Recycling used silicone and filling/extending new silicone

Please note before you start reading this older post that I have long since included a version in the mouldmaking section of my Materials pages, under silicone rubber, which can be accessed above. It is that version that may have been updated or expanded since.

There’s nothing better than silicone rubber if one wants to make reliable and long-lasting moulds. It is really second-to-none for reproducing detail, in addition to having the ideal combination of flexibility and strength. But silicone rubber isn’t cheap, averaging about £22 per kg and unlike many other materials there are no ‘cut-price’ versions. It’s rare to find one for under £20 per kg (in the UK may offer the best deals) and even the reduction one would usually expect when buying in bulk isn’t really that significant in this case. Moreover, unless one definitely needs to use a substantial amount all at once, buying silicone in bulk can often lead to wastage because its shelf life is comparatively short.

The common practice of building up a silicone ‘skin’ or layer over a form (rather than pouring a block) helps to reduce the amount and cost. See the series of posts Making a hollow 2-piece cast in fibreglass in the Methods section under Mouldmaking and casting for an account of this method. But there are additional, perhaps lesser known, ways of reducing the cost which at the same time modify or extend the capabilities of the material. Once cured, silicone cannot be simply re-melted (as vinyl can) but hardened leftovers and moulds which are no longer needed can be put to good use. In short, if these can be granulated they can be added as a filler to newly-mixed silicone.

In the photo below I am using a traditional-style kitchen mincer to turn silicone leftovers into granules. I’d heard about this being done before but didn’t really make much note of it until I came across this YouTube video below. I suppose I just hadn’t believed before that silicone could be granulated so easily! The commentary is in Portuguese, but it is fairly self-evident.

I then discovered that the practice is featured in a book that I’ve actually got! .. good old Thurston James’ The Prop Builder’s Molding & Casting Handbook 1989. I must confess that I read this book thoroughly a long time ago and probably still owe a lot to it without being conscious of that anymore. But I remember that after first reading I stowed it away with a little frustration, purely because many of the materials he features are either a little out-of-date now or only available in the US. I’m now planning to read it again with a bit more respect!

I bought my mincer ( a KitchenCraft brand ‘cast iron mincer’) at John Lewis in London for £18 last year, but Robert Dyas also has them and there are many online for between £17-£25.

cast iron mincer

I’ve found that it works best if the silicone is cut (with knife or scissors) into strips rather than chunks and that the receiving funnel is fairly continuously supplied while turning the handle. At times this is tough to turn, but ‘back pedalling’ a little before continuing will usually make it easier. These cast iron mincers usually come with a few extrusion plates with differently sized holes. I assume these will affect the size of the granules but I haven’t tested them yet. According to the YouTube demonstration the granulated silicone can be put through the mincer again to make it finer if needed. Another point to make here is that, although I cleaned the metal parts of the mincer as best I could before using it the action of the mincer appears to darken the silicone granules a little. I think it’s something in the metal itself but, as far as I can tell, it doesn’t affect the bonding of the silicone.

What I found surprising was that this doesn’t just work with the standard softer types (Shore A 20-25) but equally with a harder one. As long as it’s cut into manageable strips, as below, even a tougher Shore A 55 silicone (this is RTV-101 from Tiranti) will granulate without too much effort. Perhaps this is because the harder ones often have a lower tear resistance.

chopping up old moulds

granulated silicone

The sample to the left (above) includes a few different brands of silicone which have the same or similar Shore A value, while the pile to the right is just the one harder type. So far I’ve found that granules of different brands or even different hardnesses can be mixed into fresh silicone and the only thing I’m assuming will not work (and haven’t bothered to try) is mixing condensation cure silicones with addition cure (see Lexicon). I didn’t wash or wipe the old moulds or leftovers first. Some might have been a little dusty and I don’t think that would make much difference in the end, but as a rule it’s probably better at least to wipe them first.

