margin you used the release agent on).

Remove the fossil gently so that you don't tear the silicone. (If the silicone tears, you can repair it with a dab of more silicone.)

Voila! You have a silicone mold and a mother mold. If you think this sounds easy, you're right. It takes time and a modicum of care. It produces a very high quality cast with just a little practice.

Casting resin?? Try five-minute epoxy. Add some dry tempera pigment for color. I sometimes use wood powder -- hardwood, NOT pine sawdust. Pine resin will inhibit the curing of the epoxy.

These are some casts I made using this method.

-------Harry Pristis

[Harry Pristis]

Here's an image of the whole deal -- cast, silicone mold, and plaster mother-mold.

The dimples and ridges on the silicone and in the plaster are register keys to seat the mold in the mother mold correctly. The dark stains on the plaster to the right and left of the silicone mold are surplus resin that was squeezed from the mold when it was closed up.

[Boesse]

Hey Harry,

Thanks a million for the (extremely) detailed directions! I will most certainly give this a shot once I'm home for the summer. I really appreciate it. Thanks man.

Bobby

[jpc]

Hey Bobby-

Or you could come to the Tate Conference. We're doing a workshop on making two-part silicone molds.

jpc

[Harry Pristis]

Please note that I have edited my post suggesting materials and methods for mold-making and casting.

Note that it is GE or Dow-Corning Silicone II caulk that is used for making molds.

[matt cable]

Great information. Thanks for the tutorial!