Updated: Mar 25
Silicone moulds have become an incredibly useful tool in my process but knowing where to start was intimidating. This is my journey with making silicone moulds, why I use them and how I make them.
So let’s start with what a mother mould actually is.
A mother mould is a copy that is taken of another mould so that multiple copies of a mould can be produced.
It’s essentially what I do when I use plaster moulds to make multiple copies of a vase or mug in clay. I’m just using different materials – silicone – to make multiple copies of my object – plaster mould parts.
So what I’ve started doing in my ceramics studio is to make a master plaster mould of my object as I normally would and I then make a copy of that plaster mould in silicone so that I can produce as many moulds as I need.
I have found this to be so incredibly useful and time saving but I was daunted by it and put off trying it for a long time.
This is a lot of mould talk so I’ll show some photos to help show what my moulds of moulds look like.
*I won’t be talking about using 3d prints to make mother moulds in this post, this is focusing on the use of mother mould making with plaster moulds.
So why make a mother mould?
The great thing about slip casting is you can make multiple copies of your design but there are some drawbacks.
Plaster moulds don’t last forever.
The surface of the plaster will eventually start to break down and get rougher after many uses, it varies depending on your clay but could be around 50-100 uses. Your mould will get to the end of its useful life and will stop absorbing the moisture from the slip making it take a very long time to dry your slip cast.
It can happen very easily, your hand slips on some slip and you drop your mould and it gets damaged.
Time is precious.
Slip casting is a time consuming process. Say you leave your slip in the mould for 30 minutes, it takes a few hours to dry before demoulding, your mould isn’t ready to cast again straight away because it has absorbed the water in that slip cast and needs time to dry out. I only slip cast once a day in a mould to allow it to dry out for casting the next day (I also live in Scotland a notoriously wet country so my moulds tend to take a while to dry out). So if I had one mould it would take me 5 days to cast 5 mugs, that might be fine if I only want to make a few mugs but I make small batches so I want to have several moulds to slip cast with each day. If I had 5 moulds I could make 5 casts a day and 25 casts a week, it maximises my time and production process.
The biggest thing for me with using silicone moulds has been my time saved. I’ve identified that my time is my most valuable thing in my business. I’m the sole employee so I’m very conscious of where my time is spent.
Mould making is a time consuming process. Lets say it takes me a full day to make a plaster mould for my mug design. If I want to have 5 mug moulds it would take me a full week to make them all. That’s a full week away from other work and then another full week to allow the moulds to fully dry out. I also find it very physically demanding to make moulds, especially for a full week. It takes a toll on my body and it makes me not enjoy the process. And I’d have to spend another full week making more moulds months later when my first ones have reached the end of their use.
So I am spending the time and making an investment initially in order to save time in the future.
When you have a silicone mother mould it takes less than one hour to make your plaster, wait for it to set and demould it. One hour compared to one full day. And five hours to make five moulds compared to a full week.
Let’s talk costs. Silicone is expensive so it can feel really daunting to spend a lot of money on a new material. I will say a lot of my initial fears were in spending a lot on silicone and having it go terribly wrong. If you can make a plaster mould you can make a silicone mould!
I’ve been trying to do cost breakdowns for my own moulds, everything I’ve calculated are estimates. A 5.5kg kit of my silicone is £120 so that’s £21.82 per kilo. For the most recent moulds I made, both 3 parts, a pendant and a small vase, I did them at the same time and I used just under 3kg of silicone mix so that’s £65.46 for two three part moulds. They’re very different sizes so the pendant mould I estimate at £21.82 and the larger mould at £43.64 to make.
Here are the plaster mould sizes:
Pendant – 8 x 4.5 x 5cm both sides, 8 x 8 x 3cm
Small vase – 13 x 14.5 x 7.5cm both sides, 14.5 x 14.5 x 3.5cm
These costs don’t even consider my time. It took me around 5 hours to make both sets of silicone moulds. So if I applied this to every mould I make it takes me on average one full day to make a plaster mould, then it takes me half a day to make it’s silicone mould. When I’m casting plaster into the silicone mould its maybe an hour to mix, pour plaster, let it set and demould so I could make 5 mug moulds in 5 hours when it would take me 5 full days if I did it the traditional way.
So it will cost me a day and a half plus £20 or £40 in materials to make my practice incredibly easier and save me so much time in future. For me the silicone moulds more than pay for themselves in how much time is saved.
Another thing I want to mention here is the cost of saved plaster. By casting the same object over and over into the silicone mould you’re able to make a note of exactly how much plaster is needed each time. For the first use I pour water into the silicone moulds then pour it out and measure how much is needed, I can then calculate the plaster required based on a ratio of 100:70, 100g of plaster to 70ml water. So I only make as much plaster as I need, saving in materials and waste.
Choosing the right silicone for you
I use Polycraft GP3481-F I get it from MB Fibreglass. This is the only silicone I’ve tried, there are many choices out there so what I use might not be well suited to you. I ended up going with this one because it doesn’t require a vacuum chamber to rid it of bubbles. From my research it didn’t produce bubbles on the surface of casts and surface bubbles haven’t been a problem for me. It also has a working time of 40-60 minutes so I can mix and pour with plenty of time. It does take 18-24 hours to cure fully but there are faster silicones out there. The silicone I use has a hardness shore of 27 A, this puts it at the top end of the soft category. With silicone the higher the shore the harder it’ll be so for our purposes we want a silicone that’ll hold it’s shape but that will be easy to demould a hard bit of plaster so somewhere in the middle of the scale seems right to me.