But to find out more about what could be done I made the following test pieces.

test samples

For some of these small blocks I catalyzed an amount of new silicone first (roughly half the volume to be filled)  and poured or brushed a little of this as an initial layer in the Lego containment before mixing the remainder with its equal weight in granules, then adding it on top. The silicone should always be catalyzed before any other additions to it because otherwise these could affect the even spread of the catalyst. If a proper mould were being made this first ‘pure’ layer or detail coat would be essential, to keep any granules at a little distance from the mould surface. With some of the other blocks I waited until this first ‘pure’ layer was cured (i.e. added the recycling mix the next day) just to see if this might make a difference to the bonding of the layers.


first samples

Above are the test pieces made mixing ‘like for like’ granules and new silicone together (i.e. recycled and new silicone of the same type; Lukasil 429 Shore A 20), the one poured in two stages (with a day between) and the other all at the same time. There was no difference in feel between the cured blocks and certainly no delamination (i.e. separation of the layers) or cracks appearing when strongly flexed.

But the recycled granules do seem to have quite an impact in terms of flexibility! I made a test block of the same size using the Lukasil 429 pure to compare this, and the first photo below shows how this can be flexed with ease compared to exerting the same amount of pressure on the recycled block below it. Although the recycled granules had the same Shore A value the block was much tougher.

flexing pure Lukasil 429

flexing sample with granular fill

The samples were made on a perfectly smooth ceramic tile surface and the blocks match this without a blemish when relaxed, but I did notice that the underlying granular structure is noticeable when flexed (if this comes across at all in the photo below). I would assume however that this wouldn’t affect the surface during casting.

pitting on surface when flexed

Below are similar flexing tests on samples made using granules of the harder silicone (Tiranti RTV-101 Shore A 55) in which the layers can be better seen. Although I expected more problems since the silicones are not ‘like for like’, again there was no fissuring or delamination even under strong flexing. Also as expected the block was tougher than the previous one but, interestingly, not by that much considering the RTV-101 is so much harder.

flexing RTV-101 sample

flexing RTV-101 sample2

While I was about it I thought I might as well see what happened mixing other materials with the silicone instead of the recycled granules. It provided an interesting comparison and led to a surprise discovery. The sample block below contains cork granules and, as expected, the silicone doesn’t bond with them. The block started to fracture when flexed.

cork granules

But then I tried Fillite and the results were different. Fillite is an industrially produced ash commonly used as a filler for resins and plasters. I use it a great deal for extending polyurethane resin.

mixing Fillite with silicone

Fillite sample blocks

As before I made one block laying a thick detail coat of pure silicone first which was left to cure and for the other I just poured in the mix. I mixed almost the same weight of Fillite to silicone (actually 10g of Fillite to 12g of silicone because it already seemed quite a lot by volume). It mixed in very well and the mixture was still pourable though very thick. As with all the other tests, there was no effect on the normal curing time.

flexing block with Fillite layer

flexing Fillite block

On curing the silicone appeared to have bonded firmly with the Fillite, including the Fillite-filled layer with the pure silicone layer, with no sign of fracturing when flexed. In the process the composite had become substantially tougher, changing the Shore A 20 silicone into, at a guess, a Shore A 70 or more. The block composed wholly of silicone/Fillite mix above was very resilient, very slightly flexible but quite difficult to bend.

The Fillite and silicone had fused so well in fact that I suspected that some form of bonding was taking place at a molecular level i.e. not just ‘mechanical gripping’ but rather that the silicone polymer chains were linking directly with the Fillite. I have to say that before this I had been largely ignorant of what silicone rubber (i.e. what we buy off the shelf) actually is! I know at least a little more now. For example .. yes, ‘silicone’ is related to ‘silica’. Silicone rubber is derived from quartz sand and one reliable scientific source I looked at defined silicone rubber as ‘made from crosslinked silicon-based polymer reinforced with filler’. So it makes perfect sense that the long chain molecules of silicone rubber .. the flexible version of silica .. will attach themselves naturally to any less flexible version of silica, but because they are long will still keep much of their flexibility. Incidentally, this is also the reason why silicone rubber, which owes its popularity to the fact that it will not stick to most things, will stick fast to clean glass! Fillite is, as I now know, alumino-silicate, and the silicone rubber we buy off the shelf contains similar fillers as standard. Cabosil for example, which Thurston James also mentions as something which can be used to extend or thicken silicone, is so-called ‘fumed silica’. The silicone rubber manufacturers I looked at list fumed silica, precipitated silica, aluminum silicate or alumina trihydrate as their standard fillers.

