Making Moulds: Silicone

Making Moulds: Silicone – Water Clear
I wasn’t planning on posting this for another week but I’m so tired I can barely move and have a tummy bug so I wouldn’t be able to concentrate. When you’re working with keepsakes they’re so precious that you need everything to be exactly right, from the right resin, to the right colour shade of shimmer, perfect moulds and the ability to keep your eyes open and not throw up every ten minutes…

You can download or pin a printable version of our silicone types here:

silicone mould making chart free from Nikki Kamminga at Tree of Opals
silicone mould making chart free from Nikki Kamminga at Tree of Opals. Click to open then Pin or right click (Windows)/Ctrl+click (Mac) to save.

Preparing Silicone for Mould Making

Once you have put all of the spheres or charm bead masters in the shot glasses, or your cabochons and flatbacks are walled, or you’ve prepared any other kind of embedded object, you need to work out the silicone you need. I try to make a little less than I think I would need because of the cost of the silicone. If you mix up too much it can be devastating to see it sitting there. You could prepare some extra mould masters in shot glasses, but if you go to that trouble you might as well make them. My little trick is so simple. I get a few clean side plates and have them on hand in case I have a little leftover silicone, if so I just pour it onto the plate and I have a piece of reusable cling film (just make sure the resin is food-safe). You could even use the silicone to protect the base of your UV lamp if you’re working with UV resin.

So that I pour the right amount of silicone in each mould I like to make a couple of marks on the side with a Sharpie, which I buy by the dozen for the studio. I mark one line just above the embedded object and one a little higher. I know to stop pouring when the silicone reaches the first mark and it then rises a little higher, I top it up to the second mark if necessary.

sphere silicone mould in progress Tree of Opals Nikki Kamminga
sphere silicone mould in progress

To work out the volume of silicone needed you can put a prepared glass/box on the weighing scales you’re going to use to weigh the silicone, then tare so it goes to zero. Fill it to the top line with water and you’ll have the weight for one. The weight to volume ratio for silicone is close enough to water here. Times this by the amount of that mould you’re making, then add 10% to allow for waste and spillage. In the video here I mixed up around 280g silicone to 28g hardener (I’m using a 10:1 ratio silicone, follow the manufacturers’ guidelines to the closest half gram!) and it was perfect for 18 spheres and four charm beads. It worked out to about 15g per mould, but I imagine I’d have needed nearly twice that if I’d used larger shot glasses. I don’t do the water trick, I eyeball it, but I gained experience with inexpensive blue/green RTV silicone then the middle-priced cloudy clear silicone before investing in the water-clear. If you want to eyeball it, make less than you think you’ll need then you can always mix more up later. Have a look at the chart at the top of the page to find out which silicone to use for your project.

Silicone Mixing

I use SORTA-Clear 18 which is soft enough for making spheres, here’s a list of their international distributors. You could try this brand of translucent silicone but I can’t recommend it as I’ve not tried it myself. In all silicone and resin work you’ll need vinyl gloves, which I double up on; that means you can rip off a pair and carry on working without breaking your workflow to put on another pair. I save my gloves when working with silicone and UV resin for other projects (or when our puppy Loki has an accident…) Refuse, reuse and recycle folks!

Once you’ve worked out how much silicone you’ll need (see above) grab your scales and a couple of bowls and cover your workstation with newspaper*. The first bowl doesn’t need to be much bigger than the silicone volume, I sometimes use a plastic Christmas pudding bowl from about eight years ago. The base fits on the scales nicely and it takes up to 300g. I have a huge tub of silicone so I open it up and coax a bit out then use a lollipop stick (we get through tons of these in the studios) to stop it dribbling down the side. As I said, my ratio is 10:1 (but yours might be different) so if I pour 280g part A I know I need 28g part B. I don’t want to waste part B so I grab a clean shot glass and pour roughly the amount I’ll need in it, then tare the scales and weigh out the part B (hardener). Any I don’t use can be put back in the bottle.

