DIY casting silicone from caulk, pourable version

Science Technology

Our post from three weeks back about Instructables user mikey77’s “Oogoo” formulation generated some really great discussions in the comments. Now, reader Iceman086 has reported some successful experiments combining caulk, cornstarch and a solvent to make homebrew silicone with a pourable consistency. Perhaps we can call it “Ooze-oo?”

I found that when making a mould you can use a 4 to 1 ratio of paint thinner to caulk (2 oz of caulk to 1/2 oz of paint thinner) to make a small mould using the bottom of a plastic cup. I added in about 3 drops (literally drops) of paint to the mix in order to help show the details of the mould. Once I had a consistent mix of paint thinner to caulk I added in about 1 tbs of corn starch as the hardening agent. This gave me about 5 to 10 minutes worth of pot time and fully set within 45 minutes.

I gave it an hour and a half just to be sure before I mixed up some casting silicone. This time I used a ratio of 3/4 to 1 (silicone to paint thinner) in order to get a mix that was able to flow. You can go 1 to 2 (silicone to paint thinner) if you want a thinner mix that is pourable but its up to you. I added in my paint (again 3 drops but yellow this time) and mixed until the 3 parts were consistent. I then added in 1/2 tbs of corn starch to the mix as a hardener. After doing this though the mix thickened and I ended up having to spoon it out. This batch still worked how ever and I was able to get some very nice
results. The pot time was about 15 to 20 minutes with this mix and the dry time was around 2 1/2 hours.

If you want a casting consistency that is still pourable I would recommend using the same ratio of Silicone to Paint Thinner but you can reduce the amount of corn starch to 1/4 tbs. HOWEVER! Reducing the amount of corn starch will increase the set time.

In some of the posts above people talked about shrinkage. I didn’t see any noticeable shrinkage given the amount of paint thinner that I used.

His full comment also includes some interesting reports about his experiences with improvised mold releases. Well done, sir!


31 thoughts on “DIY casting silicone from caulk, pourable version

  1. cancerouspete says:

    Ratios given confusing because first it is said 4:1 thinner to caulk…but amounts given are caulk to thinner.

    So later when you say 3/4:1 caulk to thinner, we don’t know which way you are reading it as. Caulk:thinner, or thinner:caulk.

    Finally….given ratios, saying 1/4 tablespoons of starch, does not help us.

  2. Iceman086 says:

    I am sorry for the confusion in the things that I wrote above.

    All of the ratios are Caulk to Paint Thinner. The one that was 3/4:1 was to make the mix pourable. The thicker ratios of 4:1 are for making moulds. You can use something thinner like 2:1 or 1:1 and paint the mix on then use a thicker 4:1 for a more solid mould.

    Alot of the mixes can be done by feel but you have to over thin the mix slightly in order to compensate for adding in the corn starch, if you want to make it pourable. Atleast, thats what I have found.

    As for the starch that was meant to be Teaspoons (tsp).

    Again, I apologize for my errors.

    1. Sean Michael Ragan says:

      No need to apologize; thanks for clarifying.

      When you give ratios of volumes, like 2 to 1 or 4 to 1 or whatever, and then give an absolute volume of cornstarch (like 1/4 teaspoon), the missing information is what is the absolute volume of caulk/solvent involved? For example, if you say 2:1 paint thinner:caulk + 1/4 teaspoon cornstarch, how much caulk are you using? 10 milliliters? 20 milliliters? Or in oz.?

      Thanks again! This is great info, an experiment I wanted to try but probably would’ve been on the back burner until I forgot about it.

  3. Wilson! says:

    What do you mean by “paint thinner” ?? Mineral spirits? Lacquer thinner? I know some cans just say Paint Thinner, but one brand may be different from another.

    I really want to try this. I have always wanted to be able to make molds of stuff to cast in resin. The local hobby shop has resin for a reasonable price but RTV silicone molding stuff is too pricey for my taste.

    1. Sean Michael Ragan says:

      …but my guess is it won’t matter too much. When I (eventually, hopefully, someday) get around to trying this, I will probably use whatever hydrocarbon solvent I’ve got lying around: Lighter fluid or toluene or generic paint thinner. Other users have reported that xylenes are effective. Would probably veer away from acetone or alcohol or anything that might introduce water earlier than you intend. Or, that might turn out to be a good thing: Thin it with damp acetone and you might not have to use cornstarch at all, and might end up with a product that’s translucent.