What I’m suggesting this means is that one can modify any standard silicone rubber oneself, without fear of ‘weird’ consequences, whether you want to economise by extending the volume or whether you just want to increase the hardness. It’s what the manufacturers do anyway!

To sum up

My tests were not 100% scientific i.e. not rigorously controlled and I only tried a few variations, but at least they gave me some indications of possibilities and drawbacks. For me they definitely suggested that;

– addition of silicone rubber granules will toughen the resultant composite even if both the recycled and the fresh silicones used are relatively soft and of the same type. I would assume that when a fresh silicone with a low Shore A value (soft) is being used this will be toughened up in proportion to the amount of recycled silicone added, although I haven’t tested the range specifically.

– recycled silicone and fresh silicone don’t necessarily need to be ‘like for like’ or the same hardness to bond together

– the old silicone moulds or leftovers don’t need to be rigorously cleaned if they’re going through the mincer anyway.

– there was no noticeable effect on the normal cure time of the fresh silicone. There was no obvious effect on the general strength of the cured composite but the effect on tear strength in practice and in the long term would have to be further examined.

– if other fillers are used they should be ‘silicate’ and there is a range of these available. Fillite is one of the cheapest, will bond firmly with fresh silicone and, dependant on the ratio, can be used to adjust the flexibility/hardness of the composite from just a little tougher to almost plastic hardness. I’m assuming that fillers will always alter the flexibility of the cured silicone in some measure.

– according to all my tests so far ‘silicone will bond with silicone’, whether the first layer is still fresh or tacky or whether it’s been left to fully cure (as long as the surface has been kept clean and free from dust!) and this applies equally between different silicone brands, different hardness, filled or unfilled. It is however unlikely to apply between condensation cure and addition cure silicones!

33 thoughts on “Recycling used silicone and filling/extending new silicone

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  2. What if u had some clear rubber silicone and u wanted to use it but it’s all dried up how would u soften it back up to where you would be able too actually use it …or is it this even possible at all

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  4. Dear Mr Neat,
    Thanks so much for your excellent website – so much useful information! May I ask a quick question: what actually happens to old, unused silicone rubber? Will it not cure properly? I have a quantity that is well past it’s use by date but it’s just enough to do the job at hand. Normally I would just do a test – but I think I will need all that I have, and trying to accurately catalyse a really small test amount (e.g. a teaspoon full) is likely to be tricky. Obviously I don’t want to go ahead and coat my sculpture with a sticky mess of silicone that never cures, but it’s such expensive stuff that I’m loathe to waste it! Can you offer any helpful advice? Many thanks in advance if you can!

    As a thank you for all the excellent free info you provide, perhaps I can point you in the direction of a superb material that you might not have tried before: “Thistle Multi-finish” wall plaster. Although it’s intended for walls, I have found it has lots of excellent qualities: very strong; a long working time (@1 hour); can be mixed into a thick, highly thixotropic paste that can be applied even onto vertical surfaces; VERY, VERY cheap (@£6 per 25kg) and available from all diy sheds and builders merchants. I’ve used it for all manner of applications – mould jackets, casts, “direct” sculpture. I imagine it would be superb for many model-making purposes – especially creating terrain or texture. Anyway, just thought I’d mention it, because it would be easy to overlook and assume it would only work on walls.
    All the best,

    • Hi Ross,

      Thanks for the tip re the plaster .. I haven’t seen this but will look out for it. As for the old silicone .. you’ll have to test it (no sure alternative)! It’s possible to mix up a very small amount i.e 10-20 grams by dosing the catalyst using a pipette (since catalysts for silicone are likely to weigh 1g per 1ml). That’s all I can say!

  5. To add on silicone compatible fillers. I’ve tried sand with great success, also have sprinkled it on an uncured surface to assist in coupling coats (think fiber reinforced jacket molds with a permanently bonded silicone face). A cheap light weight solution, Poraver recycles glass into a bubbled filler, larger than the traditional micro variety, ranging 0.04mm to 8mm. Used for industrial filling applications, they’re cheap, about $350 delivered for 3(55 gal) sacks of various sizes selected to match natural beach sand grain sizing. A problem I’ve encountered with sand filler in loading epoxy was tracked to water absorption, both the resin and the sand had been in storage. Put the sand in a pan bake 220 let the mass get up to temp then let it cool on it’s own. That fixed it.

