How-To: Make cheap castable silicone from caulk


Sean Michael Ragan of Make: Online says:

Simple, elegant garage casting hack here from Instructables user mikey77, who calls the stuff "Oogoo," which is, I think, a portmanteau of "Oobleck" and "Sugru."

If you've ever bought casting silicone, you know it can be pretty expensive. If, seeking a cost-saving alternative, you've ever experimented with making castings using the dirt-cheap silicone caulk they sell at the hardware store, you know that it doesn't work very well. Large volumes set up very slowly, if at all.

Turns out, though, that mixing in some cornstarch accelerates the drying process; just how much depends on how much starch you add. The author recommends starting with a 1:1 mix. I'm not sure about his explanation that the process works because the hygroscopic starch carries moisture into the internal volume of the silicone, but in any case there are probably other additives that will accelerate the process as well as or better than cornstarch. You might even find one that's not opaque and allows for translucent castings.

How-To: Make cheap castable silicone from caulk


  1. Obvious question: Would this be suitable for d.i.y. sex toys, or could I end up in the hospital in some unforeseen embarrassing way?

      1. When in doubt, it’s usually best not to put the caulk in your mouth. At least, that’s how I was raised. :D

    1. The trick is to make it non-porous, or water (among other fluids) will accumulate and bacteria will grow in the cracks. From the looks of the funnel, it wouldn’t be. Maybe if you played around with settling it very nicely in a low friction mold? Or different additives / temperatures might help.

    2. Don’t take this as a straight up (heh) endorsement, but “Standard silicone caulk is generally considered safe to use on countertops or other food-preparation areas.” Bathroom caulk however, apparently has anti-bacterial stuff in it to prevent mold. If you did make anything out of it, you’d be advised to wait at least a week (and perhaps bake it at 200F) to get rid of any remaining solvents in the silicone caulk. That’s assuming those solvents aren’t flammable.

    3. To make sex toys, you simply make toy from this mixture and when it cures just cover it with a layer of pure caulk. Caulk has high surface tension and the cured surface would be bumpy but super smooth. You can smooth uncured silicone caulk with fingers wet in soapy water.

    4. As Steve Martin’s carnie girlfriend said in The Jerk (in reference to a corn-dog, no less) “put a rubber on it!”

  2. using an appropriate silicone is the deciding factor there macho, some of them are chemically treated for weather applications. You’d need a caulk that is 100% silicon. Cornstarch (while odd) wont hurt you any.

    More concerned by your molds…

  3. Awesome – pure awesomeness. I’m a science teacher, and I’ve wanted to use to this for a long time, but could never afford casting silicon. We are so making customized tin soldiers in earth science this year…

  4. The extra surface area of the corn starch may also catalyze the reaction (or the hydroxl groups of sugar molecules may react similarly to H20 with the benefit of being bound to a solid substrate).

  5. There must be something to do wit starch as an additive. The great wall of china had rice starch added to the mortar which vastly increased the durability of the mortar.

  6. Given the rather large amount of catalyst (1:1?) being used, how strong is the stuff? That said I might have to try it out just to see…

  7. oh man, macho, that is an excellent question. Most dildos I’ve looked at are always too expensive, this could be a great thing if it could work. Anon in #2 stated it has to be 100% silicone, the mold would be an issue (roughness, “flash” you can see on toy soldier bases), and the porousness mention by Anon in #9. Maybe you could first go in with a brush and do just a thin coat in your mold that’s well spread on the surface, then go back later and fill it. Maybe that would help prevent air pockets, but I don’t know. It would be nice to know if someone knows if it’s feasible and safe.

    1. the mold would be an issue

      The way I’ve heard of this being done in the past is to make a plaster of Paris cast of a commercial toy (or I suppose you could also use a vegetable, real life phallus, etc.), and then use that as the mold in which to pour your silicone. Would that resolve the concerns?

      I’d think you’d (I’d?) want something more solid than most silicone caulk, but maybe it comes in more varieties than I’m aware of.

      I imagine you could also just use it in combination with a condom to resolve all of these concerns, but maybe that defeats the appeal in the first place.

  8. I love how most of the comments are about discussing how to use the silicone material for sex toys. Oh internet (:

  9. Come on, folks. I can’t believe there aren’t already some reasonably-priced make-your-own sex-toy mold kits out there. In fact, I’m confident there are. What you’re paying for is the confidence that the silicone provided is pure and appropriate for your needs, and that the instructions for use are comprehensive and reliable.

    Sounds to me like the best use for this cheap castable silicone is for ad-hoc messing around projects, like quick-and-dirty alpha-test prototyping, where eventual degradation of the material isn’t a buzzkill.

    It is really cool, though. Make your own M.U.S.C.L.E. men!

  10. I’ve found food-grade silicone at aquarium stores – if it doesn’t harm fish living in water that touches it 24 hrs/day, it shouldn’t harm you by eating food that’s touched it for shorter times, or by relatively brief contact with mucous membranes. It’s not much more expensive than the hardware store stuff – might even be the same price if it didn’t have the higher markups that tend to go with being sold at a smaller shop.

    All the silicone sold at my local hardware stores is not rated for food – it generally has some sort of antifungal agents to slow mildew.

  11. I second the advice about using aquarium silicone if you’re going to be doing anything intimate with your silicone. Dragonfrog is correct that just about anything from the hardware store will have non-food-grade nastiness.

    Even the silicone sealant that’s sold for kitchen use is labeled as being inappropriate for surfaces that will come into contact with food (which seems a bit silly, but hey).

    Anyway, read the warning labels very carefully. The mold inhibitor in silicone sealant is often arsenic, although I think there are probably lots of other irritants in there that you’d need to watch out for.

  12. The most awesome benefits of this DIY Sugru are:

    (1) Sugru has a shelf life of just 6 months. So…by the time it sits on their shelves; gets shipped overseas to you; then sits on your workbench for a few weeks, it’s…time to toss it.

    (2) The stuff is only available abroad right now. Who wants to pay and wait for overseas shipping?.

    Far smarter to make it closer to when and where you’ll use it.

    Nice hack.

  13. for halloween?

    using the wrong stuff on bare skin burns I know when I tried this without the corn starch and it stuck to everything inc me.

    pure silicone would be totally food safe or food safe possible I would think since I have oven mitts cookie sheets muffin tins and ice cube trays made out of the stuff.

    as for the obligatory dildo reference here is a NSFW link although I would expect animal anatomy (while not in use)would fall under simple biology LOL

  14. do not put this stuff on your skin! it may not burn you but it is an adhesive and will take quite some time and scrubbing yourself raw to get it off. I believe it’s only food safe after the solvent has evaporated and that may take more than the recommended drying time in thick applications.

  15. could you use it to fill a bicycle tire, to make it solid, instead of buying a solid bike tire which run about $40 a piece

  16. Add 3 to 4 drops of glycerin per ounce of silicone, stir it up, ad a drop of acrylic paint, continue stirring, will assure a rapid and even cure throughout the thickness of the silicone. Cornstarch is mentioned because it is hygroscopic (holds water) which the silicone needs to cure. A thick pour of silicone on its own will develop a cured skin on the outside and leave liquid silicone on the inside. If you get the mix right, you can cast items several inches thick, and get a perfect cure throughout.

    Using 100% pure silicone (no additives) is perfectly safe around food and the body. Silicone is chemically inert, IE: it absorbs nothing, and it leeches nothing, including out-gassing.

    An excellent article on casting with silicone caulks can be found at:

    Cheers, Doug

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