  4. Iceman086 says:

    I will say thank you to everyone for their responces. I have never written any thing quite like this so I am glad for people responding and helping to correct me.

    @ Sean Michael Ragan
    I was workin in Ounces. I wasn’t sure how to put that into a raio because I used an approximation when adding in the corn starch.

    @ Wilson!
    I was using generic “Paint Thinner.” Thats what it said on the can. It was a blue and white container.

    Just in case anyone wanted to know the products that I used:
    Argo Corn Starch
    GE 100% Silicone
    Rosco Laytex Paint (Its a theaterical stage paint that is very high in pigment, though im sure that most kinds of laytex based paints would work)
    Generic Pastic Cups (mother mould)

    I casted in some Microcrystaline Wax (a medium hard wax used for casting) to see how the hot wax would respond with the silicone. I used a really thin layer of Vaseline and after pouring in the wax I stuck it in a freezer to let it all cool down. The mould was fine and the casting turned out great. I am egar to try a 2 part mould next using layered pours of the silicone mix. I will let you guys know how it turns out once I get a change to try it out.

  5. Anonymous says:

    I tried several cominations as suggested,  nothing worked or was even close to working as described.

    1. Bon says:

      Be sure to use silicone I caulk II DOES NOT work for this!

  6. Adric Menning says:

    Quelab (our makerspace), just did a big “Make a creepy thingy” class for halloween, 

    We pretty much used the original oogoo instructible.   

    First off it has to be 100% silicone caulking.  we found 2 teaspoons of conrstarch ere plenty for one tube of caulking.

    Im interested in the paint thinner option, for making it less cake frosting like and more pourable.

    you can see some of the creations here,

  7. Adric Menning says:

    Quelab (our makerspace), just did a big “Make a creepy thingy” class for halloween, 

    We pretty much used the original oogoo instructible.   

    First off it has to be 100% silicone caulking.  we found 2 teaspoons of conrstarch ere plenty for one tube of caulking.

    Im interested in the paint thinner option, for making it less cake frosting like and more pourable.

    you can see some of the creations here,

  8. Curious says:

    I tried an expreiment tonight.
    I had an old tube of 100% silicone clear caulk that would not squeeze out so I cut the tube in two to get at the unhardened material inside.
    I mixed some 1:1 with corn starch (roughly) and thought I would try an experiment at the same time. Based on the ingregients of materials like cyberskinI read about which are based on elastomer gel and mineral oil, I thought I would try adding some mineral oil. I looked around the house for baby oil and couldn’t find any but did find a bottle of paraffin oil used for oil lamps. I read the ingredients and it said it contained mineral oil.
    I mixed about equal amounts of caulk, corn starch and oil. The oil was a reddish color and the result came out a nice skin color and was very easy to mix and thin enough to use easily in a mold.
    It setup quickly and seemed to be about twice as soft as the version without the oil.
    I will see what it looks like tomorrow and experiment some more.
    Speaking of thinners, I read on another web site using Naptha as a thinner.

  9. charl says:

    Methelated spirit also works well as a thinner.

  10. Melanie says:


    I’m new to all this and I’m not sure if this is a stupid question or not but, does anyone know the melting point for this type of mixture? Like if i wanted to cast little figurines and then hot glue gun them to something else, would that be an issue?

    1. brian says:

      Silicone has no melting point, it doesn’t melt.

      1. Travis says:

        Yes. Silicone does go through a glass transition rather than melting. Many silicones list a working temperature of up to 400 deg. F. I expect it becomes too fluid beyond this point to hold form, but I’ve never done it.

        I use plaster to mold molten rubber, rather than silicone.

  11. Tom Ghoul says:

    Here (in Denmark), paint thinner is a mix of acetone, toluene/toluol and either methylisobutylketone or methylethylketone. Sounds a bit harsh, no?
    I tried the mentioned ratios on clear sil caulk. When using enough thinner (white spirits or mineral turpentine, not clear about which) to make the mix pourable, there was considerable shrinking, not to mention a several days long curing time for hardness.
    I would love to see a standardized system of naming all those volatile compounds sometime.