  6. Hi I was wondering what is the ratio that you can mix silicone with fresh is it 50/50? or something like 70/30? I have quit a bit of scrap silicone laying around and would like to use some in my mixture for my next mold project.



  7. Hi David,
    I really appreciate all the information you have gathered here.
    I keep stumbling into your site when I am searching for obscure sculpting knowledge.

    I am casting a bit of a tricky object and wanting to brush tin cure silicone onto the vertical surfaces with thixotropic and hopefully make my silicone go a bit further with some recycled/failed moulds. Do you have any tips for this?
    I am thinking I will need to at least initially build up a few mm thickness with pure silicone before moving onto the recycled mix? I am also thinking there should be a decent amount of curing time between layers?

    Silicone is expensive in here in Australia as well but my theory is because it is so specialised with no competition as far as art suppliers go here. I can get resins much cheaper through a boat building supplier than the art supplier (they have mentioned silicone as coming soon though so fingers crossed)

    Cheers and thanks again for your efforts

    • Hello Rob,

      Yes, 2-3mm new silicone should be applied before using the recycled mix. This first layer doesn’t have to cure fully before the recycled mix is applied, it only needs to be fairly firm. I can’t believe that one can’t obtain decently priced silicone outside art suppliers in Australia ..surely? Where do prop-makers or film effects people get it from then?

      • Thanks for responding David,
        I now await my thixotropic to arrive in the mail.

        The major supplier here for casting materials is ‘Barnes’ and I was lumping them in with the art suppliers but they mostly consist of casting materials. The cheapest I have found on their website is condensation cure silicone when purchased in a bulk of 22kg works out at $27 aud per kg which converts to 14.3 british pound. However I can buy much smaller quantities off ebay (marketed to people casting soap and candles) for $30 aud and I am trying to figure out their source.
        Perhaps this is OK and not going to find much cheaper.

  8. I add cabosil to tin cured silicone rubber, it does affect the curing time.
    Usually the rubber will cure around 12 hours. After adding cabosil, it took another 30 hours for the silicone rubber to fully cure. For 500 grams of silicone rubber i add about 200ml cup full of cabosil. The rubber can flow, but it is not as viscous as usual. After 24 hours, the silicone rubber is still tacky.

      • There are many type of cabosil out there? maybe the one i use is just not quite suitable. This is the only one i can find near my place, the shop owner only label it Cabosil with no other info. I didn’t notice huge increament in shore hardness, maybe i will et the mold sit there for another few days.

      • Of course Cabosil should not be having this effect on the silicone because (I always assumed) it was supposed to be inert? I have to say, I’ve never had to use it so far so I haven’t really looked into it. I’ll try to find out if there are any issues .. let me know how things turn out?

      • Hi, all molds cured after few days. Since i am not hurry into casting i just let it sit there to fully cured. I read a pdf document from maker of cab-o-sil reguarding shore hardness and elongating strength increament with cabosil added. What i was after is shore hardness increament, but i can’t find any fillite here. Is fillite powder, light weight like cabosil?

        I also recycle my old mold by cutting them into huge chunk and arrange them next to the pattern, to fill the space, after a first layer of rtv rubber was pour.

  9. Thanks for sharing your knowledge! I am wondering if anyone can help me find videos or books on how to make molds for small boxes. My children and I are wanting to make many silicone small boxes:) . Also can silicone be cured in sheets then stamped to form a shape like plastic? For instance how is the silicone bake wear made and ice trays?

    • I’m afraid I don’t know of any instruction that would help specifically. Have you looked at my articles on making moulds .. particularly ‘Beginner’s Basics’ .. under ‘Mouldmaking and casting’ in the ‘Methods’ section? Also, I don’t fully understand what you want to do .. do you want to make silicone moulds to cast ‘box’ forms in something else, or cast box forms in silicone? Silicone will cure in any form you pour it .. so for example if you pour it onto some kind of flat containment it will cure as a sheet. But as far as I know it’s not a good material for ‘stamping’ once cured .. at least not using home methods .. you’d need a lot of force and special cutters to cut it cleanly. The silicone bake wear you speak of is machine-made i.e. silicone probably injection-moulded under pressure etc. A lot of forms are possible industrially which would be beyond anyone working in the home or studio .. one can’t really compare them.