Stir slowly and carefully, trying to avoid adding too much air to the silicone. It’s impossible to get zero bubbles but you can minimise it. You need the two parts totally incorporated, scraping down the sides, just as you would for epoxy resin. I mix and scrape for at least two minutes. Perfect time to do your pelvic floors, any time I’m stirring I’m squeezing; if you think that’s TMI then you should know how normal stress incontinence is, especially after birth, and how opening up the discussion takes away the stigma frees us from the patriarchy… Anyway, when you’ve stirred it well pour it into the second container, which needs to be big enough to fit in the 1.5 gallon degassing chamber with vacuum pump and stir for another two minutes. Make sure you have enough vacuum pump oil because this machine is an investment you need to make the most of. TIP: I invested in mine the first Black Friday I was in business, the following year was a kiln. This year I’m taking fewer orders because I’ve been so ill so no treats, oh well.

Follow the manufacturers’ instructions for using the chamber and don’t re-pressurise it too quickly! Learn from my mistakes because you don’t want your lid covered in your expensive resin or silicone. There are lots of videos on using these machines on YouTube so have a look around when you get one. They are optional, but necessary (I think) for making good moulds for keepsakes.

degassing process of mould making Nikki Kamminga Tree of Opals
degassing process of mould making
degassing silicone to make moulds in a vacuum chamber Tree of Opals Nikki Kamminga
degassing silicone to make moulds in a vacuum chamber

Pouring Silicone Moulds

When you’re pouring silicone moulds you need to do it slowly and carefully with a very thin stream. Bubbles are easily trapped especially around the plasticine, and are a pain to get out with a toothpick. If you want to you can use a large pipette or even better a kids’ medicine syringe (which you can reuse for silicone next time but not meds obv) because it can be hard to control the pour rate.

pouring silicone onto sphere mould masters Tree of Opals Nikki Kamminga
pouring silicone onto sphere mould masters

Stop when you get to the first line and top up a while later if you need to. The silicone needs a long time to ease itself around the masters without trapping air so I like to add a little bit to each shot glass then go back and repeat again and again until they’re all full. You can paint on the silicone if you can get to all areas but I’ve knocked off too many carefully-placed charms and spheres to recommend it!

Finishing Silicone Moulds

I don’t recommend putting the moulds on a heat mat because the heat reacts with the plasticine and turns otherwise-clear silicone cloudy, like it will go after it’s been used with resin after a while. Every type of silicone will start to degrade with use; coloured ones will go pale and blotchy and they all lose their shine. When they get really bad the silicone will stick to the resin and have to be scraped off with a knife then sanded and the piece recast (or start from scratch, not always possible with keepsakes).

I find overnight in a warm home enough to cure silicone (instead of my cold studio) so I bring in the tray and put it somewhere away from kids and pets. To finish the moulds you check one to test that it’s firm by pressing your finger on the top. If it seems ok, go ahead and crack the shot glass. Sadly there’s no way to reuse these but you can recycle them… refuse, reuse and recycle is my motto and I urge everyone reading this to think about how they can make their business more sustainable and eco-friendly. Anyway, check the silicone isn’t tacky round the base which is where I sometimes find they’re the slowest to cure. Gently pull apart the silicone to get to the plasticine dot and take this out. If it’s a bit tricky you can put the moulds, dot down, once removed from the shot glasses onto something cold like a marble chopping board, or in the fridge. Hard plasticine is more solid and easier to pull off. Remove the numbers if you’ve added them and then take out the spheres. I like to work in stages, especially if I have 18 of them, and I must admit my mum often does this bit for me in front of the TV while she’s babysitting my little ones. We crack all the shot glasses, then remove all the plasticine (this can be reused).

Remove the mould master and trim off any excess silicone, wash up in warm soapy water and you have some beautiful silicone moulds. Ideally, you should do a test cast with each but you’ll soon learn to spot any duds.

I’m currently writing a series on making keepsake jewellery:
How to Make Breastmilk Jewellery
How to Make Cremation Ash Jewellery
How to Make Lock of Hair Jewellery

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mould (UK) = mold (US) thanks for the heads up Robin! Mwahhh x
calliper (UK) = caliper (US)
jewellery (UK) = jewelry (US)

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*We don’t actually have newspapers (or physical books, only for the kids) so my wonderful mum gets them on Freecycle for me.

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