    1. brian says:

      Pure toluene will probably work even better than paint thinners

  12. Christopher says:

    I made this yesterday and it came out excellently well…except I *really* need to find a way for there to not be the vinegar/acetic acid smell. I’d like to make products with this, but I can’t have it smelling of vinegar. Has anyone found a way to neutralize the vinegar smell?

    I know that only silicone I will work…but what about using silicone II, or some other substance and cornstarch?

  13. sidney says:

    I have done this a couple of times and it always shrinks and curls. I think mineral spirits is not the way to go. causes shrinkage

    1. Prince says:

      I agree. I used xylene on the first molds that I made, and the shrinkage rendered them nearly unusable. I spent a lot of time sanding with a dremel to make my cast pieces fit together. I made that first mold about a year ago, and it was about 6 inches long. It is now about 5 inches long. The most shrinkage occurred within the first few weeks, but I think that it is still shrinking as the xylene deep in the structure evaporates. I have come up with a better way that works for my applications. I start by coating my model with a thin 1/8″ coat of silicone caulk to pick up the detail of my model. You could probably mix a little thinner into this coat. Once that coat has cured, I pack regular “unthinned” oogoo over the thin silicon coat to make my mold

  14. Michel NL says:

    Going to experiment with this the next few days (rather than “paint thinner” I’m going to try it with some terpentine as it has less of a foul smell and I suspect the latter may also be purer (not a mix(?)).
    I do wonder if this technique/material can be used for both the mold and the cast? (Ideally I’d like the mold to be flexible, but the cast, the end result as well. I expect I’d need to cover the mold with some grease, oil or perhaps talcon powder before pouring the cast, or do I need to take more precautions? e.g. make sure there is at least a certain difference in flexibility between mold and cast (by adding oil to the mixture of either one or the other?)


    1. Elizabeth Dawson says:

      Try using Crisco. It’s cheap and very easy to use. In my experience it also leaves less residue than oil.

  15. Naushad Sufyan says:
  16. Naushad Sufyan says:

    It’s a great piece of info. But need a little bit more information on making a silicon paint to print on silicon wrist bands. Check this site for jobs

  17. Tom Mort says:

    This article is titled DIY casting silicone, but, as often as not the comments that follow are about using it to make molds.

    I am looking to make a casting. I plan on using a funnel with a cone angle of 45 degrees as the mold with a 1-1/4 inch dowel down the middle of the cone. The large end will be 2-1/2 inches in diameter and the smaller end will be 1-1/4 inch.

    I am trying to make a gasket to replace one on the drain of an old concrete laundry tub. Replacements do not appear to be available.

    I don’t want it to shrink and need it to resistant to deterioration.

    Does anyone have any suggestions for my application?

  18. Annan says:

    Perfectly pourable and simple:
    By Volume:
    1 Part Cornstarch : 1 Part Silicone Caulk : 1 Part Mineral Spirits

    Smells horrible… do it outside.

    Also, a huge difference between some mineral spirits:
    A: Do NOT use the 1 gallon stuff from home depot (comes in a plastic bottle):

    B: Use the 1 qt stuff that is a metal can:

    The “B” type results in a nice pourable mixture that has a pot life of 5 to 10 minutes.. The “A” type has a pot life of 1 minute or less (before you finish stirring)… though it may be good for quick putty.

  19. omar figueroa says:

    Specifically is this diluent chemical name?

  20. Youkaii says:

    I don’t like the idea of using paint thinner. I say this because, the item I am trying to cast has been already painted and I do not want to mess with the current paint. When I was looking around for an alternative, I found this stuff called silicone oil. It is used to thin out casting silicone to a pour-able form when it has become thick. Has anyone tried this? I’m trying to figure out if the airsoft and treadmill belt “silicone oil” is the same because it’s cheaper.

  21. Sharon says:

    can we use oil instead of paint thinner?

  22. abercrombie paris soldes says:

    let the cold wind blowing all the Acacia sleep a simple person really do not know what I will become.

  23. selenafuentes24 says:

    Man this really was a life saver, thank you soooooo much!!

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I am descended from 5,000 generations of tool-using primates. Also, I went to college and stuff. I am a long-time contributor to MAKE magazine and My work has also appeared in ReadyMade, c't – Magazin für Computertechnik, and The Wall Street Journal.

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