  10. many thanks for this, should prove a great help! recently got my own mincer off ebay…and been busy grinding….just one word of caution for folk though. I got the type with a suction bottom….very problematic, no way to fix it securely enough so makes the work a lot harder. Anybody thinking of getting one should definately get the type that bolts/ screws down.

    also, has anybody tried the electric type? i do have a lot to grind up…but they are quite expensive though.I don’t have the money right now….and worry it cd prove a waste if it burns the motor out to quickly on them? but i wdn’t have thought silicon wd be any harder wearing than meat…

      • 😉 i was thinking more the gristle and lumps of fat maybe….i’m not that much of a fancy cook so never tried grinding meat…found one on ebay, #20 brand new…turns out it 600 watt though, where most tend to be 12oo or higher….but you gnerally talking more than double the price….i could buy more silicon for that kind of money 😉

  11. Hi there.

    I use silicone rubbers in my mouldmaking (I sculpt mini figures and parts for articulated toys and mould and then cast them under pressure) and have been wondering what is safe to use to increase the shore A hardness of silicones. While I do know of the condensation (tin) cure rubber you’ve used (I’ve bought from Tiranti before too), I now exclusively use addition (platinum) silicones (I use TOMPS’s Viscolo range) as they have a longer shelf life, are more dimensionally stable (the solvents used in condensation based silicones will evaporate over time and cause shrinkage and that’s not good if you want your parts to fit together as they’re supposed to!), easier to mix (these have a 1:1 ratio by weight that I can’t get wrong) and less toxic with no odour. While addition cure silicones are apparently easy to “poison” so that the cure is inhibited, I don’t have that issue, but I guess it’s because I work very cleanly compared to some others?

    But because addition cure silicones are not as tolerant as condensation cure, I’m wondering if you have ever tried Fillite (or other fillers) with one to see if it will cure successfully? I’d like to harden my silicones, but I don’t use resin fillers as I use specialised resins and blends of different ones and don’t really want to buy a bag of Fillite I’ll never use if it doesn’t work (no one I know round where I live does resin casting as I know). What I can add to this article however is that I’ve done that reusing method for years and found funnily enough with the addition cure silicone I use it doesn’t seem to change the hardness for me. Maybe it’s because I cut mine up into little cubes (thought about mincing, but didn’t know how, so thanks for sharing that information). I’ll be definitely trying that out very soon!

    • Thanks for your post! I see no reason why Fillite shouldn’t have the same thickening/hardening effect I’ve achieved with the condensation cure silicones I’ve used, because it is totally inert. I know that cabosil for example is often used with silicones of either type. The ‘downside’ is that although there’s an increase in hardness there’s a loss of flexural strength i.e. the material can fracture much more easily (bits can break off under stress) .. and this happens with a regular hard silicone like Tiranti’s RTV 101 anyway! And yes .. the more finely minced the recycled silicone the more it will toughen the silicone mix, as far as I’ve found so far!

      • That’s good to know. Unfortunately, I broke my mincer the other day trying to mince my old Viscolo 22 moulds. Well it WAS an old mincer, but I should never have underestimated the rubber! Shouldn’t have cranked the handle so hard!

    • i did notknow that about condensation cure silicones! i have prob nearly 60 quids worth that at least a year old now…s***…..have i wasted my money? i’ve been saving it—just didn’t know it had a shelf life

      • Do a test with some of it now and if it cures as it should it may be fine for another 6 months .. as a broad guess! I’ve also found that often silicone rubber well past its shelf life will work roughly as normal if you can buy new catalyst.

      • i was wondering if might just be the cat…already bought extra of it…but noticed it seems to have separated out—was hoping a good shake wd fix it…but i’ll try that, ta! (another bit to grind up if it works) 😉

  12. I imagine I might have come up with this idea on my own if I had another 10 years or so, but reading about certainly will now make my mold making much cheaper to do thanks to you.

    Thank you.

    • Glad it will help! It’s ironic that silicone rubber is so expensive considering the raw material, quartz sand, is an unlimited resource apparently. It must mean that the processing is costly.

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