I can also cut it up into small pieces and reuse it in a new mould. I believe this can be done with most types of silicone, you can see some of the pieces sometimes but the appearance of the mould is not something I care about. Being able to do this has eased my worries about both my environmental impact and the cost of the material.
When you’re choosing the right silicone for you I recommend emailing some suppliers and telling them what you’re wanting to do and they will usually give you some recommendations. The people working for these companies know a lot more about silicone than I do so I’d definitely make use of their technical knowledge.
How it’s done and what to use
I am fairly new to silicone mould making so I am no expert. I struggled to find ceramics based tutorials online but the video I watched several times was Van Tiki’s. He’s not only entertaining but he also breaks it down really well so you can understand at beginner level. I also watched a lot of videos by more general model makers and made some changes based on what they do.
Calculate the estimated volume of silicone required
I measure the size of my plaster mould (if I’ve not made it yet I calculate it based on having 3cm around the widest point of my object and use a ruler during mould making to keep it right)
Calculate the volume of the mould, using an online calculator (I do art not maths!)
Calculate the volume of silicone needed. The box needs to be 1cm around each side and on top.
Take away the volume of the plaster mould from the volume of silicone and you’ll have an approximate volume of silicone needed for your mould. (I don’t tend to measure the object volume because I’m not a maths person). Silicone sticks to the sides of the container so having that little bit extra is good.
Prepare the mould box
I use foam boards for my mould boxes, I’ve seen a lot of ceramicist use wooden boards but I don’t have access to tools to cut wood well and it’s cheaper to buy foam board. Many model makers use foam board even for large projects. The downside of foam boards is they can only be used once because I always damage them when demoulding, wooden boards can be used again and again.
Measure and cut the foam (or wood) boards to allow to 1cm around all sides of the plaster and 1cm plus a little extra on top to cover it.
With the foam I cut the foam not all the way down to bend it and reinforce the corners with masking tape on the inside.
You’ll also need a board to sit the box on top of, a few cm bigger than the mould box.
Mould release agent
I use soft soap as my release agent as Van Tiki does in his video. Silicone technically shouldn’t stick to materials other than silicone (and glass apparently) but plaster is porous so I see it as good practice to seal it. Remember to wash your plaster mould after you’ve made the silicone mould – I made this mistake once and wondered why casts weren’t drying in big spots – it was because it was covered in soft soap! Wooden boards will need sealing too, I don’t bother on the foam boards though.
Protect your studio space
I cover my worktable and floor with plastic sheets to protect from spills (silicone without the catalyst won’t set and will just be a sticky mess). Also wear an apron and take off your nice shoes.
Build the mould boxes
I use a hot glue gun to seal around the edges of my mould boxes and hot glue to stick down the plaster mould so it doesn’t move. With 3d prints I made the mistake of using hot glue and one of my prints came unstuck and floated up but superglue works much better.
Level the table
Use a spirit level to make sure your mould boxes are level. My worktable is a very wonky foldout table to I do this with each mould and add card under the base until level.
Measure and mix the silicone
Your manufacturer should provide mixing instructions so your silicone might be different from mine.
I weigh out the silicone into a big container then weigh out the catalyst into a plastic cup (check your ratio for your silicone) mine is a ratio of 10:1. I use a thick wooden stirrer bought from my silicone supplier to mix. It does need to be a sturdy mixing stick as the silicone is very thick.
Once its all mixed fully I pour it from around 30 - 50cm ish above the box, most of the videos I watched recommended pouring from a high height to prevent bubbles but I have more control over it from this height. I think you’re recommended to do it as slow as you can which is hard when its heavy and why I went for a silicone with a longer working time. I start down the sides of the mould and let that build up so it spills up on the top surface of the mould. Doing it slow, steady and with a bit of height does seem to work. I have found that the silicone is so viscous that it will make its way into all the nooks and crannies of a mould by itself.
I have my glue gun hot and on hand just in case there’s any leaks to plug. The great thing I have found about silicone is it is very slow to leak so there’s less chance of a major disaster as can happen with plaster.
Wait and demould
I wait 18-24 hours to demould and I find it so satisfying! I trim the edges a little with a Stanley knife and the scraps can be saved to use in future moulds.
The silicone mother mould should then be ready to start using to make as many plaster moulds as you want.
Casting plaster into silicone moulds
I remake foam boards to go around the sides of the mother mould so the sides don’t bulge out when I do the plaster, if you used wooden boards they’ll do that job for you.
A tip I picked up from Van Tiki was to spray watered down window cleaner on the surface of the silicone before pouring plaster, this breaks the surface tension and helps prevent bubbles forming.
So are you convinced to try making a silicone mould yet?
Silicone moulds really do pay for themselves in how much time they save in the long term. They won’t be suited to you if you only want to make a couple pieces, there’s no need. But if you have products that you want to make in small batches with multiple moulds and plan on selling in the long term I highly recommend them.
By spending a couple of days making the master plaster mould then the mother silicone mould you will be saving yourself countless days of work in future. The breakdown of costs that I did feels worth it to me and the convenience of being able to make a new plaster mould without taking up my entire day or my entire studio is well